Hydrocarbons - Unit 13 - PDF

UNIT 13 HYDROCARBONS 1 CLASSIFICATION ⚫Depending upon the types of carbon-carbon bonds present, they can be classified into three main categories ⚫Saturated ⚫Unsaturated and ⚫Aromatic hydrocarbons. 2 ALKANES ⚫Alkanes ar...

UNIT 13 HYDROCARBONS 1 CLASSIFICATION ⚫Depending upon the types of carbon-carbon bonds present, they can be classified into three main categories ⚫Saturated ⚫Unsaturated and ⚫Aromatic hydrocarbons. 2 ALKANES ⚫Alkanes are saturated open chain hydrocarbons containing carbon - carbon single bonds. ⚫The general formula for alkanes is CnH2n+2, where n stands for number of carbon atoms and 2n+2 for number of hydrogen atoms in the molecule. ⚫All H-C-H bond angles are of 109.5°. 3 Types of carbon atoms ⚫Carbon atom attached to no other carbon atom as in methane or to only one carbon atom as in ethane is called primary carbon atom. ⚫Carbon atom attached to two carbon atoms is known as secondary. ⚫Tertiary carbon is attached to three carbon atoms and neo or quaternary carbon is attached to four carbon atoms. 4 Nomenclature 5 6  CH3 CH3 CH3 C C CH CH CH2 CH2 CH3 7  Preparation ⚫From unsaturated hydrocarbons ⚫Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes. This process is called hydrogenation. ⚫CH2═CH2+H2—Pt/Pd/Ni→CH3.CH3 ⚫CH≡C-CH3+ 2H2—Pt/Pd/Ni→CH3.CH2.CH3 8 From alkyl halides ⚫Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes. ⚫CH3Cl+H2—Zn/H+→ CH4 +HCl 9 Wurtz reaction ⚫Alkyl halides on treatment with sodium metal in dry ether give higher alkanes. This reaction is known as Wurtz reaction. ⚫CH3Cl+Na+Na+Cl.CH3→ CH3.CH3 +2 NaCl 10  From carboxylic acids ⚫Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as decarboxylation. ⚫CH3COONa+NaOH—CaO→ CH4 +Na2CO3 11 Kolbe’s electrolytic method ⚫An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane. 12 13  Physical properties ⚫Alkanes are almost non-polar molecules because of the covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms. ⚫The first four members, C1 to C4 are gases, C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odourless. 14 Chemical properties ⚫Substitution reactions ⚫One or more hydrogen atoms of alkanes can be substituted by halogens, nitro group and sulphonic acid group. ⚫Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light. 15 ⚫CH4 +Cl2 ⎯hν ⎯→ CH3Cl + HCl ⚫CH3Cl + Cl2 ⎯hν ⎯→ CH2Cl2 + HCl ⚫CH2Cl2 + Cl2 ⎯hν ⎯→ CHCl3 + HCl ⚫CHCl3 + Cl2 ⎯hν ⎯→ CCl4 + HCl 16 Mechanism ⚫Step:1 Initiation ⚫The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat... ⚫Cl-Cl ⎯hν ⎯→ Cl + Cl 17 ⚫Step2: Propagation ⚫Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl. 18 19 ⚫Step3: Termination ⚫The reaction stops after some time due to consumption of reactants and / or due to the following side reactions 20 21 Combustion ⚫Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat and hence alkanes are used as fuels.. CH4+2O2→ CO2+2H2O;ΔCH0=-840kJ/mol C4H10+13/2O2→ 4CO2+5H2O;ΔCH0=-2876kJ/mol 22 Controlled oxidation ⚫Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products. 2CH4+O2--Cu/523K/100atm → 2CH3OH CH4+O2-- Mo2O3 → H.CHO+ H2O 2CH3.CH3+3O2--(CH3COO)2Mn→2CH3.COOH+ H2O 23 Isomerisation ⚫n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes. CH3.CH2.CH2.CH2.CH2.CH3 Anhy.AlCl3/HCl CH3.CH.CH2.CH2.CH3+CH3.CH2.CH.CH2.CH3 CH3 CH3 2-Methylpentane 3-Methylpentane 24 Aromatization ⚫n-Alkanes having six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as aromatization or reforming. 25 ⚫ CH3 ⚫ CH2 CH3 Cr2O3/V2O5/Mo2O3 773K/10-20 atm ⚫ CH2 CH2 ⚫ CH2 26 Reaction with steam ⚫Methane reacts with steam at 1273 K in the presence of nickel catalyst to form carbon monoxide and dihydrogen. This method is used for industrial preparation of dihydrogen gas. ⚫CH4+H2O– Ni/Δ → CO+3H2 27 Pyrolysis ⚫Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking. ⚫ C6H12+H2 ⚫C6H14 C 4H8+C2H6 ⚫ C3H6+C2H4+CH4 28 Conformations The rotation of C-C σ bond results in different spacial arrangement in alkanes. These are called conformations, conformers or rotamers. Conformations of ethane ⚫There are infinite number of conformations of ethane. However, there are two extreme cases. ⚫ eclipsed conformation and ⚫staggered conformations. 29 ⚫Conformation in which hydrogen atoms attached to two carbons are as close as possible is called eclipsed conformation. ⚫Conformation in which hydrogen atoms are as far apart as possible is known as the staggered conformation. ⚫Any other intermediate conformation is called a skew conformation. 