HUCKEL MOLECULAR ORBITALS OF ALLYL SYSTEMS

The sizes of the 2p atomic orbitals represent the degree to which the orbitals contribute to the molecular orbitals in the allyl system. The constructive overlap of all three 2p orbitals in the lowest energy molecular orbital is bonding over the whole system. The nonbonding molecular orbital has a nodal plane at the center carbon atom. The electron configuration of the allyl radical is shown. 

The three molecular orbitals in the allyl carbocation. The bonding orbital, tIj, contains two electrons and the nonbonding orbital, n, is empty. The antibonding orbital, ti, is also empty. 

Ttie electrons of the n system occupy the lowest available orbitals. 

Allyl cation Allyl radical Allyl carbanion (2 71 electrons) (3 n electrons) (4 7t electrons) 

We can determine the electron configuration of the allyl carbanion using the allyl radical as a reference. Adding another electron to the radical gives the anion. The second electron must be paired with the electron already located in the MO. C-1 and C-3 share the electron pair equally, and each one has a formal charge of -Vi. 

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