Mesomeric effects carbonyl substituents

Steric repulsions between substituents with n bonds and other groups attached to the benzene ring may twist the planes of both n systems as outlined in Sections 3.1.3.8 and 4.7.2 for o, o -dialkylated acetophenones. This sterically induced hindrance of conjugation is not only monitored by an increase in the carbonyl shifts but also by a shielding of the para benzenoid carbon due to an attenuated mesomeric effect of the carbonyl group. This... 

This can be understood from the inductive and mesomeric effects of the electronegative substituent. For example, the +M effect of Cl is much weaker than that of the NH2 group because the lone pair on chlorine does not interact well with the 2p-orbital on carbon (see Section 13.2.2). Therefore, whereas Cl withdraws electrons from the carbonyl group, the NH2 group mesomerically donates them. 

If both substituents on a C=0 bond are carbon or heavier atoms and bond angles are normal ( 120 ") then carbonyl frequency shifts can be related directly to mesomeric and inductive effects which account for the change in force constants of C=0 bonds and substituent single bonds. In the inductive effect, electronegative substituents (like Cl) raise C==0 frequencies by reducing the weight of carbonyl bond weakening resonance forms... 

If the mesomeric stabilization is provided by a double bond, the lithiated species is a homoenolate synthon, as shown in Scheme 44a. Reaction with an electrophile typically occurs at the y-position, yielding an enamine, which can then be hydrolyzed to a carbonyl compound. An important application of this approach is to incorporate a chiral auxiliary into the nitrogen substituents so as to effect an asymmetric synthesis. 2-AzaaUyl anions (Scheme 44b), which are generated by tin-lithium exchange, can be useful reagents for inter- and intramolecular cycloaddition reactions. ... 

Infrared spectra were used for investigations on tautomerism and isomerism. The frequencies of the carbonyl stretching bands seem to depend on mesomerism and inductive effects with respect to the positions of the carbonyl group and the substituents at the triazolopyrimidine system [77HC(30)179]. 

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