On structural formulas of basic fuchsin and aldehyde-Schiff reaction products.

A variety of structural formulas has been suggested for the basic fuchsin moiety of aldehyde-Schiff reaction products. It was therefore deemed of interest to review the development of these different concepts of dye structure. Formulation of basic fuchsin as an ammonium salt preceded the quinonoid theory; however, chemists could not find such salts. The quinoid theory also could not be reconciled with chemical observations and spectroscopic data. In the 1920's the quinonoid formulas were superseded by benzenoid formulas with a positive charge at the central carbon atom. Using the resonance theory, basic fuchsin is often written with an apparently pentavalent nitrogen atom at a quinonoid ring. But such limiting structures do not exist in reality; the structure of the resonance hybrid is intermediate between the various contributing structures. The carbonium formula appears preferable to other limiting structures because it would remove temptations to endow basic fuchsin with a quinonoid ring, an imino or an ammonium group. Some formulas of aldehyde-Schiff reaction products carry two positive charges. But divalent basic fuchsin is very unstable. The divalent form of its derivatives formaldehyde- Schiff's reagent, aldehyde-fuchsin, and Crystal Violet is deep green; with decreasing H-ion concentration the divalent green compounds revert to the red or violet monovalent substance. It appears therefore highly unlikely that divalent basic fuchsin exists in PAS reaction products in washed and dehydrated sections.