Question

Rank the following molecules in increasing order of relative rate of $S_{N}1$ solvolysis with methanol and heat

• A. $III<II<IV<V<I$
• B. $II<III<IV<I<V$
• C. $V<IV<III<II<I$
• D. $I<II<III<IV<V$

Answer 1

The correct option is B $II<III<IV<I<V$

Rate of $S_{N}1$ reaction is directly proportional to the stability of the carbocation formed.
Allylic carbocations are more stable.
Here, (V) and (I) forms allylic carbocation but (V) is most stable than (I) because it forms teritary allylic carbocation whereas (I) forms secondary allylic carbocation.
$II$ is a vinylic halide which is the least reactive because positive charge on double bond is unstable.
Comparing (III) and (IV), both forms secondary carbocation but the leaving group ability of $Br$ is better than $Cl$
Therefore, rate of (IV) is higher than (III).
Hence, increasing order of relative rate of $S_{N}1$ solvolysis is,
$II<III<IV<I<V$