30 Representations 31 Representation ⚫Eclipsed and the staggered conformations can be represented by Sawhorse and Newman projections. ⚫Sawhorse projections ⚫In this projection, the molecule is viewed along the molecular axis. 32 Sawhorse projections 33 Newman projections ⚫In this projection, the molecule is viewed at the C-C bond head on. Staggered Eclipsed 34 Relative stability of conformations ⚫The repulsive interaction between the electron clouds, which affects stability of a conformation, is called torsional strain. ⚫The staggered form has the least torsional strain and the eclipsed form, the maximum torsional strain. So the staggered form is more stable than the eclipsed form. 35 ALKENE S 36  Alkenes ⚫Alkenes are unsaturated hydrocarbons containing at least one double bond. The general formula for alkenes is CnH2n ⚫Nomenclature ⚫The suffix ene is given in alkenes. 37  Isomerism ⚫Alkenes show both structural isomerism and geometrical isomerism. ⚫Structural isomerism ⚫Alkenes show position isomerism and chain isomerism. 38 Geometrical isomerism ⚫ Isomer in which two identical atoms or groups lie on the same side of the double bond is called cis isomer. ⚫Isomer in which identical atoms or groups lie on the opposite sides of the double bond is called trans isomer. 39 Cis Trans 40  Preparation ⚫From alkynes ⚫Alkynes on partial reduction with calculated amount of dihydrogen in the presence of Lindlar’s catalyst (palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline) give alkenes. 41 CR≡CR + H2 –Pd/C→ CHR═CHR Cis isomer CR≡CR + H2 –Na/liq.NH3→ CHR═CHR Trans isomer CH≡CH + H2 → CH2═CH2 42 From alkyl halides ⚫Alkyl halides (R-X) on heating with alcoholic potash eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation i.e., removal of halogen acid. This is example of β-elimination reaction 43 44 From vicinal dihalides ⚫Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal dihalides. Vicinal dihalides on treatment with zinc metal lose a molecule of ZnX2 to form an alkene. This reaction is known as dehalogenation. ⚫CH2.Br- CH2.Br + Zn → CH2═CH2 +ZnBr2 45 From alcohols by acidic dehydration ⚫Alcohols undergo dehydration to form alkenes on treating with a protic acid e.g., concentrated H2SO4 → 46 Physical properties ⚫The first three members are gases, the next fourteen are liquids and the higher ones are solids. 47 Chemical properties ⚫Alkenes show addition reactions in which the electrophiles add on to the carbon-carbon double bond to form the addition products. ⚫Addition of dihydrogen ⚫Alkenes add up one molecule of dihydrogen gas in the presence of finely divided nickel, palladium or platinum to form alkanes 48 Addition of halogens ⚫Halogens like bromine or chlorine add up to alkene to form vicinal dihalides. ⚫CH2 = CH2 + Br2 → CH2Br- CH2Br 1,2-dibromoethane 49 Addition of hydrogen halides ⚫Hydrogen halides (HCl, HBr,HI) add up to alkenes to form alkyl halides. ⚫CH2 = CH2 + HCl → CH3- CH2Cl 50 Addition reaction of HBr to unsymmetrical alkenes (Markovnikov Rule) Addition of HBr to unsymmetrical alkenes may produce two products. 51 ⚫CH3.CH═CH2+H.Br ⚫ CH3.CH2.CH2.Br ⚫ 1-bromopropane (minor) ⚫ CH3.CHBr.CH3 ⚫ 2-bromopropane (major) Markovnikov rule ⚫The rule states that negative part of the addenum gets attached to that carbon atom which possesses lesser number of hydrogen atoms. 53 Mechanism ⚫Hydrogen bromide provides an electrophile, H+, which attacks the double bond to form carbocation as shown below. ⚫CH3.CH═CH2+H+ ⚫→ CH3.CH+.CH3 +CH3.CH2.CH2+ ⚫ 20 10 ⚫The secondary carbocation is more stable than the primary carbocation 54 ⚫The secondary carbocation is attacked by Br - ion to form the product. ⚫CH3.CH+.CH3 +Br-→ CH3.CHBr.CH3 55 Anti Markovnikov addition or peroxide effect or Kharash effect ⚫In the presence of peroxide, addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. ⚫CH3.CH═CH2+H.Br (C6H5CO)2O2 CH3.CH2.CH2.Br ⚫ 1-bromopropane (major) 56 Mechanism ⚫Peroxide effect proceeds via free radical chain mechanism. 57 ⚫The secondary free radical obtained in the above mechanism (step iii) is more stable than the primary. This explains the formation of 1- bromopropane as the major product. 