Question

Choose the halogen which is most reactive towards SN1 reaction in the given compounds (A, B, C & D)

• A. A-Br_(a) ; B-I_(a) ; C-Br_(b) ; D-Br_(a)
• B. A-Br_(a) ; B-I_(a) ; C-Br_(a) ; D-Br_(a)
• C. A-Br_(b) ; B-I_(a) ; C-Br_(a) ; D-Br_(a)
• D. A-Br_(b) ; B-I_(b) ; C-Br_(b) ; D-Br_(b)

Answer 1

The Correct Option is A

The reactivity of halogens towards the SN1 reaction is primarily dependent on the stability of the carbocation formed after the departure of the leaving group. The carbocation stability is influenced by factors like resonance, hyperconjugation, and inductive effects. 

1. The SN1 mechanism involves two main steps: first, the leaving group departs, forming a carbocation, and second, the nucleophile attacks the carbocation.
2. Among the given compounds, let's examine each to determine where the most stable carbocation can form:
   • Compound A: Contains Br_(a) attached to a benzyl group. The resulting carbocation is stabilized by resonance with the aromatic ring.
   • Compound B: I_(a) is a good leaving group, and iodine forms a relatively unstable carbocation due to the lack of resonance stabilization.
   • Compound C: Br_(b) is on a bridgehead, complicating carbocation stability; however, Br_(a) is also adjacent to a space allowing for moderate resonance.
   • Compound D: The resulting carbocation is less stable due to the lack of resonance or significant hyperconjugation effects.
3. The most stable carbocation forms when Br_(a) in Compound A is the leaving group, due to resonance stabilization by the aromatic ring.

Therefore, the correct answer is A-Br_(a); B-I_(a); C-Br_(b); D-Br_(a), where Br_(a) in Compound A is the site of highest reactivity towards SN1 reaction.