Effect of Substituent in Diazotized Orthanilic Acid on the Azo Coupling with 7-Acetylamino-4-hydroxynaphthalene-2-sulfonic Acid in Citrate-Phosphate Buffers

Abstract

Effect of substituent in the para position with respect to the diazo group in diazotized orthanilic acid derivatives on the rate and selectivity of azo coupling with 7-acetylamino-4-hydroxynaphthalene-2-sulfonic acid was studied in citrate-phosