Which substrate would form a resonance-stabilized carbocation intermediate under SN1 conditions?

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Multiple Choice

Which substrate would form a resonance-stabilized carbocation intermediate under SN1 conditions?

Resonance-stabilized carbocations form when the positive charge can be delocalized into a conjugated system. In SN1, the leaving group departs to give a carbocation, and stability of that intermediate governs the reaction. Benzyl chloride is able to form a benzyl carbocation (Ph-CH2+). Once that positive charge appears, it can be delocalized into the aromatic ring, creating several resonance structures with the charge spread over the ortho and para positions of the ring. This delocalization dramatically stabilizes the carbocation, making SN1 formation of this intermediate more favorable. Methyl chloride would generate a methyl cation with no place to delocalize the charge. n-Butyl chloride would form a primary carbocation that lacks resonance stabilization. tert-Butyl chloride does form a very stable carbocation, but its stability comes mainly from hyperconjugation and inductive effects rather than resonance with a ring, so it’s not resonance-stabilized in the same sense. So the substrate that forms a resonance-stabilized carbocation under SN1 conditions is benzyl chloride.

Resonance-stabilized carbocations form when the positive charge can be delocalized into a conjugated system. In SN1, the leaving group departs to give a carbocation, and stability of that intermediate governs the reaction.

Benzyl chloride is able to form a benzyl carbocation (Ph-CH2+). Once that positive charge appears, it can be delocalized into the aromatic ring, creating several resonance structures with the charge spread over the ortho and para positions of the ring. This delocalization dramatically stabilizes the carbocation, making SN1 formation of this intermediate more favorable.

Methyl chloride would generate a methyl cation with no place to delocalize the charge. n-Butyl chloride would form a primary carbocation that lacks resonance stabilization. tert-Butyl chloride does form a very stable carbocation, but its stability comes mainly from hyperconjugation and inductive effects rather than resonance with a ring, so it’s not resonance-stabilized in the same sense.

So the substrate that forms a resonance-stabilized carbocation under SN1 conditions is benzyl chloride.

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