The effect of substituents upon the alkaline hydrolysis of esters

Abstract

The velocity coefficient of a reaction in solution may be written k = PZe -E/RT, where E is the energy of activation, Z is the collision frequency as calculated for a gaseous system, and V is the proportion of energised collisions which actually leads to the formation of products. The thesis describes a kinetic studyof the alkaline hydrolysis of (a) a series of substituted benzoic esters and (b) a series of aliphatic esters; a similar study of the base-catalysed prototropy of a number of phenyl alkyl ketones had been carried out for purposes of comparison. The results are analysed in accordance with the above equation, and their significance is considered in its relationship to two problems, viz., that of the ortho effect and that of the influence of alkyl groups upon reactivity. Previous workers have shown that the introduction of substituents into the m~ or p-position in the aromatic nucleus influences reaction velocity almost' entirely by changing the energy of activation, the variations in the P factor being relatively unimportant. This was shown, for example, in the case of the alkaline hydrolysis of p-substituted ethyl benzoates (Ingold and Nathan, J.C.S., 1936, 222). In the present work a similar constancy of P has been demonstrated for m-substituted benzoic esters. When the substituent is in the o-position, however, it reduces the reaction velocity by decreasing P. This is discussed in connection with other data relating to the ortho effect, and the conclusion is drawn that the changes In P are due to a "chelation" process. In the alkaline hydrolysis of normal aliphatic esters from ethyl acetate to ethyl octoate, the changes in velocity are due to variations in the energy of activation, which are in harmony with the known order of inductive effects of alkyl groups. Esters having a branched chain show, in addition, variations in the P factor, while this factor also changes, appreciably in passing from the prototropy of acetophenone to that of propiophenone. These changes in P are interpreted in the light of an interaction between B-hydrogen and carbonyl oxygen, already postulated by Evans (J.C.S., 1936, 785) in interpreting his results on the acid-catalysed prototropy of phenyl alkyl ketones. Peculiarities observed by other workers in the effects of alkyl groups upon reaction velocities are considered in the light of the new results.