Addition of nucleophile to bent bonds of the carbonyl group
Bimolecular nucleophilic addition (AdN2) reactions of carbonyl compounds are modelled and studied using the semilocalized approach to chemical reactivity suggested previously. The approach consists in the direct obtaining of the one-electron density matrix of the whole reacting system in the form of a power series in the basis of orbitals localized on separate bonds. The double C=O bond is represented by two equivalent bent bonds, one of them being under attack of a nucleophile. The results support the previous hypothesis about an increasing polarity of the C=O bond under the influence of an external cation attached to a lone electron pair of the oxygen atom and yield a new interpretation of this effect in terms of interorbital interactions. Coordination of the oxygen atom by a subsidiary cation is shown to ensure also an increase of charge transfer from nucleophile to the reacting C–O bond at later stages of the process. These results serve to account for the known catalytic effect of acids in AdN2 reactions. Effects of substituents of various types upon the same charge transfer are explored and interpreted too. A certain analogy is concluded between early stages of the reaction under study and of the SN2 process of alkyl halogenides. Advantages of the bent bond model of the C=O bond versus the usual σσ, ππ-model are discussed in respect of interpretation of chemical reactivity.
Keywords: bimolecular nucleophilic addition, carbonyl compounds, the bent bond model, chemical reactivity, bond orbitals
PACS: 31.15.-p, 31.15.X-, 31.15.xp