Acid strengths of various substituted formazans in ethanolic solution

Abstract

The acid strengths, pK_ae′ of a number of substituted 1,5-diarylformazans have been determined by spectrophotometric titration with sodium ethoxide in ethanol. For the yellow non-intramolecularly hydrogen-bonded isomers, with alkyl and chloro-substituents in the 3-position, pK_ae [graphic omitted] 18.0–19.0. 1,5-Diphenylformazan is substantially more acidic (pK_ae= 16.5 ± 1.0), and methyl substituents in the 1- and 5-aryl groups have the small but positive effect of lowering the acid strength. Triarylformazans exist substantially as the red intramolecularly hydrogenbonded isomers, and are generally too weakly acidic for titration with sodium ethoxide.

Evidence for intramolecular hydrogen bonding when a 3-nitro-group is present, and also for the existence of tautomeric forms of 3-diphenylmethylformazans has been obtained.