LUMO of the carbonyl group

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

This model is rationalized by a combination of steric and stereoelectronic effects. From a purely steric standpoint, an approach from the direction of the smallest substituent, involving minimal steric interaction with the groups L and M, is favorable. The stereoelectronic effect involves the interaction between the approaching hydride ion and the LUMO of the carbonyl group. This orbital, which accepts the electrons of the mcommg... 

The important interaction is between the HOMO of the ene sys-tehn and the LUMO of the carbonyl group—and a Lewis-acid catalyst can lower the energy of the LUMQ still further. If there is a choice, the more electrophilic carbonyl group (the one with the lower LUMO) reacts. 

Addition of an electron to the LUMO of the carbonyl group to form a radical anion is the first step in the reduction process. Radical anions can be characterized in aprotic solvents by electron spin resonance (esr) spectroscopy. Those derived from unconjugated carbonyl compounds are highly reactive and can only be detected in a matrix at low temperatures [3]. Decay is rapid because the excess carbonyl compound acts as a proton donor toward the basic oxygen center in the radical anion. Aromatic carbonyl compounds give less reactive radical anions in which the free electron is delocalized over the whole... 

---