58 Addition of sulphuric acid ⚫Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction. CH3.CH═CH2 + → CH3.CH.CH3 OSO2.OH 59 Addition of water ⚫Alkenes react with acidified water to form alcohols, in accordance with the Markovnikov rule. ⚫ Major Product ⚫ Minor Product 60  Oxidation ⚫Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate produce vicinal glycols. Decolorisation of KMnO4 solution is used as a test for unsaturation. 61 ⚫Acidic potassium permanganate or acidic potassium dichromate oxidises alkenes to ketones and/or acids depending upon the nature of the alkene and the experimental conditions. CH3.CH═CH.CH3 --KMnO4/H+→2CH3.COOH https://www.youtube.com/watch? v=JOKYpBpVAwA 62  Ozonolysis ⚫Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by Zn-H2O to smaller molecules. 63  Polymerisation ⚫Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers. This reaction is known as polymerisation. 64 Examples 65 ALKYNE S 66  Alkynes ⚫They contain at least one triple bond between two carbon atoms. Their general formula is CnH2n-2 ⚫They are named by the suffix yne. ⚫Position isomerism is exhibited by alkynes 67  Preparation ⚫From calcium carbide ⚫On industrial scale, ethyne is prepared by treating calcium carbide with water. ⚫CaO+3C→CaC2+CO ⚫CaC2+2H2O→Ca(OH)2+C2H2 68  From vicinal dihalides ⚫Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halide is eliminated to form alkenyl halide which on treatment with sodamide gives alkyne. 69  Physical properties ⚫First three members are gases, the next eight are liquids and the higher ones are solids. All alkynes are colourless. 70  Chemical properties ⚫Acidic character of alkyne ⚫Sodium metal and sodamide (NaNH2) are strong bases. They react with ethyne to form sodium acetylide with the liberation of dihydrogen gas. ⚫CH≡CH + Na→ CH≡C-Na+ +½ H2 71 Addition reactions ⚫Addition of dihydrogen 72 Addition of halogens 73 Addition of hydrogen halides Major Minor 74  Addition of water ⚫One molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphuric acid at 333 K to form carbonyl compounds. ⚫CH≡CH + H-OH→ CH2═C-H isomerism ⚫ OH ⚫CH3-C-H ║ ⚫ O 75 Polymerisation ⚫Linear polymerisation ⚫Under suitable conditions, linear polymerisation of ethyne takes place to produce polyacetylene or polyethyne. ⚫CH≡CH + CH≡CH → ( CH═CH- CH═CH)n 76 Cyclic polymerisation ⚫Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization. Three molecules polymerise to form benzene. ⚫ CH ⚫ CH CH ⚫ CH CH ⚫ CH 77  AROMATIC HYDROCARBON 78  AROMATIC HYDROCARBON ⚫These hydrocarbons are also known as arenes. Since most of them possess pleasant odour (Greek; aroma meaning pleasant smelling), the class of compounds was named as aromatic compounds. ⚫Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non- benzenoids. 79 Structure of Benzene ⚫Kekule suggested the concept of oscillating nature of double bonds in benzene 80 Resonance and stability of benzene ⚫All the six carbon atoms in benzene are sp2 hybridized. Two sp2 hybrid orbitals of each carbon atom overlap with sp2 hybrid orbitals of adjacent carbon atoms to form six C-C sigma bonds which are in the hexagonal plane. The remaining sp2 hybrid orbital of each carbon atom overlaps with s orbital of a hydrogen atom to form six C-H sigma bonds. Each carbon atom is now left with one unhybridised p orbital perpendicular to the plane of the ring 81 82 ⚫The unhybridised p orbital of carbon atoms are close enough to form a π bond by lateral overlap. There are two equal possibilities of forming three π bonds by overlap of p orbitals of C1-C2, C3-C4, C5-C6 or C2-C3, C4-C5, C6-C1 respectively 83 ⚫X-Ray diffraction data reveals that benzene is a planar molecule. Had any one of the above structures of benzene (A or B) been correct, two types of C-C bond lengths were expected. However, X-ray data indicates that all the six C-C bond lengths are of the same order (139 pm) which is intermediate between C-C single bond (154 pm) and C═C double bond (133 pm). Thus the absence of pure double bond in benzene accounts for the reluctance of benzene to show addition reactions under normal conditions, thus explaining the unusual behaviour of benzene. 84 85  Aromaticity A compound is aromatic if it satisfies Huckel Rule. There are 3 conditions. They are ⚫Planarity ⚫Complete delocalisation of the π electrons in the ring ⚫Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2,...). 