Answer: The allylic carbocation formed is resonance-stabilised by delocalisation over three carbons.
- A The allylic carbocation formed is resonance-stabilised by delocalisation over three carbons
- B Allylic halides generally possess a weaker, more easily broken carbon-halogen bond according to standard textbooks
- C Allylic halides are structurally usually tertiary alkyl halides in general practice as frequently described
- D Resonance stabilisation does not apply to carbocation intermediates in most textbook accounts
Correct answer: A. The allylic carbocation formed is resonance-stabilised by delocalisation over three carbons
Explanation: Loss of X⁻ from an allylic halide gives an allylic carbocation (CH₂=CH-CH₂⁺ ↔ ⁺CH₂-CH=CH₂), which is stabilised by resonance over three carbon atoms, making SN1 very facile.
SN2 proceeds in a single step with backside attack and inversion of configuration, while SN1 forms a planar carbocation intermediate first, leading to racemisation.
Concept context
Study carbon-halogen compounds: how they are made, how they react via SN1/SN2 and elimination, their stereochemistry, and their environmental impact.