Q1(f). Why nucleophilic shifts are common

Answer in brief

An adjacent electron-deficient centre can accept a migrating sigma-bond pair through a favourable three-centre interaction.

Key points

• In a nucleophilic shift the migrating group carries the bonding electron pair; an empty p orbital on the adjacent carbocation provides an accessible acceptor.
• The shift can replace a less stable cation by a more stable one or relieve ring strain. Correct orbital alignment is still required.
• An electrophilic shift towards a carbanion would require migration without the bonding pair; many ordinary alkyl frameworks lack a favourable low-energy pathway.
• Radical shifts involve three-electron bonding and are commonly less favourable than fragmentation or hydrogen transfer; the trend is not an absolute prohibition.