JEE Main 2025 Jan 29 Shift 2, Chemistry Q5: Resonance & Mesomeric Effect

The carbocation stability depends on the degree of resonance and hyperconjugation that delocalises the positive charge.

Departure of $B{r}^{-}$ from each substrate gives:

(1) Allyl cation: stabilised by one $\pi$ system.

(2) Benzyl cation: stabilised by one phenyl ring.

(3) Cyclohexyl cation: only hyperconjugation, secondary alkyl.

(4) Triphenylmethyl (trityl) cation: stabilised by extensive resonance with three phenyl rings.

$\therefore$ Trityl cation is by far the most stable, so option (4) gives the most stable carbocation.

Stability order: $P{h}_3{C}^{+}>PhC{H}_2^{+}>$ cyclohexyl${}^{+}>$ allyl${}^{+}$ (in this set).