Chemistry Notes for NEET Chapter 24 - Hydrocarbons PDF
Document Details
Uploaded by NourishingRoseQuartz
null
Tags
Summary
These notes cover hydrocarbons, specifically aliphatic and aromatic hydrocarbons, and their properties. They discuss sources, theories of origin, refining processes, and petrochemicals. The document is suitable for undergraduate-level chemistry students studying for exams, like NEET.
Full Transcript
60 Chapter E3 24 Hydrocarbon Organic compounds composed of only carbon and hydrogen are called hydrocarbons. Hydrocarbons are two types (2) Aromatic Hydrocarbon (Arenes) (1) Sources of aliphatic hydrocarbon (vi) C.N.G. : When natural gas compressed at very high pressure is called compressed natural...
60 Chapter E3 24 Hydrocarbon Organic compounds composed of only carbon and hydrogen are called hydrocarbons. Hydrocarbons are two types (2) Aromatic Hydrocarbon (Arenes) (1) Sources of aliphatic hydrocarbon (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. U (1) Aliphatic Hydrocarbon (Alkanes, Alkenes and Alkynes). D YG Mineral oil or crude oil, petroleum [Petra rock; 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. (2) Composition (0.5%) and Nitrogen (1.3%). L.P.G. Contain butanes and pentanes and used as cooking gas. It is highly inflammable. This contain, methane, nitrogen and ethane. ID Aliphatic Hydrocarbon (i) Alkanes : found 30 to 70% contain upto 40 carbon atom. Alkanes are mostly straight chain but some are branched chain isomers. (3) Theories of origin of petroleum : Theories must explain the following characteristics associated with petroleum, Its association with brine (sodium chloride solution). 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. U (ii) Cycloalkanes : Found 16 to 64% cycloalkanes present in petroleum are; cyclohexane, methyl cyclopentane etc. cycloalkanes rich oil is called asphaltic oil. (i) Mendeleeff’s carbide theory or inorganic theory (ii) Engler’s theory or organic theory (iii) Aromatic hydrocarbon : found 8 to 15% compound present in petroleum are; Benzene, Toluene, Xylene, Naphthalene etc. ST (iv) Sulphur, nitrogen and oxygen compound : Sulphur compound present to the extent of 6% include mercaptans [R-SH] and sulphides [R-SR]. The unpleasant smell of petroleum is due to sulphur compounds. Nitrogenous compounds are pyridines, quinolines and pyrroles. Oxygen compounds present in petroleum are. Alcohols, Phenols and resins. Compounds like chlorophyll, haemin are also present in it. (iii) Modern theory (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. (5) Petroleum refining : Separation of useful fractions by fractional distillation is called petroleum refining. (v) Natural gas : It is a mixture of Methane (80%), Ethane (13%), Propane (3%), Butane (1%), Vapours of low boiling pentanes and hexanes Table : 24.1 Fraction Uncondensed gases Boiling range ( C) o Upto room temperature Approximate composition Uses C–C Fuel gases: refrigerants; production of carbon black, hydrogen; synthesis of organic chemicals. 1 4 Crude naphtha on refractionation yields, 30 – 150 C –C (i) Petroleum ether 30 – 70 C –C Solvent (ii) Petrol or gasoline 70 – 120 C –C 8 Motor fuel; drycleaning; petrol gas. (iii) Benzene derivatives 120 – 150 C –C 10 Solvent; drycleaning Kerosene oil 150 – 250 C –C 16 Fuel; illuminant; oil gas Heavy oil 250 – 400 o 5 o 10 5 o 6 6 o 8 o 11 C –C o 15 As fuel for diesel engines; converted to gasoline by cracking. 18 60 Refractionation gives, (i) Gas oil, (ii) Fuel oil, (iii) Diesel oil C –C Above 400 o 17 40 (i) Lubricating oil C –C (ii) Paraffin wax C –C (iii) Vaseline C –C (iv) Pitch C –C 17 20 20 30 30 30 40 Lubrication Candles; boot polish; wax paper; etc Toilets; ointments; lubrication. Paints, road surfacing As fuel. ID Petroleum coke 20 E3 Residual oil on fractionation by vacuum distillation gives, (on redistilling tar) (6) Purification D YG U (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. (a) Bergius process : This method was invented by Bergius in Germany during first world war. (ii) Doctor sweetening process : 2 RSH Na 2 PbO2 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 U (i) Cracking : It is a process in which high boiling fractions consisting of higher hydrocarbons are heated strongly to decompose them into lower hydrocarbons with low boiling points. Cracking is carried out in two different ways. ST (a) Liquid phase cracking : In this process, the heavy oil or residual oil is cracked at a high temperature (475 – 530 C) under high pressure (7 to 70 atmospheric pressure). The high pressure keeps the reaction product in liquid state. The conversion is approximately 70% and the resulting petrol has the octane number in the range 65 to 70. o The cracking can be done in presence of some 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, kerosene oil or gas oil is cracked in vapour phase. The temperature is kept 600 – 800 C and the pressure is about 3.5 to 10.5 atmospheres. The cracking is facilitated by use of a suitable catalyst. The yields are about 70%. o (ii) Synthesis : Two methods are applicable for synthesis. Fe2 O3 Coal H 2 o Mix. Of hydrocarbons or crude oil 450 500 C 250 atm (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 Watergas xCO yH 2 Mix. Of hydrocarbon H 2 O. The best catalyst for this process is a mixture of cobalt (100 parts), thoria, (5 parts), magnesia (8 parts) and kieselguhr (200 parts). Characteristics of hydrocarbons (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. The tendency to knock falls off in the following order : Straight chain alkanes > branched chain alkanes > olefins > cyclo alkanes > aromatic hydrocarbons. (2) Octane number : It is used for measuring the knocking character of fuel used in petrol engine. The octane number of a given sample may be defined as the percentage by volume of iso-octane present in a mixture of iso-octane and n-heptane which has the same knocking performance as the fuel itself. CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 n-heptane; octane no. = 0 CH 3 By increasing the proportion of branched chain or cyclic alkanes. CH 3 | | By addition of aromatic hydrocarbons Benzene, Toluene and Xylene (BTX). CH 3 C CH 2 C CH 3 ; Octane no. = 100 | CH 3 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 3 Cetane no. = 0 -Methyl naphthalene E3 The cetane number of a diesel oil is the percentage of cetane (hexadecane) by volume in a mixture of cetane and -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. The flash point in India is fixed at 44 o C , in France it is fixed at 35 C, and in England at 22.8 C. The flash point of an oil is usually determined by means of “Abel’s apparatus”. 