The structure of Schiff reagent aldehyde adducts and the mechanism of the Schiff reaction as determined by nuclear magnetic resonance spectroscopy

Abstract

An nmr study of compounds isolated from the Schiff aldehyde reaction between pararosaniline hydrochloride, sulfur dioxide, and acetaldehyde has demonstrated that these adducts are α-anilinoalkylsulfonic acids. The evidence is incompatible with the other structures most often accepted in the literature, N-phenyl alkylsulfonamides. In combination with nmr spectra obtained from solutions of the Schiff reagent and the Schiff reaction and with literature spectrophotometric data, this result leads to a reasonable proposal for the mechanism of the colour development. The effects of the concentrations of the dye, acetaldehyde, and in particular, sulfur dioxide are explained and suggest that the dominant coloured species is a 2:1 acetaldehyde–dye adduct. The nmr results also reveal a kinetic/thermodynamic competition for acetaldehyde between the aniline of the dye and the aldehyde carbonyl leading to bisulfite addition. The relation of the Schiff test with acetaldehyde and the Feulgen test for aldehydes in biological samples is also discussed.