The Steric Effect of Ortho Substituents on the Acidic Hydrolysis of Benzamides

Article abstract of DOI:10.1021/jo00279a038

The rate constant, k, for the acidic hydrolysis of a number of substituted benzamides in water and in a dioxane water mixture (1:1 v/v) was measured.The reaction conditions were selected to exclude the electronic effects of substituents on the overall rate as far as possible.The log k value for meta derivatives was almost independent of the electronic effects of substituents.Although the log k value for para derivatives was weakly affected by the through-resonance effect of electron-donating substituents, the value for ortho derivatives was almost linear with the Taft-Kutter-Hansch (TKH) E_s steric parameter except for NO2 and Ph.The effective steric effects of these ?-bonded planar substituents were approximated by the averaged value of the two TKH E_s constants estimated from the thickness and width of each group.The log k value was also analyzed by using the Charton υ steric parameter.The differences in definition of the υ from the TKH E_s parameter seriously distorted the analysis of the ortho substituent effects on the hydrolytic reaction.