Hydrocarbon Chapter 24 PDF

1081 60 Hydrocarbon Chapter E3 24 Hydrocarbon Organic compounds composed of only carbon and quinolines and pyrroles. Oxygen compounds present in petroleum are. Alcohols, Phenols and resins. Compounds like chlorophyll, haemin are also present in it. U hydrogen are called hydrocarbons. Hydrocarbons ar...

1081 60 Hydrocarbon Chapter E3 24 Hydrocarbon Organic compounds composed of only carbon and quinolines and pyrroles. Oxygen compounds present in petroleum are. Alcohols, Phenols and resins. Compounds like chlorophyll, haemin are also present in it. U hydrogen are called hydrocarbons. Hydrocarbons are two types (1) Aliphatic Hydrocarbon (Alkanes, Alkenes and Alkynes). D YG (2) Aromatic Hydrocarbon (Arenes) unpleasant smell of petroleum is due to sulphur compounds. Nitrogenous compounds are pyridines, ID Aliphatic Hydrocarbon (v) Natural gas : It is a mixture of Methane (80%), Ethane (13%), Propane (3%), Butane (1%), (1) Sources of aliphatic hydrocarbon Vapours of low boiling pentanes and hexanes (0.5%) Mineral oil or crude oil, petroleum [Petra rock; and Nitrogen (1.3%). L.P.G. Contain butanes and oleum oil] is the dark colour oily liquid with offensive odour found at various depths in many regions below the surface of the earth. It is generally found under the rocks of earth’s crust and often floats over salted water. U (2) Composition (i) Alkanes : found 30 to 70% contain upto 40 ST carbon atom. Alkanes are mostly straight chain but some are branched chain isomers. pentanes and used as cooking gas. It is highly inflammable. This contain, methane, nitrogen and ethane. (vi) C.N.G. : When natural gas compressed at very high pressure is called compressed natural gas (CNG). Natural gas has octane rating of 130 it consists, mainly of methane and may contain, small amount of ethane and propane. (ii) Cycloalkanes : Found 16 to 64% cycloalkanes (3) Theories of origin of petroleum : Theories present in petroleum are; cyclohexane, methyl cyclopentane etc. cycloalkanes rich oil is called must explain the following characteristics associated asphaltic oil. (iii) Aromatic hydrocarbon : found 8 to 15% compound present in petroleum are; Benzene, Toluene, Xylene, Naphthalene etc. (iv) Sulphur, nitrogen and oxygen compound : Sulphur compound present to the extent of 6% include mercaptans [R-SH] and sulphides [R-S-R]. The with petroleum, Its association with solution). brine (sodium chloride The presence of nitrogen and sulphur compounds in it. The presence of chlorophyll and haemin in it. Its optically active nature. Three important theories are as follows. (i) Mendeleeff’s carbide theory or inorganic theory 1082 Hydrocarbon (ii) Engler’s theory or organic theory (5) Petroleum refining : Separation of useful fractions by fractional distillation is called petroleum (iii) Modern theory refining. (4) Mining of petroleum : Petroleum deposits occurs at varying depth at different places ranging from 500 to 15000 feet. This is brought to the surface by artificial drilling. Boiling range Uncondensed gases Approximate composition Upto room temperature C1 – C4 Fuel gases: refrigerants; production of carbon black, hydrogen; synthesis organic chemicals. 30 – 150o C5 – C10 (i) Petroleum ether 30 – 70o C5 – C6 (ii) Petrol or gasoline 70 – 120o C6 – C8 (iii) Benzene derivatives 120 – 150o Kerosene oil 150 – 250o Heavy oil 250 – 400o of Solvent Motor fuel; drycleaning; petrol gas. ID Crude naphtha on refractionation yields, Solvent; drycleaning C11 – C16 Fuel; illuminant; oil gas C15 – C18 As fuel for diesel engines; converted to gasoline by cracking. U C8 – C10 D YG Refractionation gives, Uses E3 Fraction 60 Table : 24.1 ( oC) (i) Gas oil, (ii) Fuel oil, (iii) Diesel oil Residual oil on fractionation by distillation gives, vacuum Above 400o C17 – C40 C17 – C20 Lubrication (ii) Paraffin wax C20 – C30 Candles; boot polish; wax paper; etc (iii) Vaseline C20 – C30 Toilets; ointments; lubrication. C30 – C40 Paints, road surfacing U (i) Lubricating oil (iv) Pitch ST Petroleum coke As fuel. (on redistilling tar) (6) Purification (i) Treatment with concentrated sulphuric acid : The gasoline or kerosene oil fraction is shaken with sulphuric acid to remove aromatic compounds like thiophene and other sulphur compound with impart offensive odour to gasoline and kerosene and also make them corrosive. (ii) Doctor sweetening process : 2 RSH  Na 2 PbO 2  S RSSR  PbS  2 NaOH Mercaptan Disulphide s (iii) Treatment with adsorbents : Various fractions are passed over adsorbents like alumina, silica or clay etc, when the undesirable compounds get adsorbed. (7) Artificial method for manufacture of Petrol or gasoline (i) Cracking, (ii) Synthesis Hydrocarbon 1083 (i) Cracking : It is a process in which high boiling The tendency to knock falls off in the following fractions consisting of higher hydrocarbons are heated strongly to decompose them into lower hydrocarbons order : Straight chain alkanes > branched chain alkanes > olefins > cyclo alkanes > aromatic hydrocarbons. with low boiling points. Cracking is carried out in two different ways. (2) Octane number : It is used for measuring the knocking character of fuel used in petrol engine. The high octane number of a given sample may be defined as the percentage by volume of iso-octane present in a temperature (475 – under high pressure (7 to 70 atmospheric pressure). The high pressure keeps the mixture of iso-octane and n-heptane which has the same knocking performance as the fuel itself. heavy oil Liquid phase cracking : In this process, the or residual oil is cracked at a 530oC) reaction product in liquid state. The conversion is approximately 70% and the resulting petrol has the octane number in the range 65 to 70. CH 3  CH 2  CH 2  CH 2  CH 2  CH 2  CH 3 n-heptane; octane no. = 0 CH 3 | catalysts like silica, zinc oxide, titanium oxide, ferric oxide and alumina. The yields of petrol are generally high when catalyst is used. (b) Vapour phase cracking : In this process, | CH 3  C  CH 2  C  CH 3 ; Octane no. = 100 | CH 3 2, 2, 4-Trimethyl pentane or Iso-octane. For example : a given sample has the knocking performance equivalent to a mixture containing 60% iso-octane and 40% heptane. The octane number of the gasoline is, therefore, 60. ID kerosene oil or gas oil is cracked in vapour phase. The temperature is kept 600 – 800oC and the pressure is CH 3 E3 The cracking can be done in presence of some 60 (a) Presence of following types of compounds increases the octane number of gasoline. (i) In case of straight chain hydrocarbons octane number decreases with increase in the length of the chain. (ii) Branching of chain increases the value of octane number (iii) Introduction of double bond or triple bond increases the value of octane number. (iv) Cyclic alkanes have relatively higher value of octane number. (v) The octane number of aromatic hydrocarbons are exceptionally high (vi) By adding gasoline additives (eg TEL) (3) Antiknock compounds : To reduce the knocking property or to improve the octane number of a fuel certain chemicals are added to it. These are called antiknock compounds. One such compound, which is extensively used, is tetraethyl lead (TEL). TEL is used in the form of following mixture, U about 3.5 to 10.5 atmospheres. The cracking is facilitated by use of a suitable catalyst. The yields are about 70%. (ii) Synthesis : Two methods are applicable for synthesis. D YG (a) Bergius process : This method was invented by Bergius in Germany during first world war. Fe O3 o Mix. Of hydrocarbons or crude Coal  H 2 2 450 500 C 250 atm oil (b) Fischer- tropsch process : The overall yield of this method is slightly higher than Bergius process. o 1200 C H 2 O  C   CO  H 2 U Water gas xCO  yH 2  Mix. Of hydrocarbon  H 2 O. ST The best catalyst for this process is a mixture of TEL = 63%, Ethylene bromide = 26%, Ethylene cobalt (100 parts), thoria, (5 parts), magnesia (8 parts) and kieselguhr (200 parts). chloride = 9% and a dye = 2%. Characteristics of hydrocarbons deposited in the engine. To remove the free lead, the ethylene halides are added which combine with lead to form volatile lead halides. (1) Knocking : The metallic sound during working of an internal combustion engine is termed as knocking. “The greater the compression greater will be efficiency of engine.” The fuel which has minimum knocking property is always preferred. However, there is a disadvantage that the lead is Pb  Br  CH 2  CH 2  Br PbBr 2  CH 2  CH 2 Ethy lene bromide Volatile Ethy lene However, use of TEL in petrol is facing a serious problem of Lead pollution, to avoid this a new compound cyclopenta dienyl manganese carbonyl 1084 Hydrocarbon (called as AK-33-X) is used in developed countries as antiknocking compound. (4) Other methods of improving octane number of hydrocarbon. (i) Isomerisation [Reforming] : By passing an alkane over AlCl 3 at 200 o C. CH 3 | AlCl 3 CH 3 CH 2 CH 2 CH 2 CH 3   CH 3 CHC H 2 CH 3 o (7) Petrochemicals : All such chemicals which are derived from petroleum or natural gas called petrochemicals. Some chemicals which are obtained from petroleum are summarised in table : Isopentane (Octane number  90 ) (ii) Alkylation : Table : 24.2 CH 3 CH 3 CH 3 CH 3 | | | | H SO 4 CH 3 CH  CH 2  C CH 3 2 CH 3 CCH 2 CHCH 3 Isobutylene | | CH 3 CH 3 Hydrocarbons Iso- octane (Octane number  100) CH 3 CH 3 (CH 2 )5 CH 3 Heptane  4H2 500 o C Methane Methyl chloride, chloroform, methanol, formaldehyde, formic acid, freon, hydrogen for synthesis of ammonia. Ethane Ethyl chloride, ethyl bromide, acetic acid, acetaldehyde, ethylene, ethyl acetate, nitroethane, acetic anhydride. ID (iii) Aromatisation : Pt / Al2 O3    Ethylene Ethanol, ethylene oxide, glycol, vinyl chloride, glyoxal, polyethene, styrene, butadiene, acetic acid. Propane Propanol, propionic acid, isopropyl ether, acetone, nitromethane, nitroethane, nitropropane. U Toluene The octane no. of petrol can thus be improved. D YG  By increasing the proportion of branched chain or cyclic alkanes.  