Rates of hydrogen exchange in thioamides

Abstract

The rates of acid- and base-catalysed proton exchange in N-methylthioacetamide and of protonation of NN-dimethylthioacetamide have been measured in aqueous solution by using n.m.r. line-broadening techniques. N-Methylthioacetamide shows an increase of ca. 10³-fold in the rate constant for hydroxide-catalysed exchange, and a 10-fold decrease in the rate constant for acid-catalysed exchange, relative to N-methylacetamide. Both results are consistent with a considerably greater degree of polarisation in the thioamide. The rate of N-protonation of NN-dimethylthioacetamide is ca. 50 times less than the rate of acid-catalysed proton exchange of N-methylthioacetamide. It is suggested that proton exchange in the latter may proceed via the thionium ion.