86 87 Preparation of Benzene ⚫Benzene is commercially isolated from coal tar. However, it may be prepared in the laboratory by the following methods. ⚫Cyclic polymerisation of ethyne ⚫Decarboxylation of aromatic acids: ⚫Sodium salt of benzoic acid on heating with sodalime gives benzene. ⚫R.COONa + NaOH –CaO–→ R.H+Na2CO3 88 Reaction of phenol with zinc dust ⚫Phenol is converted to benzene on heating with zinc dust. ⚫ + Zn → + ZnO 89 Physical properties ⚫Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents. 90 Chemical properties ⚫Electrophilic substitution reactions ⚫The main electrophilic substitution reactions are nitration, halogenation, sulphonation, Friedel Craft’s alkylation and acylation reactions 91  Nitration ⚫A nitro group is introduced into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture). H2SO4 92  Sulphonation ⚫The replacement of a hydrogen atom by a sulphonic acid group in a ring is called sulphonation. It is carried out by heating benzene with fuming sulphuric acid (oleum). 93  Halogenation ⚫Arenes react with halogens in the presence of a Lewis acid like anhydrous FeCl3, FeBr3 or AlCl3 to yield haloarenes. 94 Friedel-Crafts alkylation reaction ⚫When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenzene is formed. 95 Friedel-Crafts acylation reaction ⚫The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids (AlCl3) yields acyl benzene. 96 97  Mechanism of electrophilic substitution reactions According to experimental evidences, SE (S = substitution; E = electrophilic) reactions are supposed to proceed via the following three steps: ⚫Generation of the electrophile ⚫Formation of carbocation intermediate ⚫Removal of proton from the carbocation intermediate 98 Generation of electrophile E ⊕ ⚫During chlorination, alkylation and acylation of benzene, anhydrous AlCl3, being a Lewis acid helps in generation of the elctrophile Cl⊕, R⊕, RC⊕O (acylium ion) respectively by combining with the attacking reagent. ⚫Cl-Cl+AlCl3→Cl++AlCl4- ⚫CH3-Cl+AlCl3→CH3++AlCl4- ⚫CH3-C-Cl+AlCl3→CH3-C++AlCl4- ⚫ O O 99 Formation of Carbocation (arenium ion) ⚫Attack of electrophile results in the formation of σ-complex or arenium ion in which one of the carbon is sp3 hybridised. ⚫The arenium ion gets stabilised by resonance 101  Removal of proton ⚫To restore the aromatic character, σ -complex releases proton from sp3 hybridised carbon on attack by [AlCl4]- (in case of halogenation, alkylation and acylation) and [HSO4]- (in case of nitration and sulphonation). 102  Addition reactions ⚫Under vigorous conditions, i.e., at high temperature and/ or pressure in the presence of nickel catalyst, hydrogenation of benzene gives cyclohexane. 103  Addition of Cl2 ⚫Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride, C6H6Cl6 which is also called gammaxane. 104  Combustion ⚫When heated in air, benzene burns with sooty flame producing CO2 and H2O. 105 Directive influence of a functional group in mono-substituted benzene ⚫When monosubstituted benzene is subjected to further substitution, either ortho and para products or meta product is predominantly formed. This behaviour depends on the nature of the substituent already present in the benzene ring. This is known as directive influence of substituents. 106 Ortho and para directing groups ⚫The groups which direct the incoming group to ortho and para positions are called ortho and para directing groups. The electron density is more on o- and p- positions. Hence, the substitution takes place mainly at these positions. 107 108 ⚫-OH group activates the benzene ring for the attack by an electrophile. Other examples of activating groups are -NH2, -NHR, - NHCOCH3, -OCH3, -CH3, -C2H5, etc. ⚫ In the case of aryl halides, halogens are moderately deactivating. Because of their strong -I effect, 109  Meta directing group ⚫The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing groups are -NO2, -CN, -CHO,-COR, -COOH, -COOR, -SO3H, etc. 110 111 ⚫Here the overall electron density on benzene ring decreases making further substitution difficult, therefore these groups are also called deactivating groups. The electron density on o- and p- position is comparatively less than that at meta position. Hence, the electrophile attacks on comparatively electron rich meta position resulting in meta substitution. 112 CARCINOGENICITY AND TOXICITY ⚫Benzene and polynuclear hydrocarbons containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property. Such polynuclear hydrocarbons are formed on incomplete combustion of organic materials like tobacco, coal and petroleum. They enter into human body and undergo various biochemical reactions and finally damage DNA and cause cancer. 113 THE 114