Effect of methyl substituents, ring size, and oxygen on bond dissociation energies and ring-opening kinetics of five- and six-membered cyclic acetals

Abstract

The demand for clean and highly efficient fuels stimulates the development of novel fuels for the next generation. With the emergence of new catalytic methods, the selective synthesis of cyclic acetals made them promising fuel candidates. The present work provides systematic and consistent calculations of bond dissociation energies for 1,3-dioxolane and 1,3-dioxane and their substituted derivatives. Rate constants of subsequent ring-opening reactions of the formed fuel radicals have been calculated to facilitate the understanding of initial fuel consumption pathways. We found that the CH bond on the carbon that is in between two oxygens will always be the weakest bond to be broken. The inclusion of two oxygens in the ring slightly changes the strain in the ring structure. However, the trend that the BDEs increase from five to six-membered ring species still exists in cyclic acetals. The reason is a smaller eclipsing strain in the five-membered radicals relative to the parent molecules. For the subsequent ring-opening reactions of the formed radicals, the radical site and the position of the substitution have a major impact. The results of our study facilitate a deeper understanding of the effect of methyl substituents, ring size, and oxygen on the reactivity of five- and six-membered cyclic acetals as well as provide accurate kinetic data for the mechanism development of these potential fuels.