Solvent-solute interactions of isomeric phthalaldehydes in aqueous solutions

In contrast to aliphatic and pi-deficient N-heterocyclic aldehydes, which readily in aqueous solutions covalently add water, the majority of benzenoid aldehydes is present in aqueous solutions at less than 3% in the hydrated geminal diol form. The only reported exception present some nitrobenzaldehydes. No information has been available on the effect of other electron-with drawing groups on increased reactivity of the CH=O group towards nucleophilic addition of water. Considerable difference between such reactivity of the three isomeric benzenedicarboxaldehydes is reported. Combination of electrochemical and spectrophotometric evidence proved that 1,3-benzenedicarboxaldehyde is less than 3% hydrated and 1,4-benzenedicarboxaldehyde is 24% present as geminal diol. Strong hydration indicates a strong resonance interaction between the two CHO groups in para position. Polarographic investigation demonstrated that in aqueous solution the 1,2-benzenedicarboxaldehyde (orthophthalaldehyde) is present in about 10% in unhydrated, about 20% in an acyclic monohydrated and in 70% in a cyclic hemiacetal form. The latter is generally assumed to be the only form present. (C) 2006 Elsevier B.V. All rights reserved.