Which one of the carbocations from the following is most stable?

Stability of a carbocation depends mainly on four effects: (i) resonance delocalisation of the positive charge, (ii) hyperconjugation from neighbouring C-H bonds, (iii) inductive $$+I$$ effect of alkyl groups, and (iv) any possibility of forming an aromatic system. The greater the overall charge delocalisation, the more stable the carbocation.

Case A: tert-Butyl carbocation, $$(CH_3)_3C^+$$ - a simple aliphatic tertiary centre. It is stabilised only by hyperconjugation (nine α-H atoms) and the $$+I$$ effect of three methyl groups. No resonance is possible.

Case B: Triphenylmethyl (trityl) carbocation, $$Ph_3C^+$$ - the positive charge on the central carbon can be delocalised into each of the three benzene rings. Each ring offers three equivalent resonance structures, so in all the charge is distributed over nine different positions. This massive resonance delocalisation far outweighs the hyperconjugation available in aliphatic centres. Hence it is extraordinarily stable; the trityl cation can even be isolated in super-acidic media.

Case C: Allylic secondary carbocation, $$CH_2=CH\!-CH_2^+$$ - the charge is shared between two carbon atoms by resonance (two structures), making it more stable than a simple primary cation but much less than benzylic or trityl systems.

Case D: Benzylic secondary carbocation, $$Ph\!-CH_2^+$$ - the charge is conjugated with one benzene ring, giving three resonance structures. This is more stable than an allylic cation but still has far fewer resonance contributors than the trityl cation.

Comparing the four:

• Option A: only hyperconjugation
• Option C: two resonance structures
• Option D: three resonance structures
• Option B: nine resonance structures (three rings × three positions) → maximum delocalisation

Therefore, the triphenylmethyl carbocation (Option B) is the most stable.

Option B which is: triphenylmethyl carbocation