Kinetics and mechanism of the pyridinolyses of dimethyl and diethyl chloro(thiono)phosphates in acetonitrile

Kinetic studies for the reactions of dimethyl chlorophosphate ( 1 ), dimethyl chlorothionophosphate ( 2 ), diethyl chlorophosphate ( 3 ), and diethyl chlorothionophosphate ( 4 ) with substituted (X) pyridines have been carried out in acetonitrile. The pyridinolyses of the P  = O systems ( 1 and 3 ) show linear free energy correlations for substituent X variations, while those of the P  = S systems ( 2 and 4 ) show biphasic concave upward Hammett and Brönsted plots with a break region between 3‐phenyl and 3‐methoxy pyridines. A concerted mechanism with both frontside and backside nucleophilic attack is proposed for the reactions of 1 and 3 , but the fraction of a frontside attacks is somewhat larger than that of a backside attack. A concerted mechanism with a dominant backside nucleophilic attack for weakly basic pyridines and a concerted mechanism with a dominant frontside nucleophilic attack for strongly basic pyridines are proposed for the reactions of 2 and 4 . The pyridinolysis rate does not depend solely on one major factor. The concave upward free energy relationship can be diagnostic of a change in the direction of the nucleophilic attack. Copyright © 2010 John Wiley & Sons, Ltd.