Schiff test

In the currently accepted mechanism, the pararosaniline and bisulfite combine to yield the "decolorized" adduct with sulfonation at the central carbon as described and shown.

Such an imine-mediated mechanism was first proposed by Paul Rumpf (1908–1999) in 1935,[5] and experimental evidence was provided by Hardonk and van Duijn in 1964.

[6] In 1980, Robins, Abrams and Pincock provided substantial NMR evidence for the mechanism, leading to its general acceptance.

[9] The mechanism was proposed in 1921 by the eminent German organic chemist Heinrich Wieland and his student Georg Scheuing (1895–1949).

[10][11] Bisulphite was believed to react with the available aromatic amine functional groups to form N-sulfinic acid groups, Ar-NH-SO2H, followed by reaction with aldehyde to form sulfonamides, Ar-NH-SO2CH(OH)-R. The 1980 NMR data that allowed visualization of intermediates does not support this mechanism or the sulfonamides as the chromogenic product.

The structure of the fuchsin dye.
The structure of the "decolorized" Schiff reagent.