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 halides towards the S$_N$1 mechanism depends on the stability of the carbocation intermediate formed during the reaction:

Compound A forms a benzyl carbocation, which is highly stable due to resonance.

Compound B forms a primary carbocation, which is less stable but reacts due to iodine's leaving group strength.

Compound C forms a tertiary carbocation, which is very stable and reactive.

Compound D forms a primary carbocation, less stable but reacts due to bromine's moderate leaving group strength.

 Thus, the halogens are ordered as per the leaving group stability in S$_N$1.