The effect on the colour of the diarylazo chromogen, and electronic substituent constants, of the N-pyridinium group

Abstract

Para- and meta-N-pyridinium-substituted anilines have been synthesised by displacement of dinitroaniline from N-(2,4-dinitrophenyl)pyridinium chloride by p-phenylenediamine or m-acetylaminoaniline in hot DMF. Diazotisation and coupling with N,N-diethylaniline gave azo compounds, with visible absorption maxima: p-pyridinium isomer, 486 nm (neutral), and 504 nm (acid); m-pyridinium isomer, 456 nm (neutral), and 508 nm (acid). Dual-parameter relationships have been derived for a series of substituted azo compounds, connecting their halochromic shifts (difference in visible absorption maximum between neutral and protonated forms) and the electronic substituent constants, σ 1 and σ R. Via these relationships, values of σ 1 = 1.01 and σ R = 0.05 have been derived for the N-pyridinium substituent. Thus, this group is shown to be an exceedingly powerful electron-withdrawer, exerting its effect mainly by inductive/field effects. Its effect on the colour of azo dyes is roughly equivalent to that of the nitro group.