Hydrocarbons (A Level) flashcards

30.1 Arenes

Nitration of benzene: reagents, conditions, electrophile

Concentrated HNO₃ and concentrated H₂SO₄, 25–60 °C (above about 60 °C gives dinitration). Electrophile NO₂⁺: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻

Halogenation of benzene

Cl₂ with anhydrous AlCl₃ (or Br₂ with AlBr₃) as halogen carrier, room temperature: C₆H₆ + Cl₂ → C₆H₅Cl + HCl.

Friedel–Crafts alkylation and acylation

Alkylation: RCl with AlCl₃ catalyst; electrophile R⁺; gives an alkylbenzene. Acylation: RCOCl with AlCl₃, heat; electrophile RCO⁺; gives a phenyl ketone. Both add carbon to the ring.

Methylbenzene with Cl₂: AlCl₃ catalyst vs UV light

AlCl₃, no UV: ring substitution, giving 2- and 4-chloromethylbenzene. UV light, no catalyst: free-radical substitution in the side chain, giving C₆H₅CH₂Cl.

Oxidation of the side chain of an alkylbenzene

Hot alkaline KMnO₄, then dilute acid: the side chain is oxidised to –COOH on the ring whatever its length, e.g. methylbenzene → benzoic acid.

Hydrogenation of benzene

H₂ with a Pt or Ni catalyst and heat gives cyclohexane. It needs harsh conditions because the delocalised system is destroyed.

Directing effects of –OH/–NH₂/–R and of –NO₂/–COOH/–COR

–OH, –NH₂, alkyl: direct to the 2- and 4- (and 6-) positions and activate the ring. –NO₂, –COOH, –COR: direct to the 3- (and 5-) position and deactivate it.