JEE Chemistry 2023 Question with Solution

Step-by-step solution

Standard Method

Given: Four carbocations (a), (b), (c) and (d) are shown with an $\text{NH}_2$ group on the ring.

Find: The most stable carbocation.

The most stable carbocation is (c). The stability of the carbocation is enhanced by the $+M$ (mesomeric) effect of the $\text{NH}_2$ group, which donates electron density into the ring, stabilizing the positive charge on the carbocation. In this case, the resonance structures show that the positive charge is delocalized, making this carbocation the most stable.

Therefore, the correct option is A.

Common mistakes

• Choosing the structure based only on visual appearance of the ring position. This is wrong because carbocation stability here depends on resonance interaction with the $\text{NH}_2$ group. Check whether the positive charge can be delocalized by the $+M$ effect.

• Ignoring the mesomeric donation of $\text{NH}_2$ and considering only inductive effects. This is wrong because the lone pair donation into the ring provides major stabilization. Use resonance first when such a substituent is present.

• Reading option labels incorrectly. The structure (c) is the correct one, but in the given options it appears as option A. Match the listed options carefully before marking the final answer.