By addition of aromatic hydrocarbons Benzene, Toluene and Xylene (BTX).  By addition of methanol or ethanol.  By additon of tetraethyl lead (C2 H 5 )4 Pb (5) Cetane number : It is used for grading the diesel oils. CH 3  (CH 2 )14  CH 3 Cetane cetane no. = U 100 ST Glycerol, allyl alcohol, isopropyl alcohol, acrolein, nitroglycerine, dodecylbenzene, cumene, bakelite. Hexane Benzene, DDT, gammexane. Heptane Toluene Cycloalkanes Benzene, toluene, xylenes, adipic acid. Benzene Ethyl benzene, styrene, phenol, BHC (insecticide), adipic acid, nylon, cyclohexane, ABS detergents. Toluene Benzoic acid, TNT benzaldehyde, saccharin, chloramine-T, benzyl chloride, benzal chloride. “Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecules.” Cetane no. = 0 -Methyl naphthalene Alkanes are less reactive so called paraffins; The cetane number of a diesel oil is the percentage of cetane (hexadecane) by volume in a  Propylene Alkanes [Paraffines] CH3 mixture of cetane and Compounds derived E3 Isobutane Chemists have prepared some hydrocarbons with octane number even less than zero (e.g., n-nonane has octane number – 45) as well as hydrocarbon with octane number greater than 100 (e.g., 2, 2, 3 trimethylbutane. has octane number of 124). 60 200 C Pentane (Octane number  62) The flash point in India is fixed at 44 o C , in France it is fixed at 35oC, and in England at 22.8oC. The flash point of an oil is usually determined by means of “Abel’s apparatus”. -methyl naphthalene which has the same ignition property as the fuel oil under consideration. (6) Flash point : The lowest temperature at which an oil gives sufficient vapours to form an explosive mixture with air is referred to as flash point of the oil. because under normal conditions alkanes do not react with acids, bases, oxidising agents and reducing agent. General formula : Cn H 2n2 Examples are CH 4 , C2 H6 , C3 H8 , (1) General Methods of preparation (i) By catalytic hydrogenation of alkenes and alkynes (Sabatie and sanderen’s reaction) Hydrocarbon Ni Ni C n H 2 n  H 2  C n H 2 n  2 ; Cn H 2n2  2 H 2  Cn H 2n2 Alkane heat Alkyne At anode [Oxidation] :  Alkane  Methane is not prepared by this method (ii) Birch reduction :  2 R  R  R (alkane) 3 R  CH  CH 2  R  CH 2  CH 3 1. Na / NH 2. CH 3 OH At cathode [Reduction] : (iii) From alkyl halide 2 H 2O 2 Na   2e   2 Na    2 NaOH  H 2 (  ) Zn / HCl (a) By reduction : RX  H 2   RH  HX (b) With hydrogen in presence of pt/pd : RX  H 2  RH  HX Pd orPt.  Both ionic and free radical mechanism are involved in this reaction. (c) Reduction of carboxylic acid : (c) With HI in presence of Red phosphorus : RBr  2 HI  RH  HBr  I2 Purpose of Red P is to remove I2 in the form of PI3 Re duction CH 3 COOH  6 HI    CH 3 CH 3  2 H 2 O  3 I2 Zn-Cu couple (x) By reduction of alcohols, aldehyde, ketones or acid derivatives Cu   (CH 3 CH 2 O)2 Zn 2 H Zinc ethoxide Red P CH 3 OH  2 HI   CH 4  H 2 O  I2 RX  2 H  RH  HX (v) Wurtz reaction : Dry ether R X  2 Na  X R    R  R  2 NaX Alky l halide ID Acetaldehyde (Ethanal) (vii) Corey-house synthesis Acetone (Propanone ) D YG 2. CuI  Reaction is suitable for odd number of Alkanes. RMgX  HOH  RH  Mg(OH )X Water Clemmenson reduction : Zn  Hg CH 3 CHO  4 H    CH 3  CH 3  H 2 O U ST Propane  Aldehydes and ketones ( C  O) can be reduced to hydrocarbon in presence of excess of hydrazine and sodium alkoxide on heating. Alkane Wolff-kishner reduction : R O || R  C  O   Na  R 2 C  O 2   H 2O R 5 C  NNH 2 2    H NNH R (b) Kolbe’s synthesis : Ionization HCl Acetone (Propanone )  NaOH and CaO is in the ratio of 3 : 1. (Sodalime) R  C  O  Na  || O Ethane Zn  Hg CH 3 COCH 3  4 H    CH 3 CH 2 CH 3  H 2 O (ix) From carboxylic acids Electrolysis HCl Acetaldehyde (Ethanal) R  X  RMgX  R  R  MgX 2 CaO Ethane  Aldehyde and ketones when reduced with amalgamated zinc and conc. HCl also yield alkanes. (b) By reaction with alkyl halide : heat R COONa  NaOH   R  H  Na 2 CO 3 Ethane 200 o C Acetamide (Ethanamid e) Alkane (a) Laboratory method [Decarboxylation reaction or Duma reaction] Propane O || Red P CH 3  C  NH 2  6 HI   CH 3  CH 3  H 2 O  NH 3  3 I2 CH 3  CH 2  CH 2  CH 3 (a) By action of acidic ‘H’ : 150 o C 200 o C Acety lchloride (Ethanoy l chloride) CH 3  CH 2  Cl (CH 3  CH 2 )2 LiCu   (viii) From Grignard reagent Ethane O || Red P CH 3  C  Cl  6 HI   CH 3  CH 3  H 2 O  HCl  3 I2 CH 3  CH 2  Cl 1. Li 150 o C Red P CH 3 COCH 3  4 HI   CH 3 CH 2CH 3  H 2O  2 I2 U 2 RX  Zn  R  R  ZnX 2 Methane Red P CH 3 CHO  4 HI   C2 H 6  H 2 O  2 I2  R  Br or RI preferred in this reaction. The net result in this reaction is the formation of even no. of carbon atoms in molecules. (vi) Frankland’s reaction : Alkylmagnesium halide 150 o C Methanol (Methy l alcohol) Alkane Alky l halide Ethane E3 Zn p Acetic acid (iv) By Zn-Cu couple : 2CH 3 CH 2 OH   2 R  C  O   2e   2 R  C  O  2 R  2CO 2 || || O O 60 heat Alkene 1085 C H ONa R 180 o C ,  N 2 CH 2 R (xi) Hydroboration of alkenes (a) On treatment with acetic acid B2 H 6 CH 3 COOH R  CH  CH 2    (R  CH 2  CH 2 )3 B   Alkene Trialky l borane 1086 Hydrocarbon R  CH 2  CH 3 Alkane (b) Coupling of alkyl boranes by means of silver nitrate  Iodination of methane is done in presence of oxidising agent such as HNO 3 / HIO3 / HgO which neutralises HI.  Chlorination of methane : o 2 B2 H 6 AgNO 3 25 C 6[R  CH  CH 2 ]   [2 R  CH 2  CH 2 ]3 B    NaOH u. v. light 2 CH 4  2Cl  Cl   CH 2  Cl 2  u. v. light , Cl 3[RCH 2 CH 2  CH 2 CH 2 R]  2 HCl  HCl  HCl CHCl 3   CCl 4 (2) Physical Properties Cl 2 (i) Physical state : Alkanes are colourless, odourless and tasteless. State C1  C4 Alkane Liquid state [Except neo pentane which is gas] temp. Nitroalkan e Nitrating mixture : (i) (Con. HNO 3  Con. H 2 SO 4 ) at Solid like waxes E3 250 o C C18 and above : R  H  HONO 2  R  NO 2  H 2 O High Gaseous state C5  C17 60 Alkanes (ii) Reaction based on free radical mechanism (a) Nitration (ii) (HNO 3 vapour at 400 o  500 o C ). (ii) Density : Alkanes are lighter than water. (b) Sulphonation : Free radical mechanism SO 3 R  H  HOSO 3 H    R  SO 3 H  H 2 O Prolonged heating ID (iii) Solubility : Insoluble in water, soluble in 1 organic solvents, solubility  M olecular mass C3 H8 M.P.(K ): 85.9 C4 H10 C5 H12 C7 H16 C6 H14 C8 H18 D YG Alkane : 138 143.3 179 182.5 216.2  Melting points of even > Odd no. of carbon atoms, this is because, the alkanes with even number of U carbon atoms have more symmetrical structure and result in closer packing in the crystal structure as compared to alkanes with odd number of carbon atoms. C C C C C C C C C C C C C Even no. of carbons ST Odd no. of Carbons (i) Substitution reactions of Alkanes (a) Halogenation : R  H  X  X  R  X  HX The reactivity of halogen is : F2  Cl 2  Br2  I2  Fluorine can react in dark Cl 2 , Br2 require light I2 doesnot show (iii) Oxidation (a) Complete Oxidation or combustion :  3n  1  Cn H 2n  2   O 2  nCO 2  (n  1)H 2 O  Q  2   This is exothermic reaction. (b) Incomplete combustion or oxidation Burn 2CH 4  3 O 2   2CO  4 H 2 O CH 4  O 2  C 2 H 2 O (c) Catalytic Oxidation : Cu  tube CH 4  [O]  o CH 3 OH 100 atm / 200 C This is the industrial method for the manufacture of methyl alcohol.  Higher alkanes are oxidised to fatty acids in presence of manganese stearate. (3) Chemical properties energy.  Lower alkanes particularly methane, ethane, do not give this reaction. U (iv) Boiling points and Melting points : Melting points and boiling points.  