Isomerism in the 2,4‐dinitrophenylhydrazones of α‐keto‐acid esters

The 2,4‐dinitrophenylhydrazones of α‐keto‐acid esters occur in isomeric forms, here called α‐ and β‐isomers. The α‐isomers differ from the β‐isomers in melting point, UV absorption, and retention volume in partition chromatography. In the presence of hydrogen ions or on irradiation with UV light, these isomers yield equilibrium mixtures in which, at any rate after irradiation, the β‐isomers greatly predominate. The α‐ and β‐isomers of pyruvic esters of homologous normal aliphatic alcohols both form a homologous series, but it is possible to separate not too complicated mixtures of these alcohols from C1 to C18, to identify the individual terms, and to determine their amounts by UV absorption measurements. Finally, some aspects of isomerism in hydrazones and the possible structure of the isolated α‐ and β‐isomers are discussed. For the β‐isomers an extrastabilized syn structure is probable.