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). (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 : Pb Br CH 2 CH 2 Br PbBr2 CH 2 CH 2 Ethylene bromide Volatile Ethylene D YG However, use of TEL in petrol is facing a serious problem of Lead pollution, to avoid this a new compound cyclopenta dienyl manganese carbonyl (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 AlCl3 at o 200 C. U CH 3 | AlCl3 CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CHC H 2 CH 3 o 200 C Pentane (Octane number 62) Isopentane (Octane number 90 ) (ii) Alkylation : ST 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 Isobutane Iso - octane (Octane number 100) (iii) Aromatisation : CH 3 CH 3 (CH 2 )5 CH 3 o 4H2 Pt / Al2O3 Heptane o U However, there is a disadvantage that the lead is deposited in the engine. To remove the free lead, the ethylene halides are added which combine with lead to form volatile lead halides. 500 C Toluene The octane no. of petrol can thus be improved. Cetane cetane no. = 100 60 CH ID 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. 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, TEL = 63%, Ethylene bromide = 26%, Ethylene chloride = 9% and a dye = 2%. o Table : 24.2 Hydrocarbons Compounds derived Methane Methyl chloride, chloroform, methanol, formaldehyde, formic acid, freon, hydrogen for synthesis of ammonia. Ethyl chloride, ethyl bromide, acetic acid, acetaldehyde, ethylene, ethyl acetate, nitroethane, acetic anhydride. Ethanol, ethylene oxide, glycol, vinyl chloride, glyoxal, polyethene, styrene, butadiene, acetic acid. Propanol, propionic acid, isopropyl ether, acetone, nitromethane, nitroethane, nitropropane. Glycerol, allyl alcohol, isopropyl alcohol, acrolein, nitroglycerine, dodecylbenzene, cumene, bakelite. Benzene, DDT, gammexane. Toluene Benzene, toluene, xylenes, adipic acid. Ethyl benzene, styrene, phenol, BHC (insecticide), adipic acid, nylon, cyclohexane, ABS detergents. Benzoic acid, TNT benzaldehyde, saccharin, chloramineT, benzyl chloride, benzal chloride. Ethane Ethylene Propane Propylene Hexane Heptane Cycloalkanes Benzene Toluene Alkanes [Paraffines] “Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecules.” Alkanes are less reactive so called paraffins; 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 H 6 , C3 H 8 , (1) General Methods of preparation (i) By catalytic hydrogenation of alkenes and alkynes (Sabatie and sanderen’s reaction) Ni Ni Cn H 2n 2 Cn H 2n H 2 Cn H 2n 2 ; C n H 2n2 2 H 2 heat Alkyne Alkane 2 R R R (alkane) Alkane At cathode [Reduction] : Methane is not prepared by this method 2 H 2O 2 Na 2e 2 Na 2 NaOH H 2 ( ) (ii) Birch reduction : Both ionic and free radical mechanism are involved in this reaction. 3 R CH CH 2 R CH 2 CH 3 1. Na / NH 2. CH 3 OH (iii) From alkyl halide (c) Reduction of carboxylic acid : Zn / HCl (a) By reduction : RX H 2 RH HX (b) With hydrogen in presence of RX H 2 RH HX (c) With HI in presence pt/pd Re duction CH 3 COOH 6 HI CH 3 CH 3 2 H 2O 3 I2 : Red phosphorus : Red P CH 3 OH 2 HI CH 4 H 2 O I2 E3 (iv) By Zn-Cu couple : 150 o C Acetone (Propanone ) 200 o C Acetyl chloride (Ethanoyl chloride) U (vii) Corey-house synthesis D YG Ethane O || Red P CH 3 C NH 2 6 HI CH 3 CH 3 H 2 O NH 3 3 I2 CH CH Cl 1. Li Propane O || Red P CH 3 C Cl 6 HI CH 3 CH 3 H 2 O HCl 3 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 : 2 RX Zn R R ZnX 2 Ethane Red P CH 3 COCH 3 4 HI CH 3 CH 2 CH 3 H 2 O 2 I2 ID Alkane 150 o C Acetaldehyde (Ethanal) Zincethoxide Alkyl halide Methane Red P CH 3 CHO 4 HI C2 H 6 H 2 O 2 I2 Cu (CH 3 CH 2 O)2 Zn 2 H RX 2 H RH HX (v) Wurtz reaction : Dry ether R X 2 Na X R R R 2 NaX Alkyl halide 150 o C Methanol (Methyl alcohol) Purpose of Red P is to remove I2 in the form of PI3 Zn-Cu couple Ethane (x) By reduction of alcohols, aldehyde, ketones or acid derivatives of RBr 2 HI RH HBr I2 Zn p Aceticacid Pd orPt. 2CH 3 CH 2 OH 60 heat Alkene 2 R C O 2e 2 R C O 2 R 2CO 2 || || O O 200 o C Acetamide (Ethanamide) 3 2 CH 3 CH 2 Cl (CH 3 CH 2 )2 LiCu Ethane 2. CuI CH 3 CH 2 CH 2 CH 3 Reaction is suitable for odd number of Alkanes. (viii) From Grignard reagent (a) By action of acidic ‘H’ : RMgX Alkylmagnesium halide Aldehyde and ketones when reduced with amalgamated zinc and conc. HCl also yield alkanes. Clemmenson reduction : Zn Hg CH 3 CHO 4 H CH 3 CH 3 H 2 O Water Alkane U ST (ix) From carboxylic acids (a) Laboratory method [Decarboxylation reaction or Duma reaction] NaOH and CaO is in the ratio of 3 : 1. (Sodalime) (b) Kolbe’s synthesis : Electrolysis R C O Na Ionization || O At anode [Oxidation] : O || R C O Na Propane Aldehydes and ketones ( C O) can be reduced to hydrocarbon in presence of excess of hydrazine and sodium alkoxide on heating. Wolff-kishner reduction : C O C NNH 2 H 2 NNH 2 R R R R Alkane HCl Acetone (Propanone ) heat R COONa NaOH R H Na 2 CO 3 CaO Ethane Zn Hg CH 3 COCH 3 4 H CH 3 CH 2 CH 3 H 2 O (b) By reaction with alkyl halide : R X RMgX R R MgX2 HCl Acetaldehyde (Ethanal) HOH RH Mg(OH )X H 2O C 2 H 5 ONa R 180 o C , N 2 CH 2 R (xi) Hydroboration of alkenes (a) On treatment with acetic acid 6 3 R CH CH 2 2 (R CH 2 CH 2 )3 B B H Alkene CH COOH Trialkyl borane R CH 2 CH 3 Alkane (b) Coupling of alkyl boranes by means of silver nitrate NaOH 3[RCH 2 CH 2 CH 2 CH 2 R] (a) Complete Oxidation or combustion : Solid like waxes 3n 1 Cn H 2 n 2 O 2 nCO 2 (n 1)H 2 O Q 2 60 C18 and above This is exothermic reaction. (ii) Density : Alkanes are lighter than water. (iii) Solubility : Insoluble in water, soluble in organic solvents, 1 solubility Molecular mass (b) Incomplete combustion or oxidation Burn 2CH 4 3O2 2CO 4 H 2 O CH 4 O2 C 2 H 2 O 138 C6 H14 C7 H16 C8 H18 179 182.5 216.2 143.3 Cu tube (c) Catalytic Oxidation : 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. ID 85.9 C4 H10 C5 H12 E3 (iv) Boiling points and Melting points : Melting points and boiling 1 points. Molecular mass No. of branches M.P.(K) : mechanism Prolonged heating Gaseous state Liquid state [Except neo pentane which is gas] C3 H 8 radical (iii) Oxidation C5 C17 Alkane : Free R H HOSO 3 H R SO 3 H H 2 O State C1 C4 : SO 3 Lower alkanes particularly methane, ethane, do not give this reaction. (2) Physical Properties (i) Physical state : Alkanes are colourless, odourless and tasteless. Alkanes Sulphonation (b) o 2 B2 H 6 AgNO3 25 C 6[R CH CH 2 ] [2 R CH 2 CH 2 ]3 B C C C C C O2 CH 3 (CH 2 )n CH 3 CH 3 (CH 2 )n COOH o 100 160 C (d) Chemical oxidation : U Melting points of even > Odd no. of carbon atoms, this is because, the alkanes with even number of 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. KMnO 4 (CH 3 )3 CH (CH 3 )3.C.OH C Isobutane C C C C C Even no. of carbons Odd no. of Carbons (iv) C D YG C o 1000 C CH 4 C 2H 2 (i) Substitution reactions of Alkanes Methane (a) Halogenation : R H X X R X HX o 500 C C 2 H 6 CH 2 CH 2 H 2 Ethane Cr2 O3 Al2 O3 The reactivity of halogen is : F2 Cl 2 Br2 I2 Fluorine can react in dark Cl 2 , Br2 require light energy. I 2 Ethylene C 3 H 8 C 2 H 4 CH 4 or C 3 H 6 H 2 doesnot show any reaction at room temperature, but on heating it shows iodination. U This reaction is of great importance to petroleum industry. Iodination of methane is done in presence of oxidising agent such as HNO 3 / HIO3 / HgO which neutralises HI. (v) Isomerisation : CH 3 | AlCl3 HCl CH 3 CH 2 CH 2 CH 3 CH CHCH 3 3 o Chlorination of methane : ST Thermal decomposition or cracking or pyrolysis or fragmentation (3) Chemical properties