Abstract

The pyridinolysis of S-methyl chlorothioformate shows a biphasic Brønsted-type plot, in agreement with a stepwise mechanism and a change in the rate-limiting step, from formation of a zwitterionic tetrahedral intermediate (T ±) at high pK a to its breakdown at low pK a. The reaction of the same substrate with secondary alicyclic amines shows a linear Brønsted-type plot of slope 0.23, which is in accordance with a stepwise mechanism where formation of T ± is the rate-determining step for the whole pK a range examined. © 2006 Elsevier Ltd. All rights reserved.