Molecular mass 1  No. of branches any reaction at temperature, but on heating it shows iodination. O2 CH 3 (CH 2 )n CH 3   CH 3 (CH 2 )n COOH o 100 160 C (d) Chemical oxidation : KMnO 4 (CH 3 )3 CH   (CH 3 )3.C.OH Isobutane Tertiary buty l alcohol room (iv) Thermal decomposition or cracking or pyrolysis or fragmentation Hydrocarbon 1087 u.v light CH 3  CH 2  CH 3  SO 2  Cl 2    o 1000 C CH 4   C  2H 2 Methane CH 3  CH 2  CH 2 SO 2 Cl  HCl 500 o C C 2 H 6   CH 2  CH 2  H 2 Ethane Cr2 O 3  Al 2 O 3 This reaction is known as reed’s reaction. Ethy lene  This is used in the commercial formation of detergent. C 3 H 8  C 2 H 4  CH 4 or C 3 H 6  H 2  This reaction is of great importance to 2-Methyl pentane 60 (1) Methane : Known as marsh gas. (i) Industrial method of preparation : Mathane gas is obtained on a large scale from natural gas by liquefaction. It can also be obtained by the application of following methods, (a) From carbon monoxide : A mixture of carbonmonoxide and hydrogen is passed over a catalyst Isobutane AlCl3  HCl    heat 2,3 Dimethyl butane ID containing nickel and carbon at 250 o C when methane is formed. (vi) Aromatisation : Ni  C CO  3 H 2   CH 4  H 2 O o CH3 250 C     o Cr2 O3 / Al2 O 3 +4H2 CH2 Benzene (b) Bacterial decomposition of cellulose material present in sewage water : This method is being used in England for production of methane. U 600 C / 15 atm n-Hexane CH3 CH3 D YG H2   Cr2O3 / Al2O3    o 600 C nHeptane : E3 200 C , 35 atm n - Butane H2C steam Individual members of alkanes CH 3 | AlCl 3  HCl  CH 3 CHCH 3 CH 3 CH 2 CH 2 CH 3  o CH2 of 800 C (v) Isomerisation : CH3 Action Ni / Al2O3 CH 4  H 2 O  o   CO  3 H 2 petroleum industry. H2C (x) Methyl cyclo Hexane Toluen e (vii) Step up reaction (a) Reaction with CH 2 N 2 (Diazo methane) : hv R  CH 2  H  CH 2 N 2  R  CH 2  CH 2  H U (b) Reaction with CHCl 3 / NaOH :  CHCl 3 / OH R  CH 2  H    R  CH 2  CHCl 2 : CCl 2 ST (c) Reaction with CH 2  C : || O O || CH  C /  2 R  CH 2  H    R  CH 2  CH 3 :CH 2 /  CO (viii) HCN formation : N 2 / electric arc 2CH 4   2 HCN  3 H 2 or Al 2 O 3 CH 4  NH 3    HCN  3 H 2 700 o C (ix) Chloro sulphonation/Reaction with SO2+Cl2 (C6 H10 O5 )n  nH 2 O  3nCH 4  3nCO 2 Cellulose (c) Synthesis :  By striking an electric arc between carbon electrodes in an atmosphere of hydrogen at 1200oC, methane is formed. o 1200 C C  2 H 2   CH 4 By passing a mixture of hydrogen sulphide and carbon disulphide vapour through red hot copper, methane is formed. High temperatur e CS 2  2 H 2 S  8 Cu    CH 4  4 Cu 2 S (ii) Physical properties (a) It is a colourless, odourless, tasteless and non-poisonous gas. (b) It is lighter than air. Its density at NTP is 0.71 g/L. (c) It is slightly soluble in water but is fairly soluble in ether, alcohol and acetone. (d) Its melting point is  182. 5 o C and boiling point is  161. 5 o C. (iii) Uses (a) In the manufacture of compounds like methyl alcohol, formaldehyde, methyl chloride, chloroform, carbon tetrachloride, etc. (b) In the manufacture of hydrogen, used for making ammonia. (c) In the preparation of carbon black which is used for making printing ink, black paints and as a filler in rubber vulcanisation. 1088 Hydrocarbon (d) As a fuel and illuminant. (2) Ethane (i) Methods of preparation (a) Laboratory method of preparation : H | Lindlar' s Cataly st R  C  C  R  H 2    R  C  C  R Pd. BaSO 4 | H Zn  Cu couple C2 H 5 I  2 H    C2 H 6  HI  Poison’s catalyst such as BaSO 4 ,CaCO 3 (b) Industrial method of preparation : Ni CH 2  CH 2  H 2   CH 3  CH 3 300 o C Ethy lene (ethene) Ethane (ii) Physical properties (a) It is a colourless, odourless, tasteless and non-poisonous gas. (b) It is very slightly soluble in water but fairly soluble in alcohol, acetone, ether, etc. (c) Its density at NTP is 1.34 g/L (d) It boils at – 89oC. Its melting point is –172oC. (iii) Uses (a) As a fuel. (b) For making hexachloroethane which is an artificial camphor. Ascent of alkane series, (i) Methane to ethane : Cl 2 Wurtz reaction   CH 3 Cl    CH 3  CH 3 UV Heat with Na in ether Ethane (ii) Butane from ethane : (b) From vicinal dihalides : UV Wurtz reaction Ethy l chloride Heat with Na in ether Butane D YG Ethane (excess) Descent of alkane series : Use of decarboxylation reaction is made. It is a multistep conversion. Ethane to methane Cl 2 Aq. KOH [O ] C 2 H 6   C 2 H 5 Cl   C 2 H 5 OH   CH 3 CHO Ethane (excess) UV Ethy l chloride Ethy l alcohol Acetaldehyde [O ] NaOH NaOH / CaO   CH 3 COOH    CH 3 COONa   CH 4 Sodium acetate heat Methane U Aceticacid Cl 2 Aq. [O ] [O ] Higher   Alkyl   Alcohol   Aldehyde   alkane UV halide KOH ST NaOH NaOH / CaO Acid    Sodium salt of    Lower alkane heat the acid Alkenes These are the acyclic hydrocarbon in which carbon-carbon contain double bond. These are also known as olefins, because lower alkene react with halogens to form oily substances. General formula is Cn H 2n. Examples, C2 H 4 , C3 H 6 , C4 H 8. (1) Preparation methods (i) From Alkynes :  If we use alc. NaOH in place of KOH then trans product is formed in majority because of its stability. According to saytzeff rule. (iii) From dihalides (a) From Gem dihalides X Zn X   R – CH + R – CH = CH – R + CH – R   2 ZnX 2 X Zn X  If we take two different types of gemdihalides then we get three different types of alkenes.  Above reaction is used in the formation of symmetrical alkenes only. C 2 H 6   C 2 H 5 Cl   C 2 H 5  C 2 H 5 Cl 2 Alkene U CH 4 Methane used to stop the reaction after the formation of alkene. (ii) From mono halides : H H H | | | R C C R  C  C  H  Alc. KOH     H  HX | | | H H X ID (3) Interconversion of Alkanes are 60 Ethane E3 C2 H5 OH Ethyl iodide H H H H | | | |  R  C  C  H  Zn dust  R  C  C  H  ZnX 2 300 o C | | X X  Alkene is not formed from 1, 3 dihalides. Cycloalkanes are formed by dehalogenation of it. Zn dust CH 2 C H 2  CH 2  C H 2    ZnX 2 | | X X H 2C CH 2 (iv) By action of NaI on vicinal dihalide : Br Br | | C C vic dihalide I I | | C C NaI   acetone unstable   I2 CC alkene (v) From alcohols [Laboratory method] : H 2 SO 4 or H 3 PO4 CH 3 CH 2 OH   CH 2  CH 2  H 2 O Ethyl alcohol 443 K Ethene (vi) Kolbe’s reaction : CH 2 COOK CH 2 Electroly sis |  2 H 2 O  | |  2CO 2  H 2  2 KOH CH 2 COOK CH 2 Potassium succinate Ethene (vii) From esters [Pyrolysis of ester] : CH 3  CO  O H CH 3  COOH Glass wool 450 o   | |  liq. N 2 CH 2  CH 2 CH 2  CH 2 Hydrocarbon (viii) Pyrolysis compounds :  of quaternary ammonium  heat (C 2 H 5 )4 N OH   (C 2 H 5 )3 N  C 2 H 4  H 2 O Tetraethy l ammonium Triethy lamine hydroxide (Tert. amine) (i) Francis experiment : According to Francis electrophile first attacks on olefinic bond. 4 CH2 – CH2 = CH2 + Br – Br  CH2 | | Br Br CCl Ethene (ix) Action of copper alkyl on vinyl chloride : 2 H 2 C  CHCl  H 2 C  CHR CuR NaCl   CH2 – CH2 + CH2 – CH2 Viny lchloride | Br (x) By Grignard reagents : | Br | Br | Cl 60 R  X  CH  CH 2  MgX 2  R  CH  CH 2 (ii) Reaction with hydrogen : X H H H H | | | | Ni R  C  C  R  H 2   R  C  C R | | H H (xi) The wittig reaction : (Ph)3 P  CH 2  CH  R (Ph)3 P  O  R  CH || || O CH 2 O || (Ph)3 P  CH  R  CH  R (Ph)3 P  O  R  CH  CH  R (iii) Reduction of alkene via hydroboration : Alkene can be converted into alkane by protolysis H  BH 2 RCH  CH 2  (R  CH 2  CH 2 ) 3 B ID (xii) From  bromo ether [Boord synthesis] Br O  C 2 H 5 Br | | Zn R  CH  CH   R  CH  CH  R   Zn C4 H 9 OH | O  C2 H 5  R E3 Mg 1089  H / H 2O    R  CH 2  CH 3 Hydroboration : Alkene give addition reaction U with diborane which called hydroboration. In this reaction formed trialkylborane, Which is very D YG (2) Physical Properties (i) Alkenes are colourless and odourless. (ii) These are insoluble in water and soluble in organic solvents. (iii) Physical state C1  C 4  gas C 4  C16  liquid  C17  solid wax (iv) B.P. and M.P. decreases with increasing branches in alkene. ST U (v) The melting points of cis isomers are lower than trans isomers because cis isomer is less symmetrical than trans. Thus trans packs more tightly in the crystal lattice and hence has a higher melting point. (vi) The boiling points of cis isomers are higher than trans isomers because cis-alkenes has greater polarity (Dipole moment) than trans one. (vii) These are lighter than water. (viii) Dipole moment : Alkenes are weakly polar. The, -electron’s of the double bond. Can be easily polarized. Therefore, their dipole moments are higher than those of alkanes. (3) Chemical properties important and used for synthesis of different organic compound 3 R  CH  CH 2  BH 3  (R  CH 2  CH 2 )3 B Trialkyl borane CH3COOH/Z n NaOH/ H2O2 HI/H2O2 R – CH2 –CH3 R – CH2 –CH2OH R – CH2 –CH3 The overall result of the above reaction appears to be antimarkownikoff’s addition of water to a double bond. (iv) By treatment with AgNO3 + NaOH : reaction gives coupling CH 3 | B2 H 6 6 CH 3  CH 2  CH 2  C  CH 2    This CH 3 | Ag / NO 3 NaOH 2[CH 3  (CH 2 ) 2  C  CH 2 ] 3 B   | H CH 3 CH 3 | | CH 3  CH 2  CH 2  C  CH 2  CH 2  C  CH 2  CH 2  CH 3 | | H H 1090 Hydrocarbon  e   Na Et. O  H   R  C H  CH 3    R  CH 2  CH 3 e  (vi) Halogenation o 500 C CH 3 CH  CH 2  Cl 2    ClCH 2  CH  CH 2  HCl Propene Ally lchloride or 3-Chloro- 1- propene  If NBS [N-bromo succinimide] is a reagent used for the specific purpose of brominating alkenes at the allylic position. CH2 – CO CH3 CH=CH2 + | CH2 – CO N – Br NBS Propene CH 2  CH 2  H  HSO 4  CH 3 CH 2 HSO 4 Ethy lene Ethy l hydrogen sulphate CH 3 CH 2 HSO 4  CH 2  CH 2  H 2 SO 4  This reaction is used in the seperation of alkene from a gaseous mixture of alkanes and alkenes. (x) Reaction with nitrosyl chloride NO | C  C  NOCl  C  C ( NOCl is called | Cl Tillden reagent)  If hydrogen is attached to the carbon atom of product, the product changes to more stable oxime. NO | H C C ⇌ C  C  NOH | | | Cl Cl ID CH2 – CO CH2 – CH = CH2+ | N–H | CH2 – CO Br Succinimid Allyl e bromide  In presence of polar medium alkene form vicinal dihalide with halogen. H H H H | | | | CCl 4 R  C  C  H  X  X   R  C  C  H | | X X alkenes 60  Na Et  O  H R  CH  CH 2   R  C H  C H 2   R  CH  CH 3  In case of unsymmetrical markownikoff rule is followed. (ix) Reaction with sulphuric acid : E3 (v) Birch reduction : This reaction is believed to proceed via anionic free radical mechanism. Oxime C C C  C (Blue colour) | | C C NO Cl (xi) Oxidation : With alkaline KMnO 4 [Bayer’s  NOCl  U CC D YG Vicinal dihalide Reactivity of halogen is F2  Cl 2  Br2  I2 (vii) Reaction with HX [Hydrohalogenation] H | CC  HX  C  C | X alkene Alky lhalide ST U According to markownikoff’s rule and kharasch