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. The pyridinolyses of the P = O systems show linear free energy correlations for substituent X variations, while those of the P = S systems show biphasic concave upward Hammett and Bronsted plots with a break region. The pyridinolysis rate does not depend solely on one major factor.