Reversibility of the ring-opening step in the acid hydrolysis of cyclic acetophenone acetals

Abstract

The acid-catalysed hydrolysis of the vinyl ethers α-(2-hydroxyethoxy)styrene 1a or α-(3-hydroxypropoxy)styrene 1b yields a mixture of acetophenone and a cyclic acetophenone acetal, 2- methyl-2-phenyl-1,3-dioxolane 3a or 2-methyl-2-phenyl-1,3-dioxane 3b. Ratios [acetophenone]: [3] have been determined by HPLC and by UV spectroscopy as 15 (from 1a) and 10 (from 1b). The acetals undergo further hydrolysis, but this is considerably slower than that of the vinyl ethers, and the above numbers represent products of kinetic control. These product ratios are equal to the rate constant ratios k₂:k_–1 for the partitioning of the oxocarbocations 1-(2-hydroxyethoxy)-1-phenylethyl 2a or 1-(3-hydroxypropoxy)-1-phenylethyl 2b between reaction with solvent water molecules and with the internal hydroxy group. These cations are also formed as intermediates in the hydrolysis of the cyclic acetals 3. That k₂ is an order of magnitude greater than k_–1 signifies that the rate-limiting step in this hydrolysis is the ring-opening forming the oxocarbocation, and that there is little reversibility in aqueous solutions. The hydrolysis reactions of these cyclic acetals are considerably slower than that of an acyclic analogue, the dimethyl acetal of acetophenone, by factors of 1.0 × 10³ for 3a and 1.8 × 10² for 3b. This study demonstrates that these rate retardations cannot be accounted for by reversibility of the ring-opening step. The presence of the ring results in an inherent decrease in the rate constant for the H⁺-catalysed formation of the oxocarbocation.