The kinetics of cleavage of nitrophenyl alkanoates by γ-cyclodextrin and by ‘dimethyl-β-cyclodextrin’ in basic aqueous solution

Abstract

The cleavage of m- and p-nitrophenyl alkanoates (C₂ to C₈) in basic solution is accelerated modestly (7–17 times), by γ-cyclodextrin (γ-CD). The effects on the two isomeric series of esters are virtually the same and they hardly vary with the ester chain length. Cleavage of the same esters (C₂ to C₁₀) is retarded by ‘dimethyl-β-cyclodextrin’(diMe-β-CD) by up to a factor of 8, but it is not totally inhibited, as was supposed earlier. The effects of this modified cyclodextrin on the two series of esters are very similar, but they vary significantly with the acyl chain lengths. For example, second-order rate constants (k₂) for reaction of the esters with diMe-β-CD show little change from C₂ to C₅, a steep increase from C₅ to C₇, and then a levelling off. Overall, the behaviours of γ-CD and diMe-β-CD differ significantly from those found earlier for ester cleavage by α-CD, β-CD and ‘hydroxypropyl-β-CD’. These differences are discussed in terms of the relative importance of transition-stage and initial-state binding, and in relation to the structural variations among the five cyclodextrins.