Detection of Reversible Reactions in Solution Using Group Enthalpies of Transfer. Formation of Benzaldehyde Hemiacetals in Methanol

The enthalpies of transfer from methanol to dimethylformamide for a wide variety of aromatic compounds are additive properties of the groups present within the molecules. Values of ΔΔHs fail to be additive for benzaldehydes by an amount of energy which increases with increasing electron withdrawing power of the substituent. Nuclear magnetic resonance studies indicate that this results from hemiacetal formation in methanol, which varies from nil for p-dimethylamino-, p-hydroxy-, and p-methoxybenzaldehyde to 71% for the p-nitro compound. The non-additivity of group ΔΔHs values can indicate that specific interactions are occurring with a solvent, even though no stable product can be isolated.