Rationale for the acidity of Meldrum's acid. Consistent relation of C-H acidities to the properties of localized reactive orbital.

Detailed investigation on the origin of the acidity of the α-protons of a set of the carbonyl molecules was carried out on the basis of properties of the localized molecular orbital. An anomalously high acidity of Meldrum's acid, as compared with those of dimedone and dimethyl malonate, is one of the well-known but unresolved issues. The well-localized σ orbitals of the C−H bonds at the α-position of the carbonyl groups can be obtained with the reactive hybrid orbital (RHO) theory. We found that the energy levels of the unoccupied RHOs of the C−H moiety of Meldrum's acid and other carbonyl compounds showed a good linear correlation with the experimental deprotonation energies. This is probably because the deprotonation reaction to form the proposed naked anions in a polar solvent is a highly endothermic process, in which the thermodynamic energy differences between the neutral molecules and the corresponding anions approximately coincide with the activation energies. We also investigated the effect of the...