On Schiff's bases and aldehyde-fuchsin: a review from H. Schiff to R. D. Lillie

In histochemistry aldehyde-fuchsin is widely regarded as an azomethine compound, though this hypothesis cannot explain the variety of reaction products. Infrared spectroscopy did not show a C = N bond. It was therefore deemed of interest to review chemical studies of aldehyde-fuchsin and other Schiff's bases by Schiff and his contemporaries. Schiff regarded reaction products of low molecular aliphatic aldehydes, e.g. acetaldehyde, and aromatic amines as diarylamines; aldehyde-fuchsin was assigned a 2:3 (dye:aldehyde) formula. These reactions were facilitated by alcohol and HCl. Others suggested condensation of two aldehyde molecules which carried a secondary and a tertiary amine respectively. Eibner proved that these compounds were ethylenes, not azomethines, and contained two secondary amines. Condensation of such bases produced ethylenic polymers, the Schultz's bases. Aromatic aldehydes readily yielded azomethines; aliphatic aldehydes formed --C = N-- bonds only during prolonged heating. These findings are in agreement with recent chemical data. Clearly, the term Schiff's bases is not synonymous with azomethines, but denotes any reaction product of aldehydes and amines. In 1962, Lillie's histochemical studies confirmed "the secondary amine nature of aldehyde aryl amine condensation". Thus, chemical and histochemical studies from Schiff in the 1860's to Lillie in the 1960's indicate that aldehyde-fuchsin is not an azomethine compound, but contains diarylamines and their derivatives.