Second-order rate constants of reactions of HO− with phosphate, phosphinate and thiophosphinate esters, (PhO)2PO.OC6H4NO2-p, Ph2PO.OC6H4NO2-p, Ph2PO.SPh, Ph2PO.SC6H4NO2-p and Ph2PO.SEt, go through minima with decreasing water content of H2O-MeCN or H2O-t-BuOH. The rate decrease is due to stabilization of the non-ionic ester on addition of organic solvent to H2O. This inhibition is partially offset by stabilization of the anionic transition states and in the drier solvents partial desolvation of HO− increases rates.
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Bunton et al. (1996) studied this question.
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