Carbonyl compounds steric destabilization

Other carbonyl compounds exhibit rotation about sp -sp bonds, including amides.In A -acetyl-A -methylaniline, the cis conformation (111) is more stable than the trans- (112) by 3.5kcalmol (14.6 kJ mol ). ° This is due to destabilization of (5) due to steric hindrance between two methyl groups, and to electronic repulsion between the carbonyl lone-pair electrons and the phenyl 7i-electrons in the twisted phenyl orientation. ... 

Steric factors are also important in the transition state, which is tetrahedral (so it has bond angles of 109.5°). This causes the alkyl groups to be closer to one another than they are in the carbonyl compound, where the bond angles are 120°. As a result of the greater steric crowding in their transition states, ketones have less stable transition states than aldehydes have. In summary, alkyl groups stabilize the carbonyl confound and destabilize the transition state. Both factors increase AG, causing ketones to be less reactive than aldehydes. 

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... 

---