The ultraviolet spectra of nitrophenylhydrazones in neutral solution; substituent and solvent effects

Abstract

The u.v. spectra of (a) 4-nitro-, (b) 2,4-dinitro-, (c) 2,4,6-trinitro-, (d) 2-nitro-, and (e) 2,6-dinitrophenylhydrazones of substituted benzaldehydes, and of their N-methyl derivatives were measured with chloroform as solvent. Each series showed in a nearly linear λ_max.–σ(or σ⁺) relationship, with higher ρ values for the polynitrophenylhydrazones. Differences in λ_max. among the series (a)–(c) and the corresponding N-methyl analogues were constant for each series. Change of solvent resulted in substituent- and series-dependent blue or red shifts. The absorption maximum was ascribed to one of two transitions; one, for series (d) and (e) and their N-methyl derivatives, involving the o-nitro-group and the hydrazone residue, and the other, for other derivatives, involving the p-nitro-group. The spectra are discussed in relation to the basicity of the nitrogen, formation of hydrogen bonds, and steric interactions, coupled with the electronic buttressing effect.