Substituent Effects in Acetal Hydrolysis

Abstract

The rates of hydrolysis of a series of substituted benzaldehyde diethyl acetals and the corresponding 2-(substi-tuted phenyl)-1,3-dioxolanes have been measured in 50% dioxane-water (v./v.) at 30°. Both series of compounds hydrolyze by a specific acid A-1 mechanism and give plots of log vs. a with marked curvature for para-substituted compounds. By employing meto-substituted benzaldehyde diethyl acetals p was determined to be — 3.35. The diethyl acetals each hydrolyze 30 to 35 times faster than does the corresponding dioxolane derivative. This difference is in each case due to a less favorable entropy of activation for the cyclic compound. Positive values of ∆S* were found for all the diethyl acetals, but the dioxolanes have ∆S* values from —6.9 to —9.6 e.u. Enthalpies of activation are nearly constant for corresponding compounds in the two series. Within each series the large rate differences are due to differences in ∆H*. © 1965, American Chemical Society. All rights reserved.