u. v. light 2 CH 4 2Cl Cl CH 2 Cl 2 u. v. light, Cl 2 HCl Tertiary butyl alcohol 200 C , 35 atm n - Butane Isobutane HCl HCl CHCl 3 CCl 4 2-Methyl pentane Cl 2 AlCl3 HCl heat (ii) Reaction based on free radical mechanism High R NO 2 H 2 O (a) Nitration : R H HONO 2 Alkane temp. (ii) (HNO 3 vapour at 400 500 C). o (vi) Aromatisation : Nitroalkane Nitrating mixture : (i) (Con. HNO 3 Con. H 2 SO 4 ) at 250 o C 2,3 Dimethyl butane CH3 H2C CH3 H2C CH2 o Cr2 O3 / Al 2 O3 o +4H2 600 C / 15 atm CH2 n-Hexane Benzene CH3 CH3 H2 Cr2O3 / Al 2 O3 o 600 C n-Heptane Methyl cyclo Hexane Toluene (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 (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 (b) Reaction with CHCl 3 / NaOH : CHCl 3 / OH R CH 2 H R CH 2 CHCl 2 : CCl 2 60 161.5 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. (d) As a fuel and illuminant. (2) Ethane (i) Methods of preparation (a) Laboratory method of preparation : Zn Cu couple C 2 H 5 I 2 H C 2 H 6 HI (c) Reaction with CH 2 C : || O C2 H 5 OH Ethyl iodide Ethane E3 (b) Industrial method of preparation : Ni CH 2 CH 2 H 2 CH 3 CH 3 O || 300 o C Ethylene (ethene) CH 2 C/ R CH 2 H R CH 2 CH 3 :CH 2 / CO (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 – 89 C. Its melting point is –172 C. (iii) Uses (a) As a fuel. (b) For making hexachloroethane which is an artificial camphor. (3) Interconversion of Alkanes ID (viii) HCN formation : Ethane N 2 / electricarc 2CH 4 2 HCN 3 H 2 or o Al2 O3 CH 4 NH 3 HCN 3 H 2 (ix) Chloro sulphonation/Reaction with SO +Cl 2 2 CH 3 CH 2 CH 3 SO 2 Cl 2 u.v light U 700 o C D YG CH 3 CH 2 CH 2 SO 2 Cl HCl Ascent of alkane series, (i) Methane to ethane : This reaction is known as reed’s reaction. This is used in the commercial formation of detergent. (x) Action of steam : o Ni / Al2O3 CH 4 H 2 O o CO 3 H 2 800 C Individual members of alkanes U (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 containing nickel and carbon at 250 o C when methane is formed. Ni C CO 3 H 2 CH 4 H 2 O o CH 4 Methane Cl 2 Wurtzreaction CH 3 Cl CH 3 CH 3 UV Heat with Na in ether (ii) Butane from ethane : Cl 2 Wurtz reaction C 2 H 6 C 2 H 5 Cl C 2 H 5 C 2 H 5 Ethane (excess) UV Ethyl chloride Heat with Na in ether 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 Ethyl chloride Ethyl alcohol ST (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 1200 C, methane is formed. Acetaldehyde [O ] NaOH NaOH / CaO CH 3 COOH CH 3 COONa CH 4 Sodium acetate heat Methane [O ] Higher Alkyl Alcohol Aldehyde Cl 2 (b) Bacterial decomposition of cellulose material present in sewage water : This method is being used in England for production of methane. Butane Descent of alkane series : Use of decarboxylation reaction is made. It is a multistep conversion. Aceticacid 250 C Ethane alkane UV Aq. halide [O ] KOH NaOH / CaO Acid Sodium salt of Lower alkane heat the acid NaOH Alkenes o o 1200 C CH 4 C 2 H 2 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. 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, C 2 H 4 , C 3 H 6 , C 4 H 8. (1) Preparation methods (i) From Alkynes : (ix) Action of copper alkyl on vinyl chloride : H | Lindlar's Catalyst R C C R H 2 R C C R Pd. BaSO 4 | H 2 H 2 C CHCl H 2 C CHR CuR Vinyl chloride (x) By Grignard reagents : R X CH CH 2 MgX2 R CH CH 2 Mg X (xi) The wittig reaction : (Ph)3 P CH 2 CH R (Ph)3 P O R CH || || O CH 2 60 Poison’s catalyst such as BaSO4 ,CaCO 3 are used to stop the reaction after the formation of alkene. (ii) From mono halides : H H H | | | R C C H Alc. KOH R C C H HX | | | H X H Alkene X R – CH Zn X + X + Zn CH – R X 2 ZnX 2 R – CH = CH – R C1 C 4 gas U H H H H | | | | R C C H Zn dust R C C H ZnX 2 300 o C | | X X (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 (2) Physical Properties (i) Alkenes are colourless and odourless. (ii) These are insoluble in water and soluble in organic solvents. (iii) Physical state ID 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. (b) From vicinal dihalides : O || (Ph)3 P CH R CH R (Ph)3 P O R CH CH R E3 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 D YG 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 NaI acetone vic dihalide I I | | C C I2 unstable U (v) From alcohols [Laboratory method] : C C alkene H 2 SO 4 or H 3 PO4 CH 3 CH 2 OH CH 2 CH 2 H 2 O 443 K Ethyl alcohol Ethene ST (vi) Kolbe’s reaction : CH 2 COOK CH 2 Electrolysis | 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 (viii) Pyrolysis of quaternary ammonium compounds : heat (C2 H 5 )4 N OH (C2 H 5 )3 N C2 H 4 H 2 O Tetraethylammonium Triethylamine hydroxide (Tert. amine) Ethene C4 C16 liquid C17 solid wax (iv) B.P. and M.P. decreases with increasing branches in alkene. (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 (i) Francis experiment : According to Francis electrophile first attacks on olefinic bond. CCl4 CH2 – CH2 CH2 = CH2 + Br – Br | | Br Br NaCl CH2 – CH2 + CH2 – CH2 | | | | Br Cl Br Br (ii) Reaction with hydrogen : H H H H | | | | Ni R C C R H 2 R C C R | | H H (iii) Reduction of alkene via hydroboration : Alkene can be converted into alkane by protolysis H H H H | | | | CCl 4 R C C H X X R C C H | | X X H BH 2 RCH CH 2 (R CH 2 CH 2 )3 B Hydroboration : Alkene give addition reaction with diborane which called hydroboration. In this reaction formed trialkylborane, Which is very 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 R – CH –CH OH R – CH –CH 2 2 2 2 3 2 R – CH –CH 2 2 3 The overall result of the above reaction appears to be antimarkownikoff’s addition of water to a double bond. (iv) By treatment with AgNO + NaOH : This reaction gives coupling CH 3 | B2 H 6 6 CH 3 CH 2 CH 2 C CH 2 Peroxide CH 3 CH CH 2 HBr ID 3 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.) E3 2 3 Alkylhalide HI/H O NaOH/ H O CH COOH/Zn Vicinal dihalide Reactivity of halogen is F2 Cl 2 Br2 I2 (vii) Reaction with HX [Hydrohalogenation] H | CC HX C C | X alkene 60 H / H 2O R CH 2 CH 3 D YG CH 3 CH 3 | | CH 3 CH 2 CH 2 C CH 2 CH 2 C CH 2 CH 2 CH 3 | | H H (v) Birch reduction : This reaction is believed to proceed via anionic free radical mechanism. Na Et O H R CH CH 2 R C H C H 2 R CH CH 3 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 Allylchloride or 3-Chloro-1- propene U Propene ST If NBS [N-bromo succinimide] is a reagent used for the specific purpose of brominating alkenes at the allylic position. CH – CO | N – Br CH CH=CH + CH – CO 3 2 2 2 NBS Propene CH – CO | | CH – CO 2 Br 2 Succinimide Allyl bromide In presence of polar medium alkene form vicinal dihalide with halogen. Ethylene Ethylene chlorohydr in In case of unsymmetrical alkenes markownikoff rule is followed. (ix) Reaction with sulphuric acid : CH 2 CH 2 H HSO 4 CH 3 CH 2 HSO 4 Ethylene Ethyl 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 Tillden | Cl reagent) If hydrogen is attached to the carbon atom of product, the product changes to more stable oxime. NO | H C C NOH C C ⇌ | | | Cl Cl Oxime N–H 2 (major) CH 2 CH 2 H O Cl CH 2 OH.CH 2 Cl 2 CH – CH = CH + (minor) (viii) Reaction with hypohalous acids : U CH 3 | Ag / NO 3 NaOH 2[CH 3 (CH 2 ) 2 C CH 2 ] 3 B | H H H H H | | | | CH 3 C C H CH 3 C C H | | | | Br H H Br C CC C NOCl C C (Blue colour) | | C NO Cl (xi) Oxidation : With alkaline KMnO4 [Bayer’s reagent] : This reaction is used as a test of unsaturation. C H H H H | | | | Alk KMnO 4 R C C H [O] H OH R C C H OH | | HO OH glycol With acidic KMnO4 : H H O | | || acidic R C C H [O] R C O H CO 2 H 2 O NaBH 4 / NaOH (CH 3 ) 3 C CH CH 2 Hg (CH 3 )3 C CH CH 3 THF | | OH OCOCH 3 3 , 3 Dimethyl 2 butanol (xvi) Epoxidation O 1 Ag CH 2 CH 2 (a) By O2 / Ag : CH 2 CH 2 O 2 2 (b) Epoxidation by performic acid or perbenzoic acid : KMnO 4 2 - Butene 60 O | C–O–O–H | CH 3 | H C OH | HO C H | CH 3 CH 2 CH 2 CH 2 CH 2 || H C O O H CH 3 CH CH 2 CH 3 CH CH 2 (xvii) Hydroboration H Tri alkyl borane R CH 2 CH 2 OH B(OH)3 (Anti markownikoff’s rule) 4 ID (xviii) Hydroformylation : R R Trans H 2 O 2 / OH 3 R CH CH 2 BH 3 (R CH 2 CH 2 )3 B C (b) Hydroxylation by OsO : | | OsO4 NaHSO 4 I C H O O Trans (racemic) R O E3 (xii) Hydroxylation (a) Using per oxy acid : CH 3 | H 2 O2 , HCOOH H C or HCO 3 H || H C | CH 3 H HO OH H U R H H | | CoH (CO )4 R CH CH 2 CO H 2 R C C H | | H C O | H If per benzoic acid or peroxy acetic acid is used then () oxirane are formed. D YG C6 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] 3n O 2 nCO 2 nH 2 O 2 They burn with luminous flame and form explosive mixture with air or oxygen. (xiv) Ozonolysis If CO H 2 O is taken then respective acid is formed. CoH (CO )4 R CH CH 2 CO H 2 O R CH 2 CH 2 | COOH (xix) Addition of formaldehyde U ST CC I H2 C C O O O H 2 O / H / Zn ZnO II Ozonide O C+C 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 CH2 R – CH O C H2 O HOH H HCHO / H R CH CH 2 CH 2 | | OH OH 1, 3 diol Cyclic acetal O C R CH CH 2 R C H CH 2 CH 2 OH (xiii) Combustion : Cn H 2n O3 H 2 C O H [H 2 C O H H 2 C OH ] Mercuric acetate (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 zeiglernatta catalyst [(R)3 Al TiCl4 ] is used then polymerisation is known as zeigler-natta polymerisation. (xxi) Isomerisation : AlCl3 CH 3 CH 2 CH 2 CH CH 2 CH 3 CH 2 CH CH CH 3 The mechanism proceeds via carbocation. (xxii) Addition of HNO : CH 2 CH 2 HO NO 2 CH 2 OH.CH 2 NO 2 (ii) It is insoluble in water but highly soluble in acetone and alcohol. Acetylene is transported under high pressure in acetone soaked on porous material packed in steel cylinders. 3 Ethene 2-Nitroethanol (xxiii) Addition of Acetyl chloride : CH 2 CH 2 CH 3 COCl CH 2 ClCH 2 COCH 3 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 These are the acyclic hydrocarbons which contain carbon-carbon triple bond are called alkynes. General formula is Cn H 2n2. Ex. Ethyne CH CH ; Propyne CH 3 C CH (1) General methods of preparation E3 2 2 2 alc KOH, NaNH CH – CHBr 3 Vinylic carbanion (more stable) 2 2 Ag dust(Powder) CHCl Nu | C C Nu C C 3 2 2 ID Zn dust CHBr – CHBr CHBr Zn | 2 Ex. CH CH NaNH 2 H C C Na U alc KOH, NaNH CH 2 2 2 [Acetylene] HC – COONa HC – COONa D YG Kolbe’s electrolytic synthesis | H O (Laboratory method) CaC 2 2 Electric arc, 1200 C o 2C+ H (alkyl carbanion) (less stable) (i) Acidity of alkynes : Acetylene and other terminal alkynes (1alkynes) are weakly acidic in character CHBr CH = CH – Cl (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 Nu | C C Nu C C alc KOH or NaNH CH Br – CH Br air. 60 Ethene (iii) Its boiling point is 84 o C. (iv) It is lighter than air. It is somewhat poisonous in nature. (v) It burns with luminous flame and forms explosive mixture with Berthelot’s process 2 (i) Na (ii) R-X CH – C CH 3 (i) CH MgI (ii) R-X CH – CCHIn reaction with gem dihalide, Alc. KOH is not used for elimination in 2 step. 1 H2 2 (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 Dicopper acetylide (Red ppt) CH CH 2[ Ag(NH 3 )2 ]NO 3 AgC C Ag 2 NH 4 NO 3 2 NH 3 Disilver acetylide (white ppt) 3 3 nd U In reaction with vicinal dihalide, if the reactant is 2-butylene chloride then product is 2-butyne as major product. Preparation of higher alkynes (by metal acetylide) ST Acetylene gives salt with NaNH 2 or AgNO 3 (ammonical) which react with alkyl halide to give higher alkyne. NaNH 2 2 CH 3 I Na C C Na 2CH CH CH 3 C C CH 3 Butyne CH 3 C CH CH 3 Mg X RX 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. This reaction can be used to distinguish between 2-alkynes and 1alkynes. 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- propyne CH 3 C C CH 3 2[ Ag(NH 3 )2 ]NO 3 No reaction Explanation for the acidic character : It explained by sp 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%), carbon atom is quite electronegative. due to large s -character the (ii) Reaction with formaldehyde Li / NH 3 HC CH 2CH 2 O CH 2 C C CH 2 | | OH OH CH 2 CH CH CH 2 OH | OH [Trans-product ] (4) Chemical properties of acetylene C6 H 6 or Benzene NH 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 C4 H 5 N H 2 3 Pyrrole S/H S 2 C4 H 4 S 2 H2 Thiophene 40% H SO /1%HgSO 4 4 80 C o CH 3 CHO 3 Hg , 80 C 2+ 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 CH 3 CHO with CH COOH o Acetaldehyde CH 3 CO CH 3 CO O Acetic anhydride Cl Acetylene CHCl | 2 CHCl | AlCl R CC (Westrosol) CHCl | Lewisite CH 2 CHOOCCH 3 +2 CC–CC Vinyl acetate Hg /HCl 2+ CH 2 CHCl 60 C o Vinyl chloride Ba(CN) CH 2 CHCN Vinyl cyanide 2 Cu Cl , HCl 2 Chloroprene 2 NH Cl Cu O 2 Cuprene C 2 Cl 6 Hexa chloro ethane (Artificial camphor) C 2 H 4 (Cis) Ni Ethylene C2 H 6 300°C U Ethane Na ST 3 KMnO 4 Oxidation Catalyst Cis 2 6 2 12 1 2 Alkene without any hydrogen atom on the carbon forming R R CHO Glyoxal R CC the double bond | 2 Trans Tests of unsaturation Higher alkynes CHO O /SeO R Trans (a) Baeyer’s reagent : It is 1% KMnO4 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. HC CNa XR HC C R Sol. Acetylide H hydrocarbon is given by 2n1 2 n 2 , Where n1 is the number of carbon atoms; n 2 is 2 the number of hydrogen atoms. For example in C 6 H 12 , the degree of unsaturation is Lindlar’s Catalyst Li / NH H CC Degree of unsaturation : The number of degree of unsaturation in a D YG 4 3 R Lindlar U HCN Pd. / BaSO 4 / quinoline (Lindlarcatalyst) ID 3 Hg H cis 2 CH COOH H2 Li / NH 3 R C C R H (Cadet and Busen reaction) CHAsCl 3 R 2 (Westron) 2 2 CHCl 2 ClAsCl almost an exclusive product while catalytic reduction at alkyne affords mainly cis alkenes. CHCl Alc. KOH 2 E3 CH CH O || CH 3 C CH 3 (Acetone) H 2 SO 4 60 Red hot tube don't show this test. R (b) Bromine- carbon tetrachloride test : The compound is dissolved in carbon tetrachloride or chloroform and then a few drops of 5% bromine solution in carbon tetrachloride are added to it, the colour of bromine disappears. It indicates the presence of unsaturation. COOH | COOH Oxalic acid Oxidative–Hydroboration : Alkynes react with BH 3 (in THF) and finally converted into carbonyl compounds. This test also fails in the case of alkene of the R 3 2 3 CH 3 C CH (CH 3 CH CH )3 B 2 BH / THF Propyne H O OH Tautomeris es CH 3 CH CHOH CH 3 CH 2 CHO (Propanal) R CC R. R (5) Uses (i) Acetylene is used as an illuminant. (ii) It is used for the production of oxy-acetylene flame. The H 2 SO 4 CH 3 CH 2 