Kinetic study of the proton transfer reaction between (2,6-dialkyl -4-nitrophenyl)phenylcyanomethanes and MTBD in acetonitrile

Three (2,6-dialkyl-4-nitrophenyl)phenylcyanomethanes (alkyl ¼ Me-1, Et-2, i-Pr-3) have been synthesized and used in kinetic studies of proton abstraction by strong MTBD base in acetonitrile. These carbon acids: 1, 2 and 3 revealed in acetonitrile similar strength, having pKa ¼ 25:78; 25.94, 26.19, respectively. The fastest reaction has been observed for 1 k258 H ¼ 141:2 dm3 mol21 s21 and the slowest for 2 k258 H ¼ 65:9 dm3 mol21 s21, while for 3 was close to diethyl derivative k258 H ¼ 67:9 dm3mol21s21. The values of primary deuterium kinetic isotope effects kH=kD are moderate and not sensitive to the bulk of alkyl substituent, ranging from 11 to 16 for 1 and 12 to 13 for 3, at the temperature range 10–35 8C. The relation between acidity constants of C-acids and kinetic and thermodynamic parameters are discussed in terms of stereoelectronic influence of alkyls on charge distribution between 4-nitrophenyl and cyano substituents. The observed kinetics reflect superposition of the properties of substituents, including ortho-alkyls and solvation effect on the transition state (TS) in acetonitrile. The general view is that the TS comes early along the reaction path. The possible structure of the TS and the general discussion of the mechanism are given. q 2003 Elsevier B.V. All rights reserved.