Chemical Properties of Aromatic Hydrocarbons

NOTE ON THE SOURCE TEXT: the book's own page prints this heading as '13.5. Chemical Properties:' (reusing the number 13.5, the aromatic-hydrocarbons unit number itself) -- a verified duplicate-numbering slip, filed here as 13.5.6 for the same reason as 13.5.5 above. Benzene's delocalised pi electrons make its ring an electron-rich centre, so it readily undergoes ELECTROPHILIC SUBSTITUTION reactions; though the ring is highly stabilised by this delocalisation, benzene can also be made to undergo addition and oxidatio …

NITRATION: heating benzene at 330 K with a nitrating mixture (conc. HNO3 + conc. H2SO4) substitutes one ring hydrogen with the nitronium ion (NO2+, the electrophile) to give nitrobenzene, C6H6 + HNO3 --conc. H2SO4, 330K--> C6H5NO2 + H2O; the concentrated H2SO4's role is specifically to generate the NO2+ electrophile. HALOGENATION: benzene reacts with a halogen (Cl2 or Br2) in the presence of a Lewis-acid catalyst (FeCl3, FeBr3 or AlCl3) to give the corresponding halobenzene, e.g. C6H6 + Cl2 --FeCl3--> C6H5Cl + HCl; fluorine reacts violently with benzene even without a catalyst, while iodine is very unreactive even with one. SULPHONATION: benzene reacts with fuming sulphuric acid (conc. H2SO4 + SO3) to give benzenesulfonic acid; the electrophile here, SO3, is a neutral molecule but still a strong electrophile because sulfur's octet is not satisfied -- and the reaction is reversible, readily undergoing desulphonation in aqueous medium. FRIEDEL-CRAFTS ALKYLATION: benzene treated with an alkyl halide in the presence of anhydrous AlCl3 gives an alkylbenzene, e.g. C6H6 + CH3Cl --anhyd. AlCl3--> C6H5CH3 (toluene) + HCl. FRIEDEL-CRAFTS ACYLA …

Benzene undergoes electrophilic substitution, rather than addition, because it is an electron-rich system thanks to its delocalised pi electrons, and is readily attacked by electrophiles -- but the aromatic ring's extra stability means the intermediate ultimately loses a proton to REGENERATE the aromatic system, giving a substitution product rather than a saturated addition product. The general three-step mechanism: Step 1 -- FORMATION OF THE ELECTROPHILE: the reagent (E-Nu) reacts with a catalyst to generate the electrophile E+ (e.g. HNO3 + 2H2SO4 --> NO2+ + H3O+ + 2HSO4-; Cl2 + AlCl3 --> Cl+ + AlCl4-; SO3 + 2H2SO4 --> SO3(as electrophile) + H3O+ + HSO4- via the fuming-sulphuric-acid equilibrium; CH3Cl + AlCl3 --> CH3+ + AlCl4-; CH3COCl + AlCl3 --> CH3CO+ + AlCl4-). Step 2 -- ATTACK ON THE RING: the electrophile E+ attacks the aromatic ring, using two of the six delocalised pi electrons to form a new sigma bond to one ring carbon; this converts that carbon to sp3 (bearing both E and the original H) and leaves a positively charged, resonance-stabilised carbocation intermediate (the arenium ion / sigma complex), whose positive charge is delocalised over the three remaining ring carbons that retain conjugation. Step 3 -- LOSS OF A PROTON: a base (the …

HYDROGENATION: benzene adds three moles of H2 over platinum or palladium to give cyclohexane, C6H6 + 3H2 --Pt/Pd--> C6H12 (the same reaction used earlier as structural evidence for benzene's ring). CHLORINATION: benzene adds three molecules of Cl2 under sunlight or UV light to give benzene hexachloride (BHC, C6H6Cl6) -- known co …

VAPOUR-PHASE OXIDATION: although benzene strongly resists ordinary oxidising agents, its vapour, mixed with oxygen and passed over V2O5 at 773 K, undergoes ring-opening oxidation to maleic anhydride (via maleic acid, with loss of water) -- an industrially important route to maleic anhydride. BIRCH REDUCTION: benzene can be PARTIALLY reduced to 1,4-cyclohexadiene by treatment with sodium or lithium in a mixture of liquid ammonia and an alcohol (ROH) -- a convenient, controlled …