Question

(i)$C_6{H}_5\stackrel{\oplus }{C}{H}_2$ (ii)$C_6{H}_{5}C{H}_2\stackrel{\oplus }{C}{H}_2$ (iii)$C_6{H}_5\stackrel{\oplus }{C}HC{H}_3$ (iv)$C_6{H}_5\stackrel{\oplus }{C}(C{H}_3{)}_2$

Stability order of the following carbocations is

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

Answer 1

The correct option is A $II<I<III<IV$

The order of stability of carbocations are
$Primary<secondary<tertiary$
Compounds $III$ and $IV$ are secondary carbocations. Between these two $IV$ is more stable when compared to $III$ because $IV$ is stabilized by two methyl groups and one benzene ring whereas in the $III$ compound carbocation is stabilized by only one methyl and one benzene ring.
In $I$ and $II,I$ is more stable than $II$ because it is stabilized by benzene rind which is adjacent to carbocation whereas in the compound $II$ the carbocation is stabilized by the methyl group and benzene ring is a bit far from the carbocation. so, it contributes to the stabilization of carbocation is less when compared to the compound $I$
So the correct order is
$II<I<III<IV$