Hydrocarbon. Aromatic Hydrocarbons (Arenes)

Hydrocarbon AROMATIC HYDROCARBONS (Arenes) Special points about Arenes : • Arenes are cyclic, planar and follow Huckel's rule. • There should be cyclic resonance in Arenes. • General formula of Arenes are CnH2n–6y where n = Number of carbons (iii) Partial bond structure Thiels (iv) Prismitic structure Ladenberg y = Number of rings • Characteristic reaction of arenes is Electrophilic substitution reaction (ESR). (v) Parallel bond structure Dewar • Arenes are cyclic unsaturated compounds but do not give test of unsaturaion with Br2/CCl4 or alk. KMnO4. (vi) Alternate bond structure Kekule • Main source of Arenes is coaltar. General methods of preparation : • They have higher precentage of carbon so burn with (i) From alkanes (By cyclisation or Aromatisation) smoky flame. Hydroforming or catalytic reforming Special points about Benzene : [Ph–H or C6H6] • • • • • • • n – Hexane   Benzene + 4H2  Benzene was discovered by Faraday from whale (ii) From alkyne (By cyclic polymerisation) When oil. acetylene is passed through red hot metallic tube Benzene was prepared by Hoffman from coaltar. cyclic polymerisation takes place and benzene is Benzene has cyclic, planar and hexagonal structure. formed. All carbons are sp2 hybridised and bond angle is 120° 3CH CH  Number of bond angle of 120° = 3 × 6 = 18 Number of bonds = 12 and number of bonds = 3 (iii) From Haloarenes (By reduction) : C—C [bond length] = 1.39Å and C—H [bond length] = 1.09Å • Resonance energy = 36.6 Kcal. • Heat of hydrogenation = 49.8 Kcal.     Defferent structure of benzene : S.No. Name of structure     Structure Given by (iv) From Phenolic compounds (By deoxygenation) (i) Diagonal structure (ii) Centric structure Clause +Zn   + ZnO Baeyer and + Zn    +ZnO Armstrong 192 Hydrocarbon (v) From carbonyl compounds (By reduction) : (xi) From petroleum : Petroleum        Mixture of lower  CH2    CH2 + CH2 CH — CH CH2 hydrocarbons (vi) From carboxylic acids (By decarboxylation) : + CO2        + CO2    (vii) From sulphonic acids (By hydrolysis) : (xii) From coaltar :       + H2SO4       + H2SO4 (viii) By diazonium salts :       Physical properties : (ix) By Grignard reagent :       (i) Benzene is colourless liquid [B.P. is 80°C] (ii) Benzene is insoluble in H2O and density less than H 2O (iii) Benzene is used as a solvent and it is soluble in organic solvents. (iv) It is highly inflammable and burns with smoky flame. (x) By Wurtz fittig reaction : + CH3—Cl    + CH3—Cl    Chemical properties : Benzene show following types of reaction – (A) Addition reactions (B) Electrophilic substitution reactions (C) Oxidation reactions (D) Extra special reactions 193 Hydrocarbon (b) Partial hydrogenation : (A) Addition reaction : (i) Addition of X2         (iii) Addition of O3 : + 3O3  Formation of BHC from C6H6 is an example of free radical addition reaction.         Addition of 3 mole Cl2 or 3 mole H2 or 3 mole O3 on benzene show presence of 3 double bonds in benzene. Benzene does not give addition reaction with Br2/ CCl4 or alkaline KMnO4 (B) Electrophilic substitution reaction [ESR] : Characteristic reaction of arenes is ESR     + E—Nu  ? Mechanism :   Formation of (ii) Addition of H2 : (a) Complete hydrogenation : + 3H2 Attack of    ............   + 3H2      Abstraction of  + H—Nu 194 Hydrocarbon (i) Halogenation : Formation of  + Cl2  ? Mechanism : Formation of Attack of    ....... Abstraction of  + H—Cl + FeCl3 (iii) Sulphonation :    Mechanism :     Note: CH3 group in toluene is o/p directing and activating group.       (ii) Nitration :       + 195 Hydrocarbon (iv) Friedel crafts reaction [FCR] : Alkylation or acylation of arenes in presence of lewis acid [FeCl3, AlCl3 or ZnCl2...] is called as FCR. CS2 or nitrobezene is used as solvent in FCR. Intermediate carbocation is formed in FCR so rearrangement may be possible. (v) Chloromethylation : 196 Hydrocarbon (vi) Gatterman's Kosch Reaction : (C) Oxidation reactions : (i) Combustion : C6H6 + O2 6CO2 + 3H2O (ii) Catalytic Oxidation : (vii) Gatterman's Aldehyde synthesis : (iii) Side chain Oxidation : The important electrophiles used in the aromatic substitution are the following Electrophilic Source Atleast one benzylic –H containing alkyl benzene gives benzoic acid in presence of strong oxidising agent. Name of substitution reaction  Cl2 + AlCl3 (Chloronium)  or FeCl3 Br2 + AlBr3 (Bromonium)  Chlorination Bromination or FeBr3 HNO3 + H2SO4 Nitration (Nitronium) SO 3 conc. H2SO4, (Sulphertrioxide)  Sulphonation fuming sulphuric acid RX + AlX3 Friedel crafts (Alkyl (X= Cl or Br) (Alkylation) carbonium) ROH + R—  O ( Acyl carbonium)  ROCl + AlCl3   Friedel crafts [Etard reaction] (Acylation) 197 Hydrocarbon (iii) Formation of sandwitch or –complex compound : Ex.29 t–butyl benzene does not gave benzoic acid in presence of H /K2Cr2O7 Sol. t–butyl benzene does not cantain benzylic–H  Bis benzene chromium (D) Extra special reactions : (i) Pyrolysis :   (Biphenyl) (iv) Reduction with HI – (ii) Mercuration : 2 + 12HI  + Cyclohexane + 6I2 Methyl cyclo pentane 198