effect. H H | | CH 3  CH  CH 2  HBr  CH 3  C  C  H | | Br H According to Anti Markownikoff rule (Based on F.R.M.) CH 3  CH  CH 2  HBr  Peroxide H H H H | | | | CH 3  C  C  H  CH 3  C  C  H | | | | Br H H Br (minor) (viii) Reaction with hypohalous acids :   CH 2  CH 2  H O Cl  CH 2 OH.CH 2 Cl Ethy lene Ethy lene chlorohy drin (major) reagent] : This reaction is used as a test of unsaturation. H H H H | | | | Alk KMnO 4 R  C  C  H  [O]  H  OH   R  C  C  H OH | | HO OH gly col With acidic KMnO 4 : H H O | | || acidic R  C  C  H  [O]  R  C  O  H  CO 2  H 2 O KMnO 4 (xii) Hydroxylation (a) Using per oxy acid : CH 3 | H 2 O2 , HCOOH H  C   or HCO 3 H || H C | CH 3 2 - Butene CH 3 | H  C  OH | HO  C  H | CH 3 Trans (racemic) R H (b) Hydroxylation by C | |  OsO4  NaHSO 4  I C R H Trans OsO4 : R H HO OH H R () Hydrocarbon 1091  H 2 O 2 / OH 3 R  CH  CH 2  BH 3  (R  CH 2  CH 2 ) 3 B   C 6 H 5 CO 3 H  H 2O R  CH  CH  R    R  CH  CH  R    or CH 3 CO 3 H | | OH OH R  CH  CH  R O [Oxirane] (xiii) Cn H 2n Combustion : 3n  O 2  nCO 2  nH 2 O 2 Tri alky l bora ne R  CH 2  CH 2  OH  B(OH )3 (Anti markownikoff’s rule) (xviii) Hydroformylation : H H | | CoH (CO )4 R  CH  CH 2  CO  H 2   R  C  C  H | | H C O | H  If CO  H 2 O is taken then respective acid is 60  If per benzoic acid or peroxy acetic acid is used then oxirane are formed. formed. CoH (CO )4 R  CH  CH 2  CO  H 2 O   R  CH 2  CH 2 | COOH (xix) Addition of formaldehyde E3 They burn with luminous flame and form explosive mixture with air or oxygen. (xiv) Ozonolysis    H 2 C  O  H [H 2 C  O H  H 2 C  OH ]  R  CH  CH 2   R  C H  CH 2  CH 2  OH O3   I O O O  C C O O H 2 O / H / Zn    ZnO  II C+C Ozonide H 2 D YG U  Application of ozonolysis : This process is quite useful to locate the position of double bond in an alkene molecule. The double bond is obtained by Joining the carbon atoms. of the two carbonyl compounds. (xv) Oxy – mercuration demercuration : With mercuric acetate (in THF), followed by reduction with NaBH 4 / NaOH is also an example of hydration of alkene according to markownikoff’s rule. (CH 3 )3 C  CH  CH 2  (CH 3 COO )2 Hg  3,3- dimethyl-1- butene Mercuric acetate NaBH 4 / NaOH (CH 3 )3 C  CH  CH 2  Hg   (CH 3 )3 C  CH  CH 3 THF | | OH OCOCH 3 3, 3 Dimethy l 2  butanol U (xvi) Epoxidation (a) By ST O || C–O–O– H | CH 2  CH 2     CH 2  CH 2 O H  C O O  H CH 3  CH  CH 2   CH 3  CH  CH 2 (xvii) Hydroboration zeigler-natta polymerisation. (xxi) Isomerisation : CH 3  CH 2  CH 2  CH  CH 2 AlCl3 CH 3  CH 2  CH  CH  CH 3 2- Nitroethan ol (xxiii) Addition of Acetyl chloride : CH 2  CH 2  CH 3 COCl  CH 2 ClCH 2 COCH 3 Ethene O || 2 Ethene (b) Epoxidation by performic acid or perbenzoic O Cyclic acetal (xx) Polymerisation H H  H H H H   | | | | |   | Trace O 2  Catalyst      C  C  C  C  C  C  1500 o / high pressure   | | | | |   |  H H H H H H  n  If in polymerisation zeigler- natta catalyst [(R)3 Al  TiCl 4 ] is used then polymerisation is known as The mechanism proceeds via carbocation. (xxii) Addition of HNO3 : CH 2  CH 2  HO  NO 2  CH 2 OH.CH 2 NO 2 O 1 Ag CH 2  CH 2  O 2   CH 2  CH 2 2 acid : R – C CH2  HCHO / H    R  CH  CH 2  CH 2 CH O O | | C OH OH H 1, 3 diol : O2 / Ag HOH  H ID CC 4 -Chlorobuta none - 2 (4) Uses (i) For the manufacture of polythene – a plastic material; (ii) For artificial ripening of fruits; (iii) As a general anaesthetic; (iv) As a starting material for a large number of compounds such as glycol, ethyl halides, ethyl alcohol, ethylene oxide, etc; (v) For making poisonous mustard gas (War gas); (vi) For making ethylene-oxygen flame. Alkynes 1092 Hydrocarbon These are the acyclic hydrocarbons which contain carbon-carbon triple bond are called alkynes. General formula is Cn H 2n2. Ex. Ethyne CH  CH ; Propyne CH 3  C  CH (v) It burns with luminous flame and forms explosive mixture with air. CHCl3 Ag dust(Powder)  Zn dust  CHBr2 – CHBr2 CHBr || CHBr Zn alc KOH, NaNH2 CH2 = CH – Cl HC – COONa || HC – COONa C2H2 [Acetylene] Kolbe’s electrolytic synthesis Nu |   C  C   Nu   C  C   ID H2O (Laboratory method) CaC2 Electric arc, 1200oC Berthelot’s process 2C+ H2 U (i) CH3MgI (ii) R-X CH3 – C CH D YG  In reaction with gem dihalide, Alc. KOH is not used for elimination in 2nd step.  In reaction with vicinal dihalide, if the reactant is 2-butylene chloride then product is 2-butyne as major product. Preparation acetylide) of higher alkynes (by metal  Acetylene gives salt with NaNH 2 or AgNO 3 U (ammonical) which react with alkyl halide to give higher alkyne. ST NaNH 2 2CH 3 I  2CH  CH   Na  C  C  Na     Vinylic carbanion (more stable) Nu |   C  C   Nu    C  C  (i) Na (ii) R-X CH3 – C CH (3) Chemical reactivity of alkynes : C  C is less reactive than the carbon-carbon double bond towards electrophilic addition reaction. This is because in alkyne carbon has more S-character so more strongly will be the attraction for  electrons. Alkyne also undergo nucleophilic addition with electron rich reagents. Ex. Addition of water, cyanide, carboxylic acid, alcohols. Nucleophilic addition can be explained on the basis that alkynes form vinylic carbanion which is more stable than alkyl carbanion formed by alkene E3 alc KOH, NaNH2 60 alc KOH or NaNH2 CH3 – CHBr2 (iii) Its boiling point is  84 o C. (iv) It is lighter than air. It is somewhat poisonous in nature. (1) General methods of preparation CH2Br – CH2Br high pressure in acetone soaked on porous material packed in steel cylinders. (alkyl carbanion) (less stable) (i) Acidity of alkynes : Acetylene and other terminal alkynes (1- alkynes) are weakly acidic in character Ex. CH  CH  NaNH 2  H  C  C Na   (Monosodium acetylide) The acetylenic hydrogen of alkynes can be replaced by copper (I) and silver (I) ions. They react with ammonical solutions of cuprous chloride and silver nitrate to form the corresponding copper and silver alkynides. CH  CH  2[Cu ( NH 3 )2 ]Cl  Cu  C  C  Cu  2 NH 4 Cl  2 NH 3 CH 3  C  C  CH 3 Butyne Dicopper acetylide (Red ppt) CH  CH  2[ Ag ( NH 3 )2 ]NO 3  AgC  C  Ag  2 NH 4 NO 3  2 NH 3 CH 3  C  CH  CH 3  Mg  X  RX CH 3  C  C  Mg  X  CH 4  CH 3  C  C  R  MgX 2 Alkyne (2) Physical properties (i) Acetylene is a colourless gas. It has a garlic odour. The odour is due to presence of impurities of phosphorous and hydrogen sulphide. However, pure acetylene has pleasant odour. (ii) It is insoluble in water but highly soluble in acetone and alcohol. Acetylene is transported under 1 H2 2 Disilver acetylide (white ppt) This reaction can be used to distinguish between 2-alkynes and 1-alkynes. 1-alkynes will give this test while 2-alkynes, will not give this test. CH 3  C  CH  2[ Ag (NH 3 )2 ] NO 3  CH 3  C  C  Ag 1- propy ne CH 3  C  C  CH 3  2[ Ag ( NH 3 )2 ]NO 3  No reaction Hydrocarbon Explanation explained by sp 1093 for the acidic character : It hybridisation. We know that an electron in s  orbital is more tightly held than in a p orbital. In sp hybridisation s -character is more (50%) as compared to sp 2 (33%) or sp 3 (25%), due to large s -character the carbon atom is quite electronegative. (ii) Reaction with formaldehyde CH 2  CH  CH  CH 2 OH 60 Li / NH 3 HC  CH  2CH 2 O  CH 2  C  C  CH 2   | | OH OH [Trans-product ] | (4) Chemical properties of acetylene Red hot tube C6 H 6 Benzene BH 3 C4 H 5 N  H 2 NH3  ID 3 2 3 CH 3  C  CH  (CH 3  CH  CH )3 B 2  S/H2S o Hg , 80 C CH 3 CHO CH 3 CHO  Acetaldehyde U 2+ CH 3 CO CH 3 CO O Acety lene D YG Acetic anhy dride CH  CH Cl2 CHCl2 | CHCl2 CHCl2 || CHCl2 Alc. KOH (Westron) ClAsCl2 AlCl3 (Westroso l) CHCl (Cadet and Busen || reaction) CHAsCl2 Lewisite CH3COOH CH 2  CHOOCCH 3 U Hg+2 Hg2+/HCl ST 60oC HCN Ba(CN) Viny lacetate CH 2  CHCl NH4Cl Cu2O OH  or (Propanal) O || H 2 SO 4   CH 3  C  CH 3 (Acetone) HgSO 4 Thus it is useful for preparing aldehyde from terminal alkyne. Reduction of Alkyne : Alkynes add on hydrogen in presence of suitable catalysts like finely divided Ni, Pd. Ni Ni CH  CH  H 2   CH 2  CH 2   CH 3  CH 3 H2 If the triple bond is not present at the end of the carbon chain of the molecule, the alkene formed may be cis and trans depending upon the choice of reducing agents. With Na / NH 3 or Li / NH 3 in (liquid ammonia) trans alkene is almost an exclusive product while catalytic reduction at alkyne affords mainly cis alkenes. Viny lchloride R CH 2  CHCN R CC Viny lcy anide 2 Cu2Cl2, HCl H Chloroprene H2 Li / NH 3    R  C  C  R   H Pd. / BaSO 4 / quinoline (Lindlarcataly st) cis R Cuprene C2 Cl6 Hexa chloro ethane Ni 300°C Na H CC H (Artificial camphor) R Trans CC–CC 3  Li / NH Trans Lindlar’s Catalyst H O Tautomeris es CH 3  CH  CHOH    CH 3 CH 2CHO Thiophene with CH3COOH BH / THF Propy ne C4 H 4 S 40% H2SO4/1%HgSO 4 80oC Oxidative–Hydroboration : Alkynes react with (in THF) and finally converted into carbonyl compounds. Py rrole  E3 OH C 2 H 4 (Cis) Ethy lene Lindlar    Cataly st C2 H 6 Ethane HC  CNa  XR HC  C  R Sol. Acety lide Higher alky nes Cis 1094 Hydrocarbon is  2  6  2  12 1 2 Tests of unsaturation (a) Baeyer’s reagent : It is 1% KMnO 4 solution containing sodium carbonate. It has pink colour. An aqueous solution of the compound, a few drops of Baeyer’s reagent are added, the pink colour of the solution disappears. The decolourisation of pink colour indicates the presence of unsaturation in the compound.  