VJC 2024 Arenes Lesson Tutorial

23 5 Summary (a) Mechanism Similarities and differences between alkene and benzene Like alkenes, benzene possesses high π electron density and is hence attacked by electrophiles. However, benzene differs from alkenes in that: (1) delocalisation of π electrons renders the benzene ring less nucleophilic compared to the C=C bond in alkenes. Hence, benzene can only be attacked by strong electrophiles. (2) benzene undergoes substitution instead of addition to preserve its resonance stabilised ring system. Hence, typical reaction of benzene is electrophilic substitution. Step 1: Halogenation / Friedel-Crafts alkylation Catalyst acts as a Lewis acid by accepting a lone pair of electrons from Cl2 / RCl, thereby generating a strong electrophile, Cl+ / R+. Cl2 + FeCl3 → Cl+ + FeCl4– RCl + AlCl3 → R+ + AlCl4– Nitration H2SO4 catalyst acts as a Br ønsted-Lowry acid by donating a proton which leads to generation of the strong electrophile, NO2+. 2H2SO4 + HNO3 → NO2+ + H3O+ + 2HSO4– Step 2: + Cl+ + H Cl + R+ + H R + NO2 + + H NO2 Step 3: Cl + H Cl + + HCl + FeCl3FeCl4 - R + H R + + HCl + AlCl3AlCl4 - NO2 + H NO2 + + H2SO4HSO4 -

24 (b) Effect of substituents on benzene ring reactions (given in the Data Booklet) 2, 4–directing 3–directing activating deactivating deactivating –R (alkyl group) –Cl –CHO, –COR –OH or –OR (highly activating) –Br –CO2H, –CO2R –NH2, –NHR or –NR2 (highly activating) –I –NH3+ –NHCOR –NO2, –CN (c) Complete the following tables. • Reactions common to both benzene and methylbenzene Type of reaction Reagents and conditions Products for benzene methylbenzene Cl2, anhydrous FeCl3 catalyst Cl CH3CH2Br, anhydrous AlBr3 catalyst, excess benzene / methylbenzene (for monosubstitution) Concentrated HNO3, concentrated H2SO4 catalyst, 55 oC for benzene / 30 oC for methylbenzene • Reactions for alkylbenzene only (side-chain reactions) Type of reaction Reagents and conditions Reactant Products Br2, uv light or heat / high temperature (excess organic compound for monosubstitution) CH3

25 Type of reaction Reagents and conditions Reactant Products KMnO4(aq), H2SO4(aq), heat Note: K2Cr2O7(aq), H2SO4(aq), heat cannot affect this reaction CH3 Side chain with benzylic H but not alkene: CH(CH3)2 CH2CH2OH CHClCH2CH3 Side chain with no benzylic H: C CH2CH3 CH3 CH3 Side chain with alkene: CH2CH=CHCH2CH3 Side chain with alkene: CH2CH C CH3 CH3 N.B. For side chain with C=C bond, consider oxidative cleavage of alkene followed by oxidation to benzoic acid due to presence of benzylic hydrogen. Note: Alkaline KMnO 4 gives the salt of carboxylic acid which must be first acidified before solid benzoic acid can be obtained. CH2CH2OH CO2 heat KMnO4(aq), OH (aq) H+ (aq) CO2H

26 Victoria Junior College JC2 H2 Chemistry Arenes (Aromatic Hydrocarbon) Tutorial Nomenclature