Question

Which of the following is the most stable carbocation?
A. $C{H_3}\mathop C\limits^ \oplus {H_2}$
B. $C{H_3}\mathop C\limits^ \oplus = O$
C. $C{H_3} - \mathop C\limits^ \oplus = NH$
D. $C{H_2} = \mathop C\limits^ \oplus H$

Answer 1

Hint: Carbocations are stabilized by their neighboring carbon-carbon bonds. Primary carbocations are less stable than secondary, tertiary carbocations. Alkyl groups are electron-donating and they stabilize the carbocations because the electrons present around the neighboring carbons are drawn towards the nearby positive charge, thus reducing the electron poverty of the positively-charged carbon. Groups like =NH, =O are electron-withdrawing groups.

Complete step by step answer:
We are given to find the most stable carbocation out of the given options.
The first carbocation is $C{H_3}\mathop C\limits^ \oplus {H_2}$. $C{H_3}\mathop C\limits^ \oplus {H_2}$ is also called as ethyl cation. Carbon is bonded with a methyl group. Alkyl groups are electron donating pairs. By gaining electrons from the alkyl group, the positive charge of the carbocation gets stabilized.
The second carbocation is $C{H_3}\mathop C\limits^ \oplus = O$. $C{H_3}\mathop C\limits^ \oplus = O$ is an acyl compound. In this carbocation, carbon is directly bonded with oxygen using a double bond. =O group is an electron-withdrawing group. So when it is bonded with carbon, it withdraws electrons from rather donating. It further destabilizes the carbocation.
In the same way, $ = NH, = C{H_2}$ are also electron-withdrawing groups and thus decrease the stability of the corresponding carbocations by withdrawing electrons from carbon.
Therefore $C{H_3}\mathop C\limits^ \oplus {H_2}$ is the most stable carbocation from among the given carbocations.

The correct option is Option A.

Note:
Alternative approach:
The stability of carbocations depends upon the inductive effect.
Inductive effect is the withdrawal of sigma electrons away from the ring towards the substituent because of the higher electronegativity of the substituent compared to the ring.
Here, the substituent is the carbon atom.
Methyl group has a positive inductive effect compared to the other rings.
Therefore, the carbocation with methyl group $C{H_3}\mathop C\limits^ \oplus {H_2}$ is more stable.