JEE Main · 2021 · 20 Jul 2021 · Shift 1 · General Organic Chemistry

A carbocation is resonance stabilised only when its empty $p$ orbital lies parallel to an adjacent $\pi$ system and can overlap with it. In the benzyl cation (A) the vacant orbital overlaps the ring $\pi$ cloud, spreading the charge to the ortho and para carbons; in the allyl cation (B) it overlaps the neighbouring double bond, so the charge is shared equally by the two terminal carbons. In (C) the cationic carbon is separated from the ring double bond by an $s{p}^3$ carbon, which breaks the conjugation and leaves a plain primary cation. In (D) the charge sits on a carbon of a double bond, and the empty orbital of such a vinyl cation lies in the nodal plane of that $\pi$ bond and cannot overlap with it, while the carbonyl further along the chain only withdraws electrons. The resonance stabilised carbocations are therefore just the benzylic species (A) and the allylic species (B).