Amide Proton Exchange in Micelles

Abstract

Rate constants for acid- and base-catalyzed NH exchange of long-chain amides have been measured in cationic and anionic micelles and compared with NH exchange of model amides in aqueous solution. The data show that the rates can be strongly influenced by the electrostatic environment. Anionic micelles, where kOH decreases by a factor of about 2500 and where kH increases by a factor of about 100, show the largest effects. The effects of cationic micelles are smaller: a 30-fold decrease in kH (for ureas, or 6-fold for ordinary amides) and essentially no change in kOH, which was unexpected. Other effects are negligible (less than a factor of about 2): counterion, nonionic surfactant, headgroup, chain length, etc. The data are discussed in terms of electrostatic effects, steric retardation, competition of counterions for the micellar surface, the Brønsted formulation of medium effects, charge exposure, and the nature of the transition state.