Alkene without any hydrogen atom on the R Ni , 300 C Ethene Br2 Br2 Alc. CH 3  CH 3   CH 3 CH 2 Br   CH 2  CH 2   hv Ethane KOH (iv) Ethyne into ethane : (Alkyne into alkane) H2 H2 o CH 2  CH 2  o CH 3  CH 3 CH  CH  Ethy ne Ni , 300 C HI KCN [H ] CH 2  CH 2   CH 3 CH 2 I   CH 3 CH 2 CN   Iodoethane Reduction Propane nitrile (Ethy l cy anide) HNO 2 CH 3 CH 2 CH 2 NH 2    CH 3 CH 2 CH 2 OH 1Propanol PBr3 CH 3 CH  CH 2   CH 3 CH 2 CH 2 Br   Alc. KOH U (5) Uses (i) Acetylene is used as an illuminant. (ii) It is used for the production of oxy-acetylene HI CH 2  CH 2   CH 3 CH 2 I 32  CH 3 CH 2 CH 3 Li (CH ) Cu Ethene Iodoethane hv 1- Chloro propane ST name naracylene. (v) Acetylene has synthetic applications. It serves as a starting material for the manufacture of a large variety of substances. (vi) On electrical decomposition acetylene KOH Propene CH 3 I / Na HI or CH 2  CH 2   CH 3 CH 2 I    CH 3 CH 2 CH 3 Propane Cl 2 Alc.   CH 3 CH 2 CH 2Cl   CH 3 CH  CH 2 hv 1- Chloro propane KOH Propene (vi) Propene into ethene : Descending an alkene series O3 / H 2 O [H ] CH 3  CH  CH 2   CH 3 CHO   Propene LiAlH 4 Ethanal flame. The temperature of the flame is above 3000 C. fruits. (iv) It is used as a general anaesthetic under the Propane Cl 2 Alc.   CH 3 CH 2 CH 2Cl   CH 3 CH  CH 2 o Is is employed for cutting and welding of metals. (iii) Acetylene is used for artificial ripening of 1  Bromopropa ne or U D YG R Ethane series  This test also fails in the case of alkene of the R Ni , 300 C (v) Ethene into propene : Ascending in alkene colour of bromine disappears. It indicates the presence of unsaturation.. Ethene 1- Aminopropa ne chloroform and then a few drops of 5% bromine solution in carbon tetrachloride are added to it, the CC Ethyne or NaNH 2 1, 2 -Dibromoeth ane Propene R CCl 4 Ethene Alc. KOH CH 2 Br  CH 2 Br   CH  CH (b) Bromine- carbon tetrachloride test : The compound is dissolved in carbon tetrachloride or R Ethane (iii) Ethane into ethyne (acetylene) : i.e., alkane into alkyne don't show this test. Ethene H2 o CH 3  CH 3 CH 2  CH 2  Ethene R KOH Ethyl bromide (ii) Ethene into ethane : (Alkene into alkane) ID R hv Ethane R CC carbon forming the double bond Br2 Alc. CH 3  CH 3   C 2 H 5 Br   CH 2  CH 2 60 For example in C6 H12 , the degree of unsaturation (i) Conversion of ethane into ethene : (Alkane into alkene) E3 Degree of unsaturation : The number of degree of unsaturation in a hydrocarbon is given by 2 n1  2  n 2 , Where n1 is the number of carbon 2 atoms; n 2 is the number of hydrogen atoms. H 2 SO 4   CH 2  CH 2 CH 3 CH 2 OH  o 170 C Ethanol Ethene (vii) Acetylene into propyne (methyl acetylene) : (Ascent) CH 3 I Na CH  CH   CH  CNa   CH  C  CH 3 Acety lene Monosodium acety lide Propy ne (viii) Propyne into acetylene : (Descent) O3 / H 2 O Lindlar' s cataly st CH 3 C  CH   CH 3 CH  CH 2   Propy ne Propy lene produces finely divided carbon and hydrogen. Hydrogen is used in airships. C 2 H 2  2C  H 2 (6) Interconversion PCl 5 Alc. CH 3 CHO   CH 3 CHCl 2   CH  CH Acetaldehyde Ethy lidene chloride KOH (ix) 1-Butyne into 2-pentyne : (Ascent) Acety lene Hydrocarbon recovered by decomposition of the precipitate with an acid. 2 3 CH 3 CH 2 C  CH  CH 3 CH 2 C  C  Na  NaNH CH I 1  Buty ne CH 3 CH 2  C  CCH 3 C2 H 2  Cu 2 Cl 2  2 NH 4 OH C2Cu 2  2 NH 4 Cl  2 H 2 O 2  Penty ne (Red ppt.) (x) 1-Butyne into 2-pentanone : (Not more than three steps) C2Cu 2  2 HNO3 C2 H2  Cu 2 (NO3 )2 The remaining gaseous mixture is passed through concentrated H 2 SO 4. Alkene is absorbed. The Hydrogen NaNH 2 CH 3 I CH 3 CH 2 C  CH   CH 3 CH 2 C  CNa   ( Liq. NH 3 ) 1 - Butyne O || H 2O , H 2 SO 4 CH 3 CH 2 C  CCH 3  CH 3 CH 2 CH 2 C CH 3 sulphate derivatives is heated at 170oC to regenerate ethene. 60 HgSO 4 2  Penty ne 2  Pentanone  C2 H 4  H 2 SO 4  C2 H 5 HSO 4  C2 H 4  H 2 SO 4 170 o C Separation of alkane, alkene and alkyne gaseous mixture is passed The methane or ethane is left behind unreacted. through ammonical cuprous chloride solution. The alkyne (acetylene) reacts with Cu 2Cl2 and forms a red precipitate. It is filtered. The alkyne or acetylene is Distinction between alkanes, Alkenes and Alkynes Property ID Table : 24.3 Alkane (Ethane) Molecular formula CnH2n+2(C2H6) Saturated Alkene (Ethene) Alkyne (Ethyne) CnH2n(C2H4) CnH2n–2(C2H2) Unsaturated Unsaturated U Nature E3 The Double bond between two carbon Triple bond between two carbon atoms. Each carbon atom is sp3-hybridized atoms. Both carbon atoms are sp2-hybridized atoms both carbon atoms are sphybridized D YG Single bond between carbon C–C Bond energy : 83 Kcal mol Burns with –1 nonluminous flame –CC– C=C Bond length 1.54 Å Burning 1095 1.34 Å 146 Kcal mol 1.20 Å –1 200 Kcal mol–1 Burns with luminous flame Burns with smoky flame C2H4+3O2 2CO2+2H2O C2H2+5/2O2 2CO2+H2O Forms alkane Forms alkene and alkane Ni CnH2n + H2  CnH2n+2 300 o C Alkane C2H4 + H2 C2H6 Ni CnH2n + H2  CnH2n+2 300 o C Alkane Addition Addition C2H4+H2SO4 C2H5HSO4 H 2O C2H2 CH3CH(HSO4)2   C2H6+7/2O2 2CO2+3H2O U Reaction with H2 ST Reation with conc. H2SO4 and hydrolysis Br2/CCl4 Baeyer’s reagent (Alk. KMnO4) – – Ni CnH2n–2 + H2  CnH2n 300 o C Alkene H 2O   C2H5OH CH3CHO Aldehyde Alcohol – – Decolourises Decolourises Dibromo derivative, Tetrabromo derivative, C2H4 + Br2 C2H4Br2 C2H2Br4 Decolourises Decolourises Glycol is formed Oxalic acid is formed CH 2 CH 2 OH ||  H 2O  O | CH 2 CH 2 OH CH COOH |||  4 O | CH COOH 1096 Hydrocarbon Ammonical Cu2Cl2 – – Red precipitate CH CCu | | |  Cu 2 Cl 2  2 NH 4 OH | | | CH CCu (Red) + 2NH4Cl + 2H2O Ammonical silver nitrate – – White precipitate 60 CH C  Ag | | |  2 AgNO 3  2 NH 4 OH | | | CH C  Ag + 2NH4Cl + 2H2O Cyclobutane > Cyclopentane. Relative ring opening of ring is explained by Baeyer strain theory. (1) Methods of preparation dihalogen CH2Cl CH2 +2Na compounds CH2 heat H2C CH2Cl 1,3 Dichloropropane (Freund + H2 CH2 Cyclopropan e Spiro compound Because small ring is more unstable than large ring Propene CH 2 Methy l cy clopropane D YG (iii) From Aromatic compounds Higher cycloalkanes do not give addition due to more stability. U Zn Cu alloy CH 3  CH  CH 2  CH 2 I 2   CH 3  CH  CH 2 Benzene cycloalkane fused with one another then addition take place in small ring +2NaCl (ii) From alkenes : + 3H2 (i) Addition in spiro cycloalkane : If two ID (i) From reaction): E3 Cycloalkane (ii) Free radical substitution with Cl2 hv CH 2  CH 2  Cl 2   CH 2  CH Cl  HCl CH 2 Cy clopropa ne o Ni , 200 C   under pres sure Cyclohexane (iii) Addition reaction Br2 (CCl4) dark (2) Physical properties (i) First two members are gases, next three members are liquids and higher ones are solids. U (ii) They are insoluble in water but soluble in alcohol and ether. (iii) Their boiling points show a gradual increase with increase of molecular mass. Their boiling points are higher than those of isomeric alkenes or corresponding alkanes. ST CH 2 Chlorocy clopropane (iv) Their density increase increase of molecular mass. gradually HBr H2C – CH2 CH2 Cyclopropane the tendency of forming addition compounds decreases with increase in size of ring cyclopropane > CH3 – CH2 – CH2Br 1-Bromopropane (i) Conc. H2SO4 CH – CH – CH OH 3 2 2 (ii) H2O 1-Propanol H2, Ni 80oC with (3) Chemical properties : Cycloalkanes behave both like alkenes and alkanes in their chemical properties. All cycloalkanes undergo substitution reaction with halogen in the presence of light (like alkane). All cycloalkane (lower members) undergo addition reaction (ex. Addition of H 2 , HX , X 2 ). Further BrH2C – CH2 – CH2Br 1, 3-Dibromopropane CH3 – CH2 – CH3 Propane (iv) Oxidation CH2 H2C CH2 H2C CH2 CH2CH2COOH + 5[O] CH2 Cyclohexane Cycloalkene   Alk KMnO 4 CH2CH2COOH Adipic acid Hydrocarbon Carbocyclic compounds with double bonds in the ring are called cycloalkenes. Some of the common 6 cycloalkenes are 2 Cycloalkenes can be easily obtained by DielsAlder reaction. These compounds undergo the electrophilic addition reactions which are characteristic of alkenes, while the ring remains intact. Cycloalkenes decolourise the purple colour of dilute cold KMnO 4 or red colour of bromine in carbon + Br2 Br tetrachloride. Cyclopentene Br OH 600 C n -Butane (v) From cyclohexene : CHO CH2BrCH=CH.CH2Br 1,4-Dibromo-2-butene (1, 4Addition) predominates (70%) in an ionising solvent (acetic acid) CHO CH2 (ii) Addition of halogen acids : CH3CHBrCH=CH2 (1, 2-Addition) 3-Bromo-1-butene (Major yield at low temp.) CH2 =CH–CH=CH2 +HBr CH3–CH=CH–CH2Br U These are hydrocarbon with two carbon-carbon double bonds. Dienes are of three types (1) Conjugated dienes seperated by one single bond. (1, 4-Addition) 1-Bromo-2-butene (Major yield at high temp.) : Double bonds are (iii) Addition of water : CH3CHOHCH=CH2 ST Ex : CH 2  CH  CH  CH 2 (1, 3-butadiene) But-3-en-2-ol (2) Cumulative dienes : Double bonds are adjacent to each other. Ex : CH 2  C  CH 2 Propadiene [allene] CH2 =CH–CH=CH2 +H2O (3) Isolated or Non-conjugated : Double bonds are separated by more than one single bond. (iv) Polymerisation : Ex : CH 2  CH  CH 2  CH  CH 2 (1, 4 pentadiene) The general formula Ethene 3,4-Dibromo-1-butene (1, 2Addition) predominates (62%) in non-ionising solvent (hexane) CH2 Dienes 1, 3-Butadiene (Cr2O3 used as catalyst.) CH2 = CHCH = CH2 + Br2 D YG (Cyclohexene) O Cataly st CH 3 CH 2 CH 2 CH 3    CH 2  CH  CH  CH 2 o 1, 3-Butadiene CH2 1, 3-Butadiene (iv) From butane : (2) Physical property : 1,3-butadiene is a gas. (3) Chemical properties (i) Addition of halogens : CH2BrCHBrCH=CH2 CH2 O heat 1, 4 -Butanediol U Cyclopentane 1,2diol + O3 1, 3-Butadiene ID OH O H2O 1,4 -Dichlorobu tane (iii) From 1,4-butanediol : OH OH | | H 2 SO 4 CH 2 CH 2 CH 2 CH 2   CH 2  CH  CH  CH 2 1, 3-Butadiene KMnO4(aq.) Cyclopent-1ene (ii) From 1, 4-dichlorobutane : Cl Cl | | Alc. KOH CH 2 CH 2 CH 2 CH 2    CH 2  CH  CH  CH 2 CH 2  CH  CH  CH 2  CH 2  CH 2 1, 2-Dibromo cyclopentane +O + H2O CH 2  CH  CH  CH 2 1, 3 -Butadiene 3 1, 4Cyclohexadiene Cyclohexen e Pd / BaSO 4 Vinylacetylene 60 4 NH 4 Cl E3 1 Cu 2 Cl 2 H2 2 HC  CH   HC  C  CH  CH 2   CCl4 Cyclobuten Cyclopenten e e 5 1097 is Cn H 2 n  2. predominant member of this class is 1, 3-butadiene. (1) Method of preparation (i) From acetylene : The CH3CH=CHCH2OH But –2-en-1-ol Peroxide nCH 2  CHCH  CH 2  [ CH 2 CH  CHCH 2 ]n 1, 3-Butadiene Buna rubber Diels-alder reaction : CH2 CH2 CH2 H–C + H–C CH2 CH2 1, 3-Butadiene Ethene (Dienophil e) o 200 C    CH2 HC CH2 HC CH2 Cyclohexene (Adduct) 1098 Hydrocarbon Stability of conjugated dienes : It is explained on the basis of delocalisation of electron cloud between carbon atoms. The four  electrons of 1, 3-butadiene are delocalised over all the four atoms. This delocalisation of the  electrons makes the molecule more stable.  