Looking for classes? Ksquare Career Institute, Bengaluru →
Which amongst the following is the most stable carbocation :-
A
$CH_3\overset{+}{C}H_2$
B
$\overset{+}{C}H_3$
C
$(CH_3)_3C^+$
D
$(CH_3)_2\overset{+}{C}H$
Detailed Solution
Alkyl groups are electron releasing. They stabilise a carbocation by the $+I$ effect and by hyperconjugation, which spread the positive charge.
The more alkyl groups attached to the positively charged carbon, the more stable the carbocation.
$(CH_3)_3C^+$ (tert-butyl cation) is a tertiary carbocation with three methyl groups and nine $\alpha$-hydrogens available for hyperconjugation.
$(CH_3)_2CH^+$ is secondary (6 $\alpha$-H), $CH_3CH_2^+$ is primary (3 $\alpha$-H) and $CH_3^+$ has no alkyl group.
Order of stability: $3^\circ \gt 2^\circ \gt 1^\circ \gt CH_3^+$
So the most stable carbocation is $(CH_3)_3C^+$.
The more alkyl groups attached to the positively charged carbon, the more stable the carbocation.
$(CH_3)_3C^+$ (tert-butyl cation) is a tertiary carbocation with three methyl groups and nine $\alpha$-hydrogens available for hyperconjugation.
$(CH_3)_2CH^+$ is secondary (6 $\alpha$-H), $CH_3CH_2^+$ is primary (3 $\alpha$-H) and $CH_3^+$ has no alkyl group.
Order of stability: $3^\circ \gt 2^\circ \gt 1^\circ \gt CH_3^+$
So the most stable carbocation is $(CH_3)_3C^+$.