Allyl Radical SOMO and Pi Orbitals

Allyl radicals are three-carbon conjugated systems. They often form as intermediates in alkene halogenation when halogen adds to the allylic carbon instead of the double bond. Each of the three sp2-hybridized carbon atoms has an unhybridized p orbital, and these p orbitals combine to form three pi molecular orbitals.

Just like the allyl cation and anion, the allyl radical is a three-atom conjugated system.

The molecular orbital framework is identical to that of the ions, with a node passing through the central carbon in ψ₂ and between the carbons in ψ3.

However, the difference lies in the number of π electrons.

Recall that the...