Non-linear Brønsted correlations: evidence for a levelling off in the reactivity of oximate ions in aqueous solution

A kinetic study of the reactivity of a series of pyridinium carbaldoximate ions (pKa= 7.13–9.02), which are all potential reactivators of acetylcholinesterase inhibited by organophosphorus poisons, towards p-nitrophenyl acetate in aqueous solution is reported. The corresponding Brønsted plot is non-linear, defining a plateau at pKa 8 which fits very well the data previously obtained for a number of structurally different oximate ions. This phenomenon appears to be typical for the behaviour of these bases since Brønsted plots for reactions of oximate ions with other electrophiles show a similar levelling off. These observations support an explanation in terms of a requirement for desolvation of the oximate ion prior to nucleophilic attack which becomes more difficult with increasing basicity. Possible implications of the rapid levelling off in reactivity of oximate ions on their chemical ability to act as efficient reactivators are discussed.