Flashcards · 9701 · A Level Topic 31

Halogen compounds (A Level) flashcards

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Halogenoarene vs halogenoalkane: Halogenoarene: halogen bonded directly to a benzene ring carbon (e.g. chlorobenzene). Halogenoalkane: halogen bonded to an sp³ (saturated) carbon (e.g. chloroethane).
Halogen carrier: The catalyst (AlCl₃ or AlBr₃) that polarises the halogen to make a stronger electrophile.
How does AlCl₃ generate the electrophile and get regenerated?: Cl₂ + AlCl₃ → Cl⁺ + AlCl₄⁻ C₆H₆ + Cl⁺ → C₆H₅Cl + H⁺ H⁺ + AlCl₄⁻ → HCl + AlCl₃
Chloroethane and chlorobenzene with NaOH(aq) under reflux: Chloroethane is hydrolysed to ethanol. Chlorobenzene does not react.
Why is chlorobenzene so unreactive towards nucleophiles?: A Cl lone pair overlaps with the ring π system, giving the C–Cl bond partial double-bond character (shorter, stronger). The electron-rich ring also repels nucleophiles, and substitution would disrupt the delocalised ring.
Relative ease of hydrolysis: acyl chlorides, alkyl chlorides, aryl chlorides: Acyl chlorides (fast, even with cold water) > alkyl chlorides (need heating with NaOH(aq)) > aryl chlorides (no reaction under these conditions).