16.4 : π Molecular Orbitals of the Allyl Cation and Anion

An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH₂group via a single bond. Allyl anions can be obtained by treating propene with a strong base that can deprotonate methyl groups. Allyl cations are formed as intermediates during substitution reactions involving allylic halides. In both cases, the hybridization of the allylic carbon changes from sp³ to sp², giving rise to a carbon chain with three sp²-hybridized carbons, each with an unhybridized p orbital.

A linear combination of the three atomic p orbitals gives three molecular orbitals, ψ₁, ψ₂, and ψ₃. ψ₁ is the bonding molecular orbital, ψ₂ is a nonbonding orbital characterized by a node passing through the central carbon, whereas ψ₃ is an antibonding molecular orbital with two nodes. The molecular orbital frameworks of the allyl anion and cation are identical and differ only in the number of π electrons. The allyl cation has two π electrons in ψ₁, whereas the allyl anion has four π electrons distributed between ψ₁ and ψ₂.