The three isomeric benzenedicarboxaldehydes – orthophthalaldehyde, isophthalaldehyde and terephthalaldehyde - differ considerably in their properties in aqueous solutions. Differences in two types of nucleophilic additions will be discussed – electroreduction, where the electron is the nucleophile and covalent hydration involving nucleophilic attack by water. Spectrohotometric and polarographic investigation indicate that the 1,3-isomer, the isophthalaldehyde is – similarly as the majority of other substituted benzaldehydes - less than 3% hydrated. The strong hydration of terephthalaldehyde, present in about 20% in hydrated form, results from strong resonance interaction between the two formyl groups. The substituent effect of 4-CHO is larger than that of 4-NO2. Orthophthalaldehyde exists in three forms: Easily reducible unhydrated form is present in about 10%, monohydrated acyclic form in about 20% and cyclic hemiacetal form in about 70%. Investigation of equilibria of the three forms is important in elucidation of mechanism of the reaction of this aldehyde with thiol and amines in determination of amino acids. Reduction of isophthalaldehyde resembles that of substituted benzaldehydes at twice concentration. Diprotonated form of terephthalaldehyde is reduced in a reversible two-electron step. Details of reduction processes of orthophthalaldehyde are under investigation.
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