Flashcards · 9701 · AS Topic 15

Halogen compounds (AS Level) flashcards

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SN2 and SN1: which halogenoalkanes, and how many steps?: SN2: primary; one step, through a transition state. SN1: tertiary; two steps, through a carbocation. Secondary can react by both.
Why do tertiary halogenoalkanes favour SN1 and primary favour SN2?: Tertiary: three electron-donating alkyl groups stabilise the carbocation. Primary: little steric hindrance, so the nucleophile can reach the δ+ carbon.
Halogenoalkane with NaOH: aqueous vs ethanolic: Aqueous NaOH, heat under reflux: substitution, gives an alcohol. Ethanolic NaOH, heat: elimination, gives an alkene.
Halogenoalkane with KCN and with NH₃: KCN in ethanol, heat under reflux: nitrile (chain + 1 C). Excess NH₃ in ethanol, heat in a sealed tube: amine.
Why does reactivity increase from chloroalkanes to iodoalkanes?: The C–X bond enthalpy decreases (C–Cl > C–Br > C–I), so C–I breaks most easily. Bond enthalpy, not bond polarity, decides it.
Comparing hydrolysis rates with AgNO₃(aq) in ethanol: Warm each halogenoalkane with AgNO₃(aq)/ethanol: precipitate appears fastest for iodo (yellow), then bromo (cream), then chloro (white).