Coal tar is a mixture of large numbers of arenes. (3) Distillation of coal tar : Arenes are isolated by fractional distillation of coal tar, C C Name of the fraction C C C C – (v) Ozonolysis : Zn / H 2 O CH 2  CHCH  CH 2  2O 3   2 HCHO  OHCCHO H2O D YG CH2–O–CH – CH–O–CH2 O Aromatic Hydrocarbon (1) Source of Arenes U Source of arenes is coal. It contains benzene, xylene, naphthalene etc. Arenes are obtained by destructive distillation of coal. (2) Distillation of coal ST COAL COKE (Solid residue nearly 70%). It is used as a fuel and as reducing agent in metallurgy. Condensed liquid Allowed to settle, two layers are separated AMMONICAL LIQUOR (nearly 8-10%) It is used for the preparation of (NH4)2SO4 to be used as a fertilizer. Middle oil fraction (Carbolic oil) 443-503 Phenol, naphthalene, pyridine Heavy oil fraction (Creosote oil) 503-543 Naphthalene, naphthol and cresol Green oil (Anthracene oil) 543-633 Anthracene, phenanthrene Pitch (left as residue) Benzene, toluene, xylene Non-volatile Carbon  The residue left after fractional distillation of coal-tar is called pitch. (4) Isolation of benzene cold H 2 SO 4 NaOH Light oil   Basic impurities removed    [Like py ridene] O (Diozonide) Upper layer Upto 443 U O3 O Main constituents Light oil (or crude oil) fraction ID  - Delocalised electron’s O Temperature range (K) E3 C C 60 Table : 24.4 Heated to 1273-1373 K (Destructive distillation) HOT VAPOURS AND GASES Cooled and passed through water COAL GAS (Mixture of uncondensed gases nearly 17%). It is used as a fuel. Lower layer COAL TAR (nearly 4-5%) Black, viscous liquid having unpleasant odour Phenols removed   Benzene (255 - 257 K) [Acidic impurities] Toluene (383 K) distillati on General characteristics of arenes (1) All arenes have general formula [Cn H 2n  6 y]. Where y is number of benzene rings and n is not less than 6. (2) Arenes are cyclic and planar. They undergo substitution rather than addition reactions. (3) Aromaticity or aromatic character : The characteristic behaviour of aromatic compounds is called aromaticity. Aromaticity is due to extensive delocalisation of -electrons in planar ring system. Huckel (1931) explained aromaticity on the basis of following rule. Huckel rule : For aromaticity the molecule must be planar, cyclic system having delocalised (4 n  2) electrons where n is an integer equal to 0, 1, 2, 3,------. Thus, the aromatic compounds have delocalised electron cloud of 2,6,10 or 14  electrons. Example : 4n  2  6 ; 4n  4 ; n  4 1 4 Benzene 6 electrons n=1 Naphthalene 10 electrons n=2 Anthracene 14 electrons n=3 Hydrocarbon 1099  Similarly cyclolpentadienyl anion or tropylium ion are also aromatic because of containing 6 electrons H H H (n=1)... H  H H Cyclopentadienyl anion 6  electrons (n=1) Cycloheptatrienyl anion 8 electrons H H H H  H  Tropyllium ion 6  electrons (n=1) Benzene (C6H6) Cyclopropenyl cation (n = 0) Hetrocyclic compounds also have 6 electrons (n.... O Pyrrole N S Furan Thiophen e Pyridine Cyclopentadienyl cation 4 electrons H D YG Cyclopentadien e 4 electrons  Cyclooctatetraene 8 electrons (a) Evidence in favour of Kekule's structure  Benzene combines with 3 molecules of hydrogen or three molecules of chlorine. It also combines with 3 molecules of ozone to form triozonide. These reactions confirm the presence of three double bonds. U Molecules do not satisfy huckel rule are not aromatic. H Benzene is the first member of arenes. It was first discovered by Faraday (1825) from whale oil. Mitscherllich (1833) obtained it by distillating benzoic acid with lime. Hofmann (1845) obtained it from coal tar, which is still a commercial source of benzene. (1) Structure of benzene : Benzene has a special structure, which is although unsaturated even then it generally behave as a saturated compound. (i) Kekule's structure : According to Kekule, in benzene 6-carbon atoms placed at corner of hexagon and bonded with hydrogen and double bond present at alternate position. ID N H Cycloctatetraene 8 electrons E3 = 1)...  60 H Cyclopropenyl anion 4 electrons.. Cyclopropenyl anion 4 electrons ST U (4) Antiaromaticity : Planar cyclic conjugated species, less stable than the corresponding acyclic unsaturated species are called antiaromatic. Molecular orbital calculations have shown that such compounds have 4 n  electrons. In fact such cyclic compounds which have 4 n  electrons are called antiaromatic compounds and this characteristic is called antiaromaticity. Example : 1,3-Cyclobutadiene, It is extremely unstable antiaromatic compound because it has 4 n  electrons (n  1) and it is less stable than 1,3 butadiene  Studies on magnetic rotation and molecular refraction show the presence of three double bonds and a conjugated system.  The synthesis of benzene from three molecule of acetylene also favour's Kekule's structure.  3CH  CH   Benzene gives cyclohexane by reduction with hydrogen. Ni C 6 H 6  3 H 2 O   Cyclohexane (b) Objections against Kekule's formula  Unusual stability of benzene. ;  According to Kekule, two ortho disubstituted products are possible. But in practice only one ortho disubstituted product is known. Thus, cyclobutanediene shows two equivalent contributing structures and it has n  1.  Heat of hydrogenation of benzene is 49.8 kcal/mole, whereas theoretical value of heat of hydrogenation of benzene is 85.8 kcal/mole. It means resonance energy is 36 kcal/mole. by about 83.6 KJ mol–1. 4n  4 n 4 1 4  Cyclopentadienyl 4 electrons 1100 Hydrocarbon  C  C bond length in benzene are equal, (although it contains 3 double bonds and 3 single bonds) and are 1.39 Å. Kekule explained this objection by proposing that double bonds in benzene ring were continuously oscillating between two adjacent positions.  Cyclic polymerisation takes place in this reaction. (g) Aromatisation (2) Methods of preparation of benzene : 4H2 60 Cr2O3 / Al2O3 C 6 H 14     C 6 H 6  500 C Benzene n  Hexane at high pressure (3) Properties of benzene (i) Laboratory method : (i) Physical properties COONa CaO + NaOH + Na2CO3 heat E3 Benzene Sodium benzoate (b) It is highly inflammable and burns with sooty flame. (ii) From benzene derivatives (a) From phenol : OH distill + ZnO Benzene Phenol (e) Benzene itself is a good solvent. Fats, resins, rubber, etc. dissolve in it. (f) It is a non-polar compound and its dipole moment is zero. Ni-Al alloy NaOH + HCl D YG 2H Benzene Chlorobenzen e (c) By first preparing grignard chlorobenzene and then hydrolysed C 6 H 5 Cl   C 6 H 5 MgCl Chlorobenz ene dry ether Pheny l magnesium chloride Mg reagent of +H2SO4 Substitution reactions in benzene are prefered rather than addition are due to the fact that in the former reactions resonance stabilised benzene ring system is retained while the addition reactions lead to the destruction of benzene ring. Principal reactions of benzene can be studied under three heads, Benzene Steam re Benzene ST Benzene sulphonic acid 150°200°C HCl,pressu (a) Addition reactions (e) From benzene diazonium chloride : N2Cl + 2H (b) Substitution reactions SnCl2 NaOH +N2 +HCl Benzene (f) From acetylene : + HC HC + HC Three molecules of acetylene (c) Oxidation reactions (a) Addition reactions : In which benzene behaves like unsaturated hydrocarbon. Addition of hydrogen : Benzene reacts with hydrogen in the presence of nickel (or platinum) as catalyst at 150°C under pressure to form cyclohexane. HC CH + CH (ii) Chemical properties : Due to the presence of  electron clouds above and below the plane benzene ring, the ring serves as a source of electrons and is easily attacked by electrophiles (Electron loving reagents). Hence electrophilic substitution reaction are the characteristic reactions of aromatic compounds.  