Question

The increasing order of reactivity of the following compounds in \( S_N1 \) reaction is:
A. \( (CH_3)_3CBr \)
B. \( CH_3CH_2CH_2CH_2Br \)
C. \( (CH_3)_2CHCH_2Br \)
D. \( CH_3CH(Br)CH_2CH_3 \)

• A. A < B < C < D
• B. B < C < D < A
• C. D < C < B < A
• D. B < D < A < C

Hint:

\( S_N1 \) rate follows carbocation stability \( 3^{\circ} > 2^{\circ} > 1^{\circ} \); watch for possible rearrangement in the isobutyl halide.

Answer 1

The Correct Option is B

Step 1: Link reactivity to carbocation stability.
Because the slow (rate-determining) step of \(S_N1\) is loss of the halide to form a carbocation, the more stabilised the resulting cation, the faster the reaction.

Step 2: Rank the cations.
A forms a tertiary cation (very stable). D forms a secondary cation. B forms an ordinary primary cation (least stable). C is nominally primary but rearranges by a methyl/hydride shift toward a tertiary cation, so it beats B.

Step 3: Write the increasing sequence.
Weakest first: \(B < C < D < A\), matching option (ii).
\[\boxed{B < C < D < A}\]