Thermochemical Studies of Carbonyl Reactions. 2. Steric Effects in Acetal and Ketal Hydrolysis

Article abstract of DOI:10.1021/ja00405a030

A calorimetric determination of the enthalpies of hydrolysis of a series of alkyl-substituted dimethyl acetals is reported.These data are critically compared with the enthalpies of hydrolysis from an analogous set of aliphatic dimethyl ketals derived from 2-alkanones.The acetals exhibit a significantly attenuated range in their enthalpies of hydrolysis relative to that for ketal hydrolysis.The free energies of acetal formation in solution were modeled by measurements of the corresponding free energies of hemiacetal formation from the aldehydes in neutral methanol.The observed free-energy differences are satisfactorily correlated with the Taft E_s steric substituent constant scale, but the corresponding acetal enthalpy data vary in a complex manner.The role of entropy in determining kinetic and equilibrium steric effects in a variety of other systems is discussed.Preliminary molecular mechanics calculations on these systems indicate the importance of bond angle bending in evaluating the torsional potential at a carbonyl group.Many of the compounds were found to possess several conformations having comparable energies.