Geminal diol anion

The Validity of the J Function. For aqueous sodium hydroxide solutions the validity of the J- function, describing the basicity of a solution by its ability to add hydroxide ions to a carbonyl group to form an anion of the geminal diol, has been proved earlier (19). For all substituted benzaldehydes studied in water-ethanol or water-dimethylsulfoxide (DMSO), the value of log (CArCH(OH)o-/CArCHo) determined from absorbance measurements was found to be a linear function of the J- function with a slope varying between 0.95 and 1.05 in the region of J values where measurements were possible. The Can-... 

The relatively modest increase in the value of the J- function upon increasing the ethanol content of a sodium hydroxide solution from 90 to 98 vol % indicates that either the nucleophilic reactivity of the ethoxide ion under these conditions does not differ substantially from that of the hydroxide ion (while solvation of the hydroxide ion and the geminal diol anion is similar to solvation of the ethoxide ion and the hemiacetal anion) or that compensation of effects takes place. 

If it is assumed, in a first approximation, that the activity coefficient ratios /ha/ /a- and /ArCHo//ArCH(OH)o do not differ very much, the difference between H and J for a given solution would be expressed by H- — J = —log flH2o-However a calculation of the difference by the latter formula, using the only available information (33) about the activity of water in water-DMSO mixtures, leads to values much smaller than those found in practice. This would indicate that the ratio /ha//a-> for the H indicator acids, is considerably larger than the ratio of /ArCHo//ArCH(OH)o- for the benzaldehydes used as J indicators. Probably this difference in activity coefficient ratios is caused by a larger extent of charge delocalization in the A anion compared to the geminal diol anion, which causes an extra stabilization of A- (manifested by a decrease in f ) by dispersion interaction with DMSO. 

This process could even be accelerated catalytically. If there is enzyme involvement in production of the carbonyl from the geminal diol, then these TSMs could be considered "suicide" substrates. Another alternative explanation is a "normal" SN2 type reaction between a serine anion and a protonated geminal diol with water as the leaving group. These alternative pathways are not mutually exclusive, but additional work on the kinetics and structural biochemistry of the interaction will be needed to indicate the predominant pathway. 

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