Amide proton exchange in micelles

Rate constants for acid- and base-catalyzed NH exchange of long-chain amide s 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 micell es, where k(OH) decreases by a factor of about 2500 and where k(H) increase s by a factor of about 100, show the largest effects. The effects of cation ic micelles an smaller: a 30-fold decrease in k(H) (for ureas, or 6-fold fo r ordinary amides) and essentially no change in k(OH), 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 count erions for the micellar surface, the Bronsted formulation of medium effects , charge exposure, and the nature of the transition state.