Tautomeric Equilibrium Historical Overview of an Analytical Problem

Tautomeric Equilibrium Historical Overview of an Analytical Problem 

The real fact is that many compounds were discovered in the second half of the nineteenth century, whose properties and behavior were impossible to explain with the available concepts at that time in structural chemistry. Here we can mention some of them preparation of p-bromodiazoaminobenzene 31 in two ways (Griess, 

[28]) the interaction between sulfuric acid and thrimethylcarbinol giving two isomeric di-isobutylenes 32 and 33, that is, isomerizing through addition and elimination of water (Butlerov, 1877, [29]) attempts to isolate alcohols in which the hydroxyl group is attached directly to a double-bonded carbon atom as in 34, giving, however, always isomeric carbonyl compounds (Erlenmeyer, 1880, [30]) and ethyl malonate (Conrad and Buschoff, 1880, [31]). 

In 1882, Baeyer and Oekonomides [32] found out that isatin 35 gives two isomeric (N- and 0-) methyl derivatives. They explained this fact with pseudomerie [33] - the possibility of one compound to have more than one structure obtained in the process of interaction, which, being unstable, converts very fast to the stable configuration. 

In 1884, Zincke and Bindewald [34] obtained the same orange dye by coupling benzenediazonium chloride with 1-naphthol and by condensing phenylhydrazine with 1,4-naphthoquinone. They supposed that a mobile equilibrium existed between two forms, namely azo (13b) and quinonehydrazone (13a), a phenomenon classified by them as oiiisomerie. 

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