Kinetics and Mechanism of the Pyridinolysis of Aryl Ethyl Chlorothiophosphates in Acetonitrile

The nucleophilic substitution reactions of Y-aryl ethyl chlorothiophosphates with X-pyridines are studied kinetically in acetonitrile at 55.0 ºC. The Hammett and Brönsted plots for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Me. The substituents of X = 4-CN and 4-Ac show great positive deviations from both the Hammett and Brönsted plots. The Hammett plots for substituent Y variations in the substrates exhibit biphasic concave upwards with a minimum point at Y = H. The obtained values of the cross-interaction constants (ρXY) are all null in spite of the biphasic free energy correlations for both substituent X and Y variations, since the ρX values with both the strongly and weakly basic pyridines are almost constant. A stepwise mechanism with a rate-limiting leaving group departure from the intermediate is proposed where the distance between X and Y does not vary from the intermediate to the second transition state. A frontside attack is proposed with the strongly basic pyridines based on the considerably great magnitudes of ρX and βX values and a backside attack is proposed with the weakly basic pyridines based on the relatively small magnitudes of ρX and βX. The positive deviations of the two strong π-acceptor parasubstituents,X = 4-Ac and 4-CN, from both the Hammett and Brönsted plots are rationalized by the great extents of bond formation and breaking. KCI Citation Count: 12