OH CH 2 CH 2 o temperature of the flame is above 3000 C. Is is employed for cutting and welding of metals. (iii) Acetylene is used for artificial ripening of fruits. (iv) It is used as a general anaesthetic under the 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 produces finely divided carbon and hydrogen. Hydrogen is used in airships. CH 3 I Na CH CH CH CNa CH C CH 3 Acetylene O /H O Propyne PCl5 Alc. CH 3 CHO CH 3 CHCl 2 CH CH Acetaldehyde NaNH 1 Butyne KOH CCl 4 Ethene NaNH 2 CH 3 I CH 3 CH 2 C CH CH 3 CH 2 C CNa Alc. KOH CH 2 Br CH 2 Br CH CH H2 H2 CH CH o CH 2 CH 2 o CH 3 CH 3 Ni, 300 C Ni, 300 C Ethene 2 Pentyne Ethane (v) Ethene into propene : Ascending in alkene series Iodoethane Propane nitrile (Ethyl cyanide) Reduction D YG HNO 2 CH 3 CH 2 CH 2 NH 2 CH 3 CH 2 CH 2 OH 1Propanol 1- Aminopropa ne KOH The gaseous mixture is passed through ammonical cuprous chloride solution. The alkyne (acetylene) reacts with Cu 2Cl 2 and forms a red precipitate. It is filtered. The alkyne or acetylene is recovered by decomposition of the precipitate with an acid. C2 H 2 Cu 2Cl 2 2 NH 4 OH C2Cu 2 2 NH 4 Cl 2 H 2O 1 Bromopropa ne (Red ppt.) Li (CH ) Cu HI or CH 2 CH 2 CH 3 CH 2 I 32 CH 3 CH 2 CH 3 Ethene Iodoethane C2Cu 2 2 HNO3 C2 H 2 Cu 2 (NO3 )2 Propane Alc. 2 CH 3 CH 2CH 2Cl CH 3 CH CH 2 Cl hv KOH 1- Chloro propane Propene CH 3 I / Na HI or CH 2 CH 2 CH 3 CH 2 I CH 3 CH 2 CH 3 The remaining gaseous mixture is passed through concentrated H 2 SO 4. Alkene is absorbed. The Hydrogen sulphate derivatives is heated at 170 C to regenerate ethene. o U Propane C2 H 4 H 2 SO 4 C2 H 5 HSO 4 C2 H 4 H 2 SO 4 Alc. 2 CH 3 CH 2CH 2Cl CH 3 CH CH 2 Cl hv 2 Pentanone Separation of alkane, alkene and alkyne Alc. PBr3 CH 3 CH CH 2 CH 3 CH 2 CH 2 Br Propene HgSO 4 U HI KCN [H ] CH 2 CH 2 CH 3 CH 2 I CH 3 CH 2 CN Ethene ( Liq. NH 3 ) O || H 2O , H 2 SO 4 CH 3 CH 2 C CCH 3 CH 3 CH 2 CH 2 C CH 3 (iv) Ethyne into ethane : (Alkyne into alkane) Ethyne 1 - Butyne ID Ethyne or NaNH 2 2 Pentyne (x) 1-Butyne into 2-pentanone : (Not more than three steps) Br2 Br2 Alc. CH 3 CH 3 CH 3 CH 2 Br CH 2 CH 2 1, 2-Dibromoeth ane CH I E3 (iii) Ethane into ethyne (acetylene) : i.e., alkane into alkyne Ethane Acetylene CH 3 CH 2 C CCH 3 Ethane hv KOH 2 3 CH 3 CH 2 C CH CH 3 CH 2 C C Na H2 CH 2 CH 2 o CH 3 CH 3 Ni, 300 C Ethylidene chloride (ix) 1-Butyne into 2-pentyne : (Ascent) Ethene (ii) Ethene into ethane : (Alkene into alkane) Ethene Propylene 60 KOH Propyne Lindlar's catalyst CH 3 C CH CH 3 CH CH 2 3 2 Br2 Alc. CH 3 CH 3 C 2 H 5 Br CH 2 CH 2 Ethyl bromide Monosodium acetylide (viii) Propyne into acetylene : (Descent) (6) Interconversion (i) Conversion of ethane into ethene : (Alkane into alkene) hv Ethene (vii) Acetylene into propyne (methyl acetylene) : (Ascent) C2 H 2 2C H 2 Ethane 170 C Ethanol o KOH 1- Chloro propane 170 o C Propene The methane or ethane is left behind unreacted. ST (vi) Propene into ethene : Descending an alkene series [H ] CH 3 CH CH 2 3 2 CH 3 CHO O /H O Propene Ethanal LiAlH4 Distinction between alkanes, Alkenes and Alkynes Property Molecular formula Nature Table : 24.3 Alkane (Ethane) CnH2n+2(C2H6) Saturated Alkene (Ethene) CnH2n(C2H4) Unsaturated Alkyne (Ethyne) CnH2n–2(C2H2) Unsaturated Single bond between carbon atoms. Each carbon atom is sp3-hybridized Double bond between two carbon atoms. Both carbon atoms are sp2-hybridized Triple bond between two carbon atoms both carbon atoms are sp-hybridized C–C C=C –CC– 1.34 Å 1.20 Å Bond length 1.54 Å Bond energy : 83 Kcal mol–1 Burning 146 Kcal mol–1 200 Kcal mol–1 Burns with nonluminous flame Burns with luminous flame Burns with smoky flame C2H6+7/2O2 2CO2+3H2O C2H4+3O2 2CO2+2H2O C2H2+5/2O2 2CO2+H2O Forms alkane Forms alkene and alkane Reaction with H2 – Ni CnH2n + H2 CnH2n+2 o Ni CnH2n + H2 CnH2n+2 o C2H4 + H2 C2H6 Ni CnH2n–2 + H2 CnH2n o 300 C 300 C Alkane 300 C – Alkene Addition Addition C2H4+H2SO4 C2H5HSO4 H 2O C2H2 CH3CH(HSO4)2 60 Reation with conc. H2SO4 and hydrolysis Alkane H 2O C2H5OH CH3CHO Alcohol Br2/CCl4 – Baeyer’s reagent (Alk. KMnO4) – Decolourises Dibromo derivative, Decolourises Tetrabromo derivative, C2H4 + Br2 C2H4Br2 C2H2Br4 E3 Decolourises Glycol is formed Aldehyde Decolourises Oxalic acid is formed Ammonical Cu2Cl2 ID CH 2 CH 2 OH || H 2O O | CH 2 CH 2 OH Ammonical silver nitrate – U – Cycloalkane – D YG – (1) Methods of preparation (i) From dihalogen compounds (Freund reaction): CH Cl 2 CH 2 CH heat +2Na U CH Cl 2 HC CH 2 1,3 Dichloropropane +2NaCl 2 2 Cyclopropane ST (ii) From alkenes : Zn Cu alloy CH 3 CH CH 2 CH 2 I 2 CH 3 CH CH 2 Propene CH 2 Methyl cyclopropane (iii) From Aromatic compounds + 3H2 CH COOH ||| 4 O | CH COOH Red precipitate CH CCu | | | Cu 2 Cl 2 2 NH 4 OH | | | CH CCu (Red) + 2NH4Cl + 2H2O White precipitate CH C Ag | | | 2 AgNO 3 2 NH 4 OH | | | CH C Ag + 2NH4Cl + 2H2O (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. (iv) Their density increase gradually with increase of molecular mass. (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 the tendency of forming addition compounds decreases with increase in size of ring cyclopropane > Cyclobutane > Cyclopentane. Relative ring opening of ring is explained by Baeyer strain theory. (i) Addition in spiro cycloalkane : If two cycloalkane fused with one another then addition take place in small ring + H2 o Ni, 200 C under pres sure Benzene Cyclohexane (2) Physical properties (i) First two members are gases, next three members are liquids and higher ones are solids. (ii) They are insoluble in water but soluble in alcohol and ether. Spiro compound Because small ring is more unstable than large ring Higher cycloalkanes do not give addition due to more stability. (ii) Free radical substitution with Cl 2 (1) Conjugated dienes : Double bonds are seperated by one single hv CH 2 CH 2 Cl 2 CH 2 CH Cl HCl CH 2 Cyclopropa ne bond. CH 2 Chlorocyclopropane Ex : CH 2 CH CH CH 2 (1, 3-butadiene) (2) Cumulative dienes : Double bonds are adjacent to each other. Ex : CH 2 C CH 2 Propadiene [allene] (iii) Addition reaction Br (CCl ) dark BrH C – CH – CH Br 2 2 HBr H C – CH CH – CH – CH Br 3 2 2 The general formula is Cn H 2n2. The predominant member of this class is 1, 3-butadiene. (1) Method of preparation (i) From acetylene : (i) Conc. H SO 2 CH – CH – CH OH 4 3 (ii) H O 2 H , Ni 80 C 3 2 o NH 4 Cl 3 Propane (iv) Oxidation CH 2 2 + 5[O] HC CH 2 Alk 2 CH CH COOH KMnO4 2 1,4 -Dichlorobu tane 2 2 2 Cyclohexane Cycloalkene 6 2 D YG 4 600 C n -Butane Cycloalkenes can be easily obtained by Diels-Alder 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 KMnO4 or red colour of (Cr O used as catalyst.) 2 1, 3-Butadiene CH BrCHBrCH=CH 2 CH = CHCH = CH + Br 2 2 4 2 1, 3-Butadiene CH BrCH=CH.CH Br 2 Br ST 1, 2-Dibromo cyclopentane (ii) Addition of halogen acids : 3 4 CH =CH–CH=CH +HBr (Cyclohexene) Dienes O 2 OH CH O 2 (1, 2-Addition) 3-Bromo-1-butene (Major yield at low temp.) CH –CH=CH–CH Br 3 Cyclopentane 1,2-diol CH 3 2 OH Cyclopent-1-ene +O 2 1,4-Dibromo-2-butene (1, 4-Addition) predominates (70%) in an ionising solvent (acetic acid) CH CHBrCH=CH KMnO (aq.) 2 O H2 O 2 3,4-Dibromo-1-butene (1, 2-Addition) predominates (62%) in non-ionising solvent (hexane) Cyclopentene +O + H O Ethene (2) Physical property : 1,3-butadiene is a gas. (3) Chemical properties (i) Addition of halogens : CCl U Br 3 CH 2 CH CH CH 2 CH 2 CH 2 bromine in carbon tetrachloride. 