Choose the halogen which is most reactive towards S_N 1 reaction in the given compounds (A, B, C D)

Solution

Chemistry ma’am · 3 steps

For each compound, ask which halogen leaves behind the more stable, flat carbocation.

1. What SN1 needs. In S_N1 the halide leaves first and a carbocation forms. The halogen whose carbocation is better stabilised (by resonance or by alkyl groups) and can become planar reacts fastest.

   Compound	Carbocation from (a)	Carbocation from (b)	More reactive
   A	secondary benzylic, resonance with the ring	plain secondary	Br₍ₐ₎
   B	primary allylic, resonance with C=C	primary, double bond too far away	I₍ₐ₎
   C	bridgehead, cannot become planar	tertiary, ring carbon free to flatten	Br_((b))
   D	tertiary	primary	Br₍ₐ₎

2. Bridgehead exception. In C, Br₍ₐ₎ sits on a tertiary carbon, but that carbon is a bridgehead of the bicyclic ring. The rigid cage stops it from flattening to the sp² shape a carbocation needs, so it hardly ionises at all.
3. So. The answer is A–Br₍ₐ₎, B–I₍ₐ₎, C–Br_((b)), D–Br₍ₐ₎, which is option (d).

The idea to keep. SN1 reactivity follows carbocation stability: benzylic and allylic beat plain primary or secondary, and tertiary beats primary. A bridgehead carbocation is the exception: it cannot go planar, so it barely forms.

Answer from the NTA final answer key. Solution by PrepWiser.