Kinetics and Mechanism of the Pyridinolysis of Phenyl Chloroformates in Acetonitrile

Abstract

Kinetic studies on the reactions of Y-phenyl chloroformates with X-pyridines in acetonitrile are carried out at 25.0 °C. Both the Hammett and Brönsted plots are linear with enhanced substituent constants, σ p -, and basicities, p K a -, for strong para π-acceptor X-substituents, p -CN and p -CH 3 CO. This indicates that the electron-rich formate (O−C−O) moiety overlaps with the pyridine ring π-system enabling, through conjugation with the para π-acceptors, the rate-limiting formation of a tetrahedral intermediate. The difference in the aminolysis mechanism between methyl, II, and phenyl chloroformate, III, is attributed to the much stronger electron-donating polarizability effect of C 6 H 5 than of CH 3 . The proposed mechanism is supported by a relatively small β X (≅0.3) and by the lower Δ H ⧧ (6.7 kcal mol - 1 ) and Δ S ⧧ (−44 eu) values for a stronger donor Y (Y = p -CH 3 O) coupled with a stronger para π-acceptor (X = p -CN) in pyridine.