2 1, 3-Butadiene (v) From cyclohexene : 3 1, 4-Cyclohexadiene + Br 1, 3 -Butadiene 1 5 Cyclohexene heat 1, 4 -Butanediol (iv) From butane : Catalyst CH 3 CH 2 CH 2 CH 3 CH 2 CH CH CH 2 o U Carbocyclic compounds with double bonds in the ring are called cycloalkenes. Some of the common cycloalkenes are Cyclopentene 1, 3 -Butadiene (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 Adipic acid Cyclobutene 1, 3 -Butadiene (ii) From 1, 4-dichlorobutane : Cl Cl | | Alc. KOH CH 2 CH 2 CH 2 CH 2 CH 2 CH CH CH 2 CH CH COOH 2 Pd / BaSO 4 Vinyl acetylene CH 2 CH CH CH 2 2 HC CH Cu 2 Cl 2 H2 2 HC CH HC C CH CH 2 CH – CH – CH 2 CH 2 1-Propanol 2 E3 2 Cyclopropane 60 1-Bromopropane 2 CH (3) Isolated or Non-conjugated : Double bonds are separated by more than one single bond. Ex : CH 2 CH CH 2 CH CH 2 (1, 4 pentadiene) 2 ID 2 2 1, 3-Dibromopropane 4 (iii) Addition of water : 2 CHO 2 2 (1, 4-Addition) 1-Bromo-2-butene (Major yield at high temp.) CH CHOHCH=CH 3 2 But-3-en-2-ol CH CHO CH =CH–CH=CH +H O 2 2 2 2 CH CH CH=CHCH OH 2 These are hydrocarbon with two carbon-carbon double bonds. Dienes are of three types 3 (iv) Polymerisation : 2 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 : CH H–C 2 CH Coal tar is a mixture of large numbers of arenes. 2 CH HC 2 Ethene (Dienophile) 2 CH HC 200o C + H–C CH 2 CH 2 2 Cyclohexene Stability of conjugated dienes : It is explained on(Adduct) the basis of 1, 3-Butadiene 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. C Name of the fraction Light oil (or crude oil) fraction Middle oil fraction (Carbolic oil) Heavy oil fraction (Creosote oil) Green oil (Anthracene oil) Pitch (left as residue) C C C (3) Distillation of coal tar : Arenes are isolated by fractional distillation of coal tar, Table : 24.4 Temperature range (K) Upto 443 Main constituents Benzene, toluene, xylene 60 CH 2 443-503 Phenol, naphthalene, pyridine 503-543 Naphthalene, naphthol and cresol Anthracene, phenanthrene 543-633 E3 CH 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 Lightoil Basic impurities removed C– C ID C C Phenols removed Benzene (255 - 257 K ) [Acidic impurities] Toluene (383 K ) - Delocalised electron’s General characteristics of arenes U (v) Ozonolysis : Zn / H 2O CH 2 CHCH CH 2 2O3 2 HCHO OHCCHO O D YG 3 HO 2 CH –O–CH – CH–O–CH 2 O O [Like pyridene] distillation O 2 O (Diozonide) Aromatic Hydrocarbon ST (Solid residue nearly 70%). It is used as a fuel and as reducing agent in metallurgy. ; 4n 4 ; n Heated to 1273-1373 K (Destructive distillation) Naphthalene 10 electrons HOT VAPOURS AND GASES Cooled and passed through water Anthracene 14 electrons n=2 = 3 also aromatic Similarly cyclolpentadienyl anion or tropylium ionn are because of containing 6 electrons (n=1). H H H H Condensed liquid Allowed to settle, two layers are separated Upper layer AMMONICAL LIQUOR (nearly 8-10%) It is used for the preparation of (NH4)2SO4 to be used as a fertilizer. 4 1 4 Benzene 6 electrons n=1 COAL COKE 4n 2 6 Example : U (1) Source of Arenes Source of arenes is coal. It contains benzene, xylene, naphthalene etc. Arenes are obtained by destructive distillation of coal. (2) Distillation of coal (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. 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 H.. H H H H H H H Cyclopentadienyl anion 6 Tropyllium ion 6 electrons Cyclopropenyl cation (n=61) electrons (n = 1). (n = 0) electrons (n=1) compounds also have Hetrocyclic.. N H Pyrrole.. O Furan.. S Thiophene N Pyridine 3CH CH Benzene gives cyclohexane by reduction with hydrogen. Molecules do not satisfy huckel rule are not aromatic. Ni C6 H 6 3 H 2 O Cyclohexane (b) Objections against Kekule's formula Cyclooctatetraene 8 electrons According to Kekule, two ortho disubstituted products are possible. But in practice only one ortho disubstituted product is known. 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. Cyclopropenyl anion 4 electrons –1 shows two Cycloheptatrienyl anion 8 electrons Benzene (C6H6) 4 1 4 COONa CaO heat + NaOH contributing + Na CO 2 (ii) From benzene derivatives (a) From phenol : OH Cyclopentadienyl 4 electrons Cycloctatetraene 8 electrons ST U 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. (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. 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. 3 Benzene Sodium benzoate D YG Cyclopropenyl anion 4 electrons equivalent ; n (2) Methods of preparation of benzene (i) Laboratory method : U 4n 4 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. ID (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 by about 83.6 KJ mol. Thus, cyclobutanediene structures and it has n 1. 60 Unusual stability of benzene. H.. E3 H Cyclopentadienyl cation 4 electrons Cyclopentadiene 4 electrons distill + Zn ZnO + Benzene Phenol (b) From chlorobenzene : Cl + 2H Ni-Al alloy NaOH + HCl Benzene Chlorobenzene (c) By first preparing grignard reagent of chlorobenzene and then hydrolysed OH Cl Mg H 2O C 6 H 5 Cl C 6 H 5 MgCl C 6 H 6 Mg Chlorobenz ene dry ether Phenyl magnesium chloride Benzene (d) From benzene sulphonic acid : SO H 3 + HOH Steam 150°-200°C +H SO 2 4 HCl,pressure Benzene sulphonic acid Benzene (e) From benzene diazonium chloride : N Cl 2 + 2H SnCl2 NaOH +N Benzene 2 +HCl (f) From acetylene : HC HC CH + CH HC + red hot tube 1500-2000°C Benzene Addition of ozone : HC HC Cr2O3 / Al2O3 (g) Aromatisation : C6 H 14 C6 H 6 4 H 2 500 C at high pressure +3O Benzene 3 CH C CH C CH O C H Zn O CHO + 3H O 3 Benzene triozonide 2 2 CHO (b) Substitution reactions : Glyoxal Nucleophilic substitution : Unimolecular : Mostly uncommon in aromatic substitution, there is only one example which obtain in benzene diazonium dichloride. Ar N 2 Arenediazonium cation (Slow ) N 2 Ar HOH ArOH (Fast) X Phenol – ArX Bimolecular : U D YG (a) Addition reactions 3H2O O O O Benzene (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. 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, O C ID (3) Properties of benzene (i) Physical properties (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. (b) It is highly inflammable and burns with sooty flame. (c) It is lighter than water. It's specific gravity at 20°C is 0.8788. (d) It is immiscible with water but miscible with organic solvents such as alcohol and ether. (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. (g) It is an extremely poisonous substance. Inhalation of vapours or absorption through skin has a toxic effect. CH O CH HC O E3 n Hexane O H C Three molecules of acetylene Cyclic polymerisation takes place in this reaction. 