C 6 H 6  Mg U HOH (g) It is an extremely poisonous substance. Inhalation of vapours or absorption through skin has a toxic effect. OH Cl H 2O (d) From benzene sulphonic acid : SO3H + (d) It is immiscible with water but miscible with organic solvents such as alcohol and ether. U (b) From chlorobenzene : Cl ID (c) It is lighter than water. It's specific gravity at 20°C is 0.8788. + Zn + (a) Benzene is a colourless, mobile and volatile liquid. It's boiling point is 80°C and freezing point is 5.5°C. It has characteristic odour. red hot tube 15002000°C + Benzene Benzene 3H2 Ni 150°C,press ure or C6H12 Cyclohexane Hydrocarbon Addition of halogen : H C Example : OH Cl HC CH HC hv 3Cl2 + 1101 OH+ CH Cl Cl OH H + Cl (Fas t) (Slo w) C Cl C C C C  Elimination-addition mechanism) Cl H Cl H * Cl (Benzen e) Cl  O +3O3 CH C CH C CH O C H O O 3H2O Zn CHO 3 + 3H2O2 CHO O D YG O C O Benzene triozonide Benzene *NH2   H NH3 (47%) + U Unimolecular : Mostly uncommon in aromatic substitution, there is only one example which obtain in benzene diazonium dichloride. HOH ArOH (Slow ) Phenol Ar  N 2    N 2  Ar  (Fast ) X– Arenediazo nium ArX cation ST Z Aryl halide Y + Y Z Y + Y Z  Y  E ;  or  (Resonating ofName Electrophile (Estructure ) the hexadienyl anion) Z H E E E   Resonance forms of carbonium ion (Arenium ion) H E  + Nu: Y attaches to the positively charged carbon atom (slow) Y H H E  Z (53%) Carbonium ion ( - complex)  Z.. NH2 (Slow ) E Nucleophilic substitution :  Bimolecular : *H +  Electrophilic substitution reaction : Benzene undergoes this reaction because it is an electron rich system due to delocalized - electrons.  H Glyoxa l (b) Substitution reactions : * U CH HC CH O * NH3 +  ID Addition of ozone : H C NH3 H Benzene hexachloride (BHC) O (Benzyne * – HCl C H mechanism E3 H Cl H Benzene HC 60 Phenol C H (Fast) + H – Nu Substitution product Y (Fas t) +Z Table : 24.5 Source Name of substitution reaction 1102 Hydrocarbon Chloronium Cl2  AlCl 3 or FeCl 3 Chlorination Br  Bromonium Br2  AlBr3 or FeBr 3 Bromination NO 2 Nitronium HNO3  H 2 SO 4 Nitration SO 3 Sulphur trioxide Conc. H 2 SO 4 , Fuming sulphuric acid Sulphonation R Alkyl carbonium RX  AlX 3 (X  Cl or Br), ROH  H  Friedel-Craft's (Alkylation) Acyl carbonium RCOCl  AlCl 3 Friedel-Craft's (Acylation)  Free radical aromatic substitution : The aromatic substitution reactions which follow free radical mechanisms are very few and have limited synthetic value. But some typical example of these reactions are:  heat (CH 3 )3 COOC  (CH 3 )3   2(CH 3 )3 C O  X X H... CH3+ +. H CH3 o(x = Cl, OCH3, NO2, CH3 etc.) H mIntermediate X D YG X CH3 X. + H CH3 + + m O  Strong oxidising agents converts benzene slowly into CO 2 and water on heating. 2 Benzene CH3 +6I2 + +12HI Methylcyclopent ane Cyclohexane (iii) Uses : (a) In dry cleaning (b) As a motor fuel U + CH3 Maleic anhy dride ID X O  2CO 2  2 H 2 O (d) Reduction :  2CH 3  2CH 3 COCH 3 X CHCO V2O5 C 6 H 6  9[O]  | | 450  500 C CHCO E3  R C  O 60 Cl  CH3 CH3 p when mixed with petrol. (c) As a solvent. (d) In the manufacture of gammexane (As insecticide). (e) In the preparation of nitrobenzene, chlorobenzene, benzene sulphonic acid, aniline, styrene, etc. Many of these are employed for insecticides, etc. making dyes, drugs, plastics, Directive effect in substituted benzene derivatives (1) Directive effect in mono substituted benzene derivatives : The substituent already present temperature is similar to that of the free radical aliphatic substitution on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) U The mechanism of chlorination of benzene at high position. This direction depends on the nature of the first substituent and is called directive or the.. C6 H 6  Cl  C6 H 5  HCl (H- abstraction) orientation effect... C6 H 5  Cl 2  C6 H 5 Cl  Cl (Chain propagation) decrease the rate of further substitution, i.e., it either ST.. Cl 2  Cl  Cl (Chain initiation) (c) Oxidation : 2C 6 H 6  15 O 2  12 CO 2  6 H 2 O H  6530 kJ/mole When vapours of benzene and air are passed over vanadium pentoxide at 450 – 500°C, maleic anhydride is obtained. The substituent already present can increase or activates or deactivates the benzene ring towards further substitution. These effects are called activity effects. There are two types of substituents which produce directive effect are, (i) Those which direct the incoming group to ortho- and para-positions simultaneously meta all together). (Neglecting Hydrocarbon (ii) Those which direct the incoming group to meta-position only (Neglecting ortho- and positions all together). paraMeta directors Ortho-para directors..........– , Strongly activating  NH 2 , NHR , NR – 2.OH,. –.O:. Moderately deactivating C  N ,SO 3 H, COOH ,COOR , CHO, COR........ Moderately activating  NHCOCH 3 ,  NHCOR ,. 3, –.OR –.OCH. Strongly deactivating  NO 2 , NR 3 ,CF3 ,CCl 3 60 Weakly activating CH 3 ,C2 H5 ,R,C6 H5........ Weakly deactivating –.F: , Br:. ,– Cl:.. –.. –,.I.: , O Theory of ortho – para directing group S S.. :S.. ,i.e.,..     (i) If the directive effects of two substituents reinforce, then a single product is formed. E Para product Example : CH3 D YG The above mechanism is followed when S is OH,  NH 2 ,Cl,  Br,I,OR, NR 2 , NHCOR etc. CH3 CH3 CH3 CH3...... In methyl or alkyl group, the +I effect of the methyl group or alkyl group initiates the resonance effect. methyl or alkyl U Thus,..  (2) Directive effect in disubstituted benzene + Ortho product Para attack O positive charge or a full positive charge on the atom directly attached to the ring. S E E +  N ID  S Ortho Orthattack o O All meta-directing groups have either a partial U  S O  N E3 S :S 1103 group directs all CH3 CH3 NO2 ; Nitrati on +NO2 NO2 (m) NO2 NO2 Thus, both (CH 3 , NO 2 ) direct further substitution to the same position (Ortho with respect to CH3). (ii) If the directing effect of two groups oppose each other strongly activating groups win over deactivating or weakly activating group. The sequence of directing power is  NH 2  OH  OCH 3   NHCOCH 3  C6 H 5  CH 3  Theory of meta directing group : The substituent, S withdraws electrons from ortho and para positions. Thus, m-position becomes a point of relatively high electron density and further substitution by electrophile occurs at meta position. For example, group is a meta directing (Electron  NO 2 meta directors ST electrophiles to ortho and para positions.  withdrawing). mechanism as : O  O Its O O can Obe explained O  N N N  OH OH OH CH3 CH3 Direct s Example : CH3 Direct s Directs (Powerfu l activator ) Direct s OH OH Br Br2 FeBr3  CH3 CH3 1104 Hydrocarbon (iii) There is normally little substitution when the two groups are meta to each other. Aromatic rings with three adjacent substituents are generally prepared by same other routes. CH3 Too hindered position (vii) From grignard reagent : MgBr CH3 Cl Phenyl magnesium bromide + HCl AlCl3 +CH3Cl Toluene  Alkyl halide isomeric change employed may 3 C6 H 6  ClCH 2 CH 2 CH 3  C6 H 5 CH AlCl n  Propy lchloride undergo an CH 3  HCl CH 3 D YG Isopropy l benzene (65 70%) (Cumene)  Catalysts can be used in place of anhydrous AlCl 3 are, AlCl 3  SbCl 3  SnCl 4  BF3  ZnCl 2  HgCl 2 (ii) Wurtz fitting reaction : Br+2Na+BrCH3 Ethe r Methyl bromide Bromobenzene CH3+2NaBr Toluene U (iii) Decarboxylation : CH 3 Soda lime C6 H 4  NaOH    C6 H 5 CH 3  Na 2 CO 3 COONa Toluene ST (o-,m- or p- ) Sodium toluate (iv) From cresol : CH3 OH +Zn CH3 heat o- + ZnO Toluen e acid : (v)Cresol From toluene sulphonic CH3 CH3 +HOH Boil SO3H p-Toluene sulphonic acid toluidine : (vi) From CH3 CH3 NaNO2 HCl NH2 p-Toluidine +H2SO4 Toluen e CH3 C2H5OH +N2+CH3CHO+HCl Toluene N2Cl p-Toluene diazonium to remove the bases, then with NaOH to remove acidic substances and finally with water. It is subjected to fractional distillation. The vapours collected between 80 110 C is 90% benzol which contains 70  80 % benzene and 14  24 % toluene. 90% benzol is again distilled and the portion distilling between 108 110 C is collected as toluene. (ix) From n- heptane and methyl cyclohexane CH3 | CH2 CH3 CH3 H2C U Benzene (viii) Commercial preparation From coal tar : The main source of commercial production of toluene is the light oil fraction of coaltar. The light oil fraction is washed with conc. H 2 SO 4 ID Toluene is the simplest homolouge of benzene. It was first obtained by dry distillation of tolubalsam and hence named toluene. It is commercially known as tolual. (1) Methods of preparation (i) From benzene [Friedel-craft's reaction] : CH3 Toluen e E3 Toluene, methyl benzene or phenyl methane +MgBr2 60 +CH3Br CH2 H2C Cr2O3 / Al2O3 500-550°C 150 atms Toluene CH2 n-Heptane (2) Physical properties (i) It is a colourless mobile liquid characteristic aromatic odour. having (ii) It is lighter than water (sp. gr. 0.867 at 20°C). (iii) It is insoluble in water but miscible with alcohol and ether in all proportions. (iv) Its vapours are inflammable. It boils at 110°C and freezes at –96°C. (v) It is a good solvent for many organic compounds. (vi) It is a weak polar compound having dipole moment 0.4D. (3) Chemical properties : Toluene shows the behaviour of both an alipatic and an aromatic compound.CH3 Side chain (Aliphatic) Benzene ring (Aromatic) Hydrocarbon (i) Electrophilic substitution reactions : Aromatic character (More reactive than benzene) due to electron releasing nature of methyl group. CH3 CH3 CH3  E+  (c) Hydrogenation : R + E pDerivative  may be Cl, NO 2 , SO 3 H etc. Alkyl benzene (e) Ozonolysis : (a) Side chain halogenation : CH2Cl HC Cl2 UV UV HC UV Benzal chloride Benzo trichloride D YG C6 H 5 CH 2 Cl  NaOH  C6 H 5 CH 2 OH  NaCl chloride on hydrolysis forms C6 H 5 CHCl 2  2 NaOH  C6 H 5 CH (OH )2  2 NaCl  (Benzal chloride) O CH C HOH O O CH3 –C=O CHO +2 H – C=O + 3H2O2 CHO Methyl glyoxal Glyoxa l (4) Uses (i) In the manufacture of benzyl chloride, benzal chloride, benzyl alcohol, benzaldehyde, benzoic acid, saccharin, etc. C6 H 5 CHO  H 2 O (ii) In the manufacture of trinitrotoluene (TNT), a highly explosive substance. (iii) As an industrial solvent and in drycleaning. (iv) As a petrol substitute. (v) In the manufacture of certain dyes and drugs. T.N.T. (Tri-nitro toluene) CH3 Preparation :  U C6 H 5 COOH  H 2 O Fuming Toluene (b) Oxidation : ST  With hot acidic KMnO4 : CH3 KMnO4 / H+ 3[O] Toluene CH3 O2N + 3H2O + H2O NO2  With acidic manganese or chromyl chloride (Etards CHO reaction) : CH3 + 2[O] NO2 COOH Benzoic acid CrO2C2 H2SO4 + 3HNO3 C6 H 5 CCl 3  3 NaOH  C6 H 5 C(OH )3  3 NaCl Toluene Zn O Triozonide  Benzo trichloride on hydrolysis forms benzoic (Benzotri chloride) O CH U  Benzyl chloride on hydrolysis with aqueous caustic soda forms benzyl alcohol. CH C O Toluene CCl3 Cl2 acid. O C CH ID CHCl2  Benzal benzaldehyde. O O CH C H Benzyl chloride (Benzyl chloride) H C +3O3 Cl2 Toluene Alkyl cyclohexane (d) Combustion : C 6 H 5 CH 3  9 O 2  7 CO 2  4 H 2 O (ii) Reactions of side chain CH3 R Na / liquid NH 3 – C2H5OH Birch reduction + 3H2 oDerivative  the side chain does not matter. 