Mechanisms of Lactone Hydrolysis in Acidic Conditions

The acid-catalyzed hydrolysis of linear esters and lactones was studied using a hybrid supermolecule-polarizable continuum model (PCM) approach including up to six water molecules. The compounds studied included two linear esters, four beta-lactones, two gamma-lactones, and one delta-lactone: ethyl acetate, methyl formate, beta-propiolactone, beta-butyrolactone, beta-isovalerolactone, diketene (4-methyleneoxetan-2-one), gamma-butyrolactone, 2(5H)-furanone, and delta-valerolactone. The theoretical results are in good quantitative agreement with the experimental measurements reported in the literature and also in excellent qualitative agreement with long-held views regarding the nature of the hydrolysis mechanisms at molecular level. The present results help to understand the balance between the unimolecular (A(AC)1) and bimolecular (A(AC)2) reaction pathways. In contrast to the experimental setting, where one of the two branches is often occluded by the requirement of rather extreme experimental conditions, we have been able to estimate both contributions for all the compounds studied and found that a transition from A(AC)2 to A(AC)1 hydrolysis takes place as acidity increases. A parallel work addresses the neutral and base-catalyzed hydrolysis of lactones.