Succinate esters: odd–even effects in melting points

Abstract

Dialkyl succinates show a pattern of alternating behavior in their melting points, as the number of C atoms in the alkane side chain increases, unlike in the dialkyl oxalates [Josephet al.(2011).Acta Cryst.B67, 525–534]. Dialkyl succinates with odd numbers of C atoms in the alkyl side chain show higher melting points than the immediately adjacent analogues with even numbers. The crystal structures and their molecular packing have been analyzed for a series of dialkyl succinates with 1−4 C atoms in the alkyl side chain. The energy difference (ΔE) between the optimized and observed molecular conformations, density, Kitaigorodskii packing index (KPI) and C—H...O interactions are considered to rationalize this behavior. In contrast to the dialkyl oxalates where a larger number of moderately strong C—H...O interactions were characteristic of oxalates with elevated melting points, here the molecular packing and the density play a major role in raising the melting point. On moving from oxalate to succinate esters the introduction of the C2 spacer adds two activated H atoms to the asymmetric unit, resulting in the formation of stronger C—H...O hydrogen bonds in all succinates. As a result the crystallinity of long-chain alkyl substituted esters improves enormously in the presence of hydrogen bonds from activated donors.