60 E Electrophile  All alkyl benzenes on oxidation with hot acidic KMnO 4 or Na 2Cr2O7 form benzoic acid. The length of E3 + E  1105 +H2O Benzaldehy de Properties : It is pale yellow crystalline solid (M.P. = 81°C). Uses :  It is used as an explosive in shells, bombs and torpedoes under the name trotyl.  When mixed with 80% ammonium nitrate it forms the explosive amatol.  TNT is also used as a mixture of aluminium nitrate, alumina and charcoal under the name ammonal. 1106 Hydrocarbon T.N.B. (Tri-nitro benzene) Preparation : CH3 CH3 O2N NO2 H2SO4 K2Cr2O7 HNO3 H2SO4  These can also be obtained by Friedel – craft's synthesis,  m-Xylene can be obtained from mesitylene. COOH O2N NO2 NO2 O2N Soda Xylenes characteristic isomers are, are colourless liquids having odour. The boiling points of three 60 NO2 o-Xylene=144°C; Xylene=138°C. lime NO2 NO2 T.N.B. Properties and uses: It is colourless solid (M.P. = 122°C). It is more explosive than T.N.T. and used for making explosive. m-Xylene=139°C; COOH Phthalic acid COOH Isophthalic acid ID Xylenes (Dimethyl benzene) C6H4(CH3)2 CH3 o-Xylene m-Xylene Ethyl benzene CH3 D YG p-Xylene and These are produced along with benzene, toluene ethylbenzene when aromatisation of C6  C8 fraction of petroleum naphtha is done. The xylenes are isolated from the resulting mixture (BTX) by fractional distillation. ST U These can be prepared by Wurtz – Fittig reaction. A mixture of bromotoluene and methylbromide is treated with sodium in dry ethereal solution to form 3 the desiredCH xylene. CH3 Br CH3 + 2Na + BrCH3 + 2NaBr o-Bromotoluene CH3 CH3 + 2NaBr CH3 m-Xylene CH3 + 2Na + BrCH3 Br p-Bromotoluene Ethyl benzene (C6H5C2H5) It can be prepared by the following reactions, (1) By Wurtz-Fittig reaction : C 6 H 5 Br  2 Na  BrC 2 H 5  C 6 H 5 C 2 H 5  2 NaBr (2) By Friedel-craft's reaction : AlCl 3 C 6 H 5 H  BrC 2 H 5    C 6 H 5 C 2 H 5  HBr (3) By catalytic reduction of styrene : C 6 H 5 CH  CH 2  H 2  C 6 H 5 CH 2 CH 3 (4) By alkyl benzene synthesis : AlCl 3 , HCl C 6 H 5 H  H 2 C  CH 2   C 6 H 5 CH 2 CH 3 95 C ,Pressure benzoic acid. Br CH3 Xylenes are used in the manufacture of lacquers and as solvent for rubber. o-Xylene is used for the manufacture of phthalic anhydride. It undergoes electrophilic substitution reactions in the same way as toluene. When oxidised with dil. HNO 3 or alkaline KMnO 4 or chromic acid it forms o-Xylene + 2Na + BrCH3 m-Bromotoluene COOH Terephthalic acid U The molecular formula, C8 H 10 represents four CH3 CH3 C2H5 CH3 isomers. CH3 p- Xylenes undergo electrophilic substitution reactions in the same manner as toluene. Upon oxidation with KMnO 4 or K 2 Cr2 O7 , Xylenes form COOH corresponding dicarboxylicCOOH acids. COOH , , E3 Toluene [O ] C 6 H 5 C 2 H 5   C 6 H 5 COOH Styrene (C6H5CH=CH2) It is present in storax balsam and coal-tar in traces. (1) Preparation + 2NaBr CH3 (i) Dehydrogenation of side CH2CH3 ethylbenzene : + CH2 = CH2 p-Xylene Benzene AlCl3 chain of CH = CH2 600°C Cr2O3 / Al2O3 Ethylbenzen e Styrene Hydrocarbon Quinol C 6 H 5 CH  CHCOOH    C 6 H 5 CH  CH 2  CO 2 When oxidised under drastic conditions, the side chain is completely oxidised to a carboxyl group. CH = CH2 (iii) Dehydration of 1-phenyl ethanol with H 2SO4 H 2 SO 4 : C 6 H 5 CHOHCH 3   C 6 H 5 CH  CH 2 [O]  H 2O KMnO4 (iv) Dehydration of 2-phenyl ethanol with ZnCl2 ZnCl 2 , heat : C 6 H 5 CH 2 CH 2 OH   C 6 H 5 CH  CH 2 (v) Dehydrohalogenation of 1-phenyl-1-chloro ethane : On heating with alcoholic potassium hydroxide, a molecule of hydrogen chloride is eliminated by the chloroderivative. Alc.KOH C 6 H 5 CHClCH 3   C 6 H 5 CH  CH 2 Heat Benzoic acid In presence of peroxides, styrene undergoes free radical polymerisation resulting in the formation of polystyrene – an industrially important plastic.     Peroxide nC 6 H 5 CH  CH 2  CH  CH 2   |   C6 H 5   n ID (2) Properties : It is a colourless liquid, boiling point 145°C. On keeping, it gradually changes into a solid polymer called metastyrene. The polymerisation is rapid in sunlight or when treated with sodium. It shows properties of benzene ring (Electrophilic substitution) and unsaturated side chain (Electrophilic addition). However, the side chain double bond is more susceptible to electrophilic attack as compared to benzene ring. At lower temperature and pressure, it reacts with hydrogen to produce ethylbenzene and at higher temperature and pressure, it is converted into ethyl cyclohexane. CH = CH2 CH2CH3 CH2CH3 Styrene E3  H 2O COOH 60 (ii) Decarboxylation of cinnamic acid : This is the laboratory preparation and involves heating of cinnamic acid with a small amount of quinol. 1107 D YG U Co-polymers of styrene with butadiene and other substances are also important since many of them are industrially useful products such as SBR ( A rubber substitute). H2 / Ni H2 / Ni 20°C, 3 atm 125°C, 110 atm Styrene Ethyl cyclohexane U Ethyl benzene With bromine, it gives the dibromide. CHBr.CH2Br ST CH = CH2 + Br2 Styrene dibromide C 6 H 5 CH  CH 2  HX  C 6 H 5 CHXCH (i) Fittig reaction : It consists heating of an ethereal solution of bromobenzene with metallic sodium. Br +2Na + Br + 2NaBr (ii) Ullmann biaryl synthesis : Iodobenzene, on heating with copper in a sealed tube, forms biphenyl. The reaction is facilitated if a strong electron withdrawing group is present in ortho or para position. + 2CuI 3 Preparation of ring substituted styrenes is not done by direct halogenation but through indirect route. CH2CH3 CH2CH3 FeCl3 Cl2 hv CHClCH3 Cl CH = CH2 Alc.. KOH Heat Cl (1) Methods of formation (iii) Grignard reaction : Phenyl magnesium bromide reacts with bromo benzene in presence of CoCl 2. Halogen acids add to the side chain. Cl2 It occurs in coal-tar. It is the simplest example of an aromatic hydrocarbon in which two benzene rings are directly linked to each other. I+ 2Cu + I Styrene + Bi-phenyl (C6H5 – C6H5) Cl MgBr + Br CoCl2 + MgBr2 1108 Hydrocarbon HNO3 / H2SO4 HNO3 / H2SO4 NO2 NO2 (2o) and tertiary (3o) hydrogens in alkanes follows the sequence : 3o > 2o > 1o. D YG U ID O2N  The order of reactivity of primary (1 o), secondary 60 Another special feature of the biphenyl is the behaviour towards second substitution in a monosubstituted biphenyl. The second substituent invariably enters the unsubstituted ring in the ortho and para position no matter what is the nature of substituent already present. It reduces alkenes and alkynes while other common functional groups such as C=O, NO2 and C  N remain unaffected, E3 (2) Properties : It is a colourless solid, melting point 71°C. It undergoes usual electrophilic substitution reactions. Since aryl groups are electron withdrawing , they should have deactivating and m-orientating effect. But, it has been experimentally shown that presence of one benzene ring activates the other for electrophilic substitution and directs the incoming group to o- and p- positions. It has been shown that monosubstitution in the bi-phenyl results in the formation of para isomer as the major product.  Octane number may be less than zero (e.g., nNonane has an octane number-45) and higher than 100 (e.g., Triptane or 2, 3, 3-Trimethylbutane has an octane number of 124).  To avoid lead pollution, a new compound cyclopentadienyl manganese carbonyl – Mn CO CO CO U (called as AK-33-X) is used as antiknock now a days in developed countries (unleaded pertol).  Acetylene has a garlic odour when impure due to impurities of phosphine and hydrogen sulphide. ST  Fluorination is a violent reaction and can be controlled by diluting fluorine with nitrogen.  The relative acidic character of water, alcohols acetylene, ammonia, ethylene and ethane follows the order : H2O > ROH> HC  CH >CH3 > CH2 = CH2 > CH3 –CH3. Obviously, the basic character of their conjugate bases follows the reverse order, i.e., CH3CH2– >CH2 = CH– > NH2– > HC  C– >RO– > HO–.  Wilkinson’s catalyst : (Triphenylphosphine) rhodium, (PPh3)3 RhCl is called wilkinson’s catalyst.