Hydroxy-group participation in ester hydrolysis

Abstract

Aryl esters of 4-hydroxybutyric acid, 5-hydroxyvaleric acid, 2-hydroxyphenylacetic acid, and 3-(2-hydroxyphenyl)propionic acid are lactonised in reactions whose rates are proportional to 10^{pH – pK_W}. The anchimeric assistance corresponds to rate enhancements of 10³–10⁶ over the rates of hydrolysis of analogous esters without the hydroxy-groups. The reaction of phenyl 4-hydroxybutyrate shows buffer catalysis, possibly general base catalysis, in acetate and phosphate buffers. The esters of 4-hydroxybutyric acid and 5-hydroxyvaleric acid also react with participation by the hydroxy-groups in the acid-catalysed hydrolysis but those of 2-hydroxyphenylacetic acid and 3-(2-hydroxyphenyl)propionic acid do not. The rate of hydrolysis of phenyl 4-hydroxybutyrate increases more rapidly with acid concentration than that of phenyl butyrate does. The ϕ values are 0·59 and 0·99 respectively. The hydrolyses of the lactones of 2-hydroxyphenylacetic acid and 3-(2-hydroxyphenyl)propionic acid were also studied.