60 + Z Z Y + Y Z Aryl halide.. Y + Y charged carbon atom (slow) Y Z Y attaches to the positively Y Z Z Y (Fast) or +Z (Resonating structure of the hexadienyl anion) Example : (b) Substitution reactions OH Cl Cl OH U (c) Oxidation reactions (a) Addition reactions : In which benzene behaves like unsaturated hydrocarbon. ST Addition of hydrogen : Benzene reacts with hydrogen in the presence of nickel (or platinum) as catalyst at 150°C under pressure to form cyclohexane. + 3H Ni or CH 6 Cyclohexane NH 3 * NH * 3 + *NH NH *H 2 H + + Cl 2 CH H C H Cl H Cl C C C C C 2 (53%) Electrophilic substitution reaction : Benzene undergoes this reaction because it is an electron rich system due to delocalized electrons. hv 3Cl NH 3 (47%) CH Benzene * 12 H C C H * Cl H Addition of halogen : HC Phenol Elimination-addition mechanism (Benzyne mechanism) (Benzene) Benzene HC + Cl (Fast) (Slow) – HCl 150°C,pressure 2 OH+ Cl H Cl H + E H E ; (Slow) Carbonium ion ( - complex) H E E + H H H E E (Fast) + H – Nu Substitution product E Resonance forms of carbonium ion (Arenium ion) Electrophile (E) Table : 24.5 Name 60 Nu: Source Name of substitution reaction Cl Chloronium Cl 2 AlCl3 or FeCl3 Br Bromonium Br2 AlBr3 or FeBr3 NO 2 Nitronium HNO3 H 2 SO 4 SO 3 Sulphur trioxide Conc. H 2 SO 4 , Fuming sulphuric acid Sulphonation R Alkyl carbonium RX AlX3 (X Cl or Br), ROH H Friedel-Craft's (Alkylation) Acyl carbonium RCOCl AlCl3 Friedel-Craft's (Acylation) E3 Bromination Nitration ID R C O Chlorination When vapours of benzene and air are passed over vanadium pentoxide at 450 – 500°C, maleic anhydride is obtained. U 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: D YG heat (CH 3 )3 COOC (CH 3 )3 2(CH 3 )3 C O CHCO V2O5 C6 H 6 9[O] | | 450 500 C CHCO 2CH 3 2CH 3 COCH 3 X X + CH X X H. 3.. CH + + 3. H CH o(x = Cl, OCH3, NO2, CH3 etc.) CH H U X CH ST 3 X. 3 + + m O + H CH 3 The mechanism of chlorination of benzene CH at high temperature is similar to that of the free radical aliphatic substitution p 3.. Cl 2 Cl Cl (Chain initiation).. C6 H 6 Cl C6 H 5 HCl (H- abstraction).. C6 H 5 Cl 2 C6 H 5 Cl Cl (Chain propagation) (c) Oxidation H 6530 kJ/mole : Maleicanhydride Strong oxidising agents converts benzene slowly into CO 2 and water on heating. (d) Reduction : 3 m-Intermediate X O 2CO 2 2 H 2 O 2C6 H 6 15O2 12CO 2 6 H 2 O CH + +12HI 2 Benzene Cyclohexane 3 +6I 2 Methylcyclopentane (iii) Uses : (a) In dry cleaning (b) As a motor fuel 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 making dyes, drugs, plastics, insecticides, etc. Directive effect in substituted benzene derivatives (1) Directive effect in mono substituted benzene derivatives : The substituent already present on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) position. This direction depends on the nature of the first substituent and is called directive or the orientation effect. The substituent already present can increase or decrease the rate of further substitution, i.e., it either 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, (ii) Those which direct the incoming group to meta-position only (Neglecting ortho- and para-positions all together). (i) Those which direct the incoming group to ortho- and parapositions simultaneously (Neglecting meta all together). Meta directors Ortho-para directors........–.. Strongly activating NH 2 , NHR , NR 2 , –.OH. , –.O. : Moderately deactivating C N ,SO 3 H , COOH ,COOR, CHO, COR Strongly deactivating NO 2 , NR 3 ,CF3 ,CCl 3.... –. OCH , –.OR...... Moderately activating NHCOCH 3 , NHCOR , 3 60 Weakly activating CH 3 ,C2 H5 ,R,C6 H5........ Weakly deactivating –. F.: , –.Cl. : , –.Br. : , –. I. : , Theory of ortho – para directing group :S S S (2) Directive effect in disubstituted benzene (i) If the directive effects of two substituents reinforce, then a single product is formed. :S.. E3 S.. Example : CH ,i.e., CH 3.. Ortho attack Ortho E + S S E NO + 2 Ortho product Para product S The above mechanism is followed when OH, NH 2 ,Cl, Br,I,OR, NR 2 , NHCOR etc... CH CH D YG CH 3 is 3 3 3.. ST U In methyl or alkyl group, the +I effect.of. the methyl group or alkyl group initiates the resonance effect. Thus, methyl or alkyl group directs all electrophiles to ortho and para positions. 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, NO 2 group is a meta directing (Electron withdrawing). Its mechanism can be explained as : O O O N O O N O 2 (m) NO O NO 2 2 Thus, both (CH 3 , NO 2 ) direct further substitution to the same position (Ortho with respect to CH ). (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 3 NH 2 OH OCH 3 NHCOCH 3 C6 H 5 CH 3 meta directors OH OH OH Directs (Powerful activator) Directs Example : CH CH 3 Directs OH 3 CH 3 Directs OH Br Br 2 FeBr CH 3 CH 3 3 Too hindered position 3 2 +NO CH N NO (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. N O 3 Nitration U E Para attack CH ; ID S CH 3 O O N.. All meta-directing groups have either a partial positive charge or a full positive charge on the atom directly attached to the ring. Cl Toluene, methyl benzene or phenyl methane 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] : CH 3 Benzene Alkyl halide employed may undergo anToluene isomeric change CH 3 AlCl3 C6 H 6 ClCH 2 CH 2 CH 3 C6 H 5 CH HCl CH 3 n Propyl chloride Isopropyl benzene (65 70%) (Cumene) Catalysts can be used in place of anhydrous AlCl3 are, AlCl3 SbCl 3 SnCl 4 BF3 ZnCl 2 HgCl2 (ii) Wurtz fitting reaction : Br+2Na+BrCH Ether Toluene (o-,m- or p- ) Sodium toluate CH 3 o-Cresol Toluene CH 3 3 Boil +HOH +H SO 2 + 3 CH 3 NaNO 2 HCl 4 2 o-Derivative CH 2 3 Cl UV Cl UV 2 2 Toluene Benzyl chloride CHCl CCl 2 3 3 Toluene 2 Cl UV 2 p-Toluene diazonium chloride (vii) From grignard reagent : ST MgBr CH Benzal chloride 3 +MgBr +CH Br 2 C6 H 5 CH 2 Cl NaOH C6 H 5 CH 2 OH NaCl Toluene (viii) Commercial preparation From coal tar : The main source of commercial production of toluene is the light oil fraction of coal-tar. The light oil fraction is washed with conc. H 2 SO 4 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 110C is 90% benzol which contains 70 80% benzene and 14 24% toluene. 90% benzol is again distilled and the portion distilling between 108 110C is collected as toluene. (ix) From n- heptane and methyl cyclohexane CH (Benzyl chloride) Benzal chloride on hydrolysis forms benzaldehyde. C6 H 5 CHCl 2 2 NaOH C6 H 5 CH (OH )2 2 NaCl (Benzal chloride) C6 H 5 CHO H 2 O Benzo trichloride on hydrolysis forms benzoic acid. C6 H 5 CCl 3 (Benzotri chloride) 3 NaOH C6 H 5 C(OH )3 3 NaCl C6 H 5 COOH H 2 O (b) Oxidation : 3 | CH With hot acidic KMnO : 4 2 CH HC 2 CH 3 CH HC 2 2 CH 3 2 3 KMnO / H + 2 + HO 4 3 3[O] 500-550°C 150 atms Toluene n-Heptane COOH 3 Cr O / Al O 2 CH Benzo trichloride Benzyl chloride on hydrolysis with aqueous caustic soda forms benzyl alcohol. 3 Phenyl magnesium bromide E p-Derivative (ii) Reactions of side chain (a) Side chain halogenation : CH Cl +N +CH CHO+HCl N Cl p-Toluidine 3 5 U 2 CH 3 C H OH 2 NH CH + 3 E + p-Toluene sulphonic acid (vi) From toluidine : E CH 3 Electrophile Toluene SO H CH 3 E may be Cl, NO 2 , SO 3 H etc. D YG CH + ZnO U heat (v) From toluene sulphonic acid : (i) Electrophilic substitution reactions : Aromatic character (More reactive than benzene) due to electron releasing nature of methyl group. CH 3 OH +Zn Benzene ring (Aromatic) ID (iii) Decarboxylation : CH 3 Soda lime C6 H 4 NaOH C6 H 5 CH 3 Na 2 CO 3 COONa Toluene (iv) From CHcresol : 3 Side chain (Aliphatic) 3 Methyl bromide Bromobenzene (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. CH CH +2NaBr 3 odour. 60 +CH Cl (2) Physical properties (i) It is a colourless mobile liquid having characteristic aromatic + HCl 3 E3 AlCl 3 Toluene 2 Benzoic acid TNT is also used as a mixture of aluminium nitrate, alumina and charcoal under the name ammonal. T.N.B. (Tri-nitro benzene) Preparation : With acidic manganese or chromyl chloride (Etards reaction) : CH CH CH 3 CHO 3 3 ON NO 2 + 2[O] 2 +H O 2 2 2 3 Benzaldehyde Na 2 Cr2 O7 form benzoic acid. The length of the side chain does not 2 NO 2 7 4 NO COOH ON 2 2 60 Toluene NO ON 2 2 2 Soda matter. lime (c) Hydrogenation : R NO R 3 2 5 Birch reduction Alkyl benzene Alkyl cyclohexane (d) Combustion : C6 H 5 CH 3 9 O2 7 CO 2 4 H 2 O O C H CH C CH O O Zn O HOH O C o-Xylene Triozonide Toluene CH –C=O CHO 3 + 3H O D YG +2 H – C=O 2 2 CHO U Methyl glyoxal Glyoxal (4) Uses (i) In the manufacture of benzyl chloride, benzal chloride, benzyl alcohol, benzaldehyde, benzoic acid, saccharin, etc. (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) CH CH m-Xylene CH 3 2 5 3 CH Ethyl benzene 3 when aromatisation of C 6 C 8 fraction of petroleum naphtha is done. The xylenes are isolated from the resulting mixture (BTX) by fractional distillation. These can be prepared by Wurtz – Fittig reaction. A mixture of bromotoluene and methylbromide is treated with sodium in dry ethereal solution to form the desired xylene. CH CH 3 Br o-Bromotoluene CH 3 CH + 2Na + BrCH 3 + 2NaBr 3 o-Xylene CH 3 3 3 Preparation : + 2Na + BrCH H SO + 3HNO ST CH 3 p-Xylene These are produced along with benzene, toluene and ethylbenzene O O ID CH C CH 3 U 3 3 CH CH O CH CH O +3O HC Xylenes (Dimethyl benzene) C6H4(CH3)2 The molecular formula, C 8 H 10 represents four isomers. (e) Ozonolysis : H C 2 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. Na / liquid NH – C H OH 2 NO 2 E3 + 3H 2 CH m-Bromotoluene CH ON CH 3 m-Xylene CH 3 3 3 NO 2 + 2NaBr 3 Br 4 3 Fuming Toluene + 2Na + BrCH 2 + 2NaBr 3 + 3H O 2 Properties : It is pale yellow crystalline solid NO (M.P. = 81°C). Uses : It is used as an explosive in shells, bombs and torpedoes 2 under the name trotyl. When mixed with 80% ammonium nitrate it forms the explosive amatol. H SO 4 Toluene All alkyl benzenes on oxidation with hot acidic KMnO4 or HC K Cr O HNO 2 CrO C 2 H SO Br CH 3 p-Xylene p-Bromotoluene These can also be obtained by Friedel – craft's synthesis, m-Xylene can be obtained from mesitylene. Xylenes are colourless liquids having characteristic odour. The boiling points of three isomers are, o-Xylene=144°C; m-Xylene=139°C; p-Xylene=138°C. Xylenes undergo electrophilic substitution reactions in the same manner as toluene. Upon oxidation with KMnO4 or K 2 Cr2 O7 , Xylenes form corresponding dicarboxylic acids. COOH COOH 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. COOH CH = CH2 CH CH3 CH CH3 2 , COOH H / Ni H / Ni 20°C, 3 atm 125°C, 110 atm 2 COOH Phthalic acid 2 Isophthalic acid Styrene Ethyl benzene Ethyl cyclohexane With bromine, it gives the dibromide. Ethyl benzene (C6H5C2H5) CH = CH It can be prepared by the following reactions, (1) By Wurtz-Fittig reaction : 60 Xylenes are used in the manufacture of lacquers and asCOOH solvent for Terephthalic acid rubber. o-Xylene is used for the manufacture of phthalic anhydride. CHBr.CH Br 2 2 + Br C 6 H 5 Br 2 Na BrC2 H 5 C 6 H 5 C 2 H 5 2 NaBr Styrene dibromide E3 Styrene 2 (2) By Friedel-craft's reaction : Halogen acids add to the side chain. AlCl3 C 6 H 5 H BrC2 H 5 C 6 H 5 C 2 H 5 HBr C 6 H 5 CH CH 2 HX C 6 H 5 CHXCH 3 Preparation of ring substituted styrenes is not done by direct halogenation but through indirect route. (3) By catalytic reduction of styrene : CH CH3 CH CH3 C 6 H 5 CH CH 2 H 2 C 6 H 5 CH 2 CH 3 ID 2 (4) By alkyl benzene synthesis : + AlCl3 , HCl C 6 H 5 H H 2 C CH 2 C 6 H 5 CH 2 CH 3 95 C ,Pressure Cl D YG Styrene (C6H5CH=CH2) It is present in storax balsam and coal-tar in traces. (1) Preparation (i) Dehydrogenation of side chain of ethylbenzene : CH CH 2 + CH = CH 2 AlCl 2 CH = CH 3 Cr O / Al O 2 3 2 3 Ethylbenzene Styrene U (ii) Decarboxylation of cinnamic acid : This is the laboratory preparation and involves heating of cinnamic acid with a small amount of quinol. ST Quinol C 6 H 5 CH CHCOOH C 6 H 5 CH CH 2 CO 2 (iii) Dehydration of 1-phenyl ethanol with H SO : 2 4 H 2 SO 4 C 6 H 5 CHOHCH 3 C 6 H 5 CH CH 2 H 2O (iv) Dehydration of 2-phenyl ethanol with ZnCl 2 , heat C 6 H 5 CH 2 CH 2 OH C 6 H 5 CH CH 2 ZnCl 2 : H 2O (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 Cl2 hv Cl 3 CH = CH 2 Alc.. KOH Heat Cl is completely When oxidised underCldrastic conditions, the side chain oxidised to a carboxyl group. CH = CH2 COOH [O] KMnO 2 600°C 3 FeCl3 CHClCH U C 6 H 5 C 2 H 5 C 6 H 5 COOH [O ] 2 2 It undergoes electrophilic substitution reactions in the same way as toluene. When oxidised with dil. HNO 3 or alkaline KMnO4 or chromic acid it forms benzoic acid. Benzene 2 , 4 Benzoic acidfree radical Styrene In presence of peroxides, styrene undergoes 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 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). Bi-phenyl (C6H5 – C6H5) 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. (1) Methods of formation (i) Fittig reaction : It consists heating of an ethereal solution of bromobenzene with metallic sodium. Heat (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 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. common functional groups such as C=O, NO and C N remain unaffected, 2 The order of reactivity of primary (1 ), secondary (2 ) and tertiary o + 2CuI I+ 2Cu + I o (3 ) hydrogens in alkanes follows the sequence : 3 > 2 > 1. o o o o (iii) Grignard reaction : Phenyl magnesium bromide reacts with bromo benzene in presence of CoCl 2. 2 + MgBr 2 ID (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. 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. HNO / H SO 3 2 60 CoCl Br + E3 MgBr NO 4 HNO / H SO 3 2 U 2 ON 4 NO 2 D YG 2 Octane number may be less than zero (e.g., n-Nonane has an octane number-45) and higher than 100 (e.g., Triptane or 2, 3, 3Trimethylbutane has an octane number of 124). To avoid lead pollution, a new compound cyclopentadienyl manganese carbonyl U CO CO CO (called as AK-33-X) is used as antiknock now a days in developed countries – Mn ST (unleaded pertol). Acetylene has a garlic odour when impure due to impurities of phosphine and hydrogen sulphide. 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 : H O > ROH> HC CH >CH > CH = CH > CH –CH. 2 3 2 2 3 3 Obviously, the basic character of their conjugate bases follows the reverse order, i.e., CH CH >CH = CH > NH > HC C >RO > HO. – 3 2 – 2 – – – – 2 Wilkinson’s catalyst : (Triphenylphosphine) rhodium, (PPh ) RhCl is 3 3 called wilkinson’s catalyst. It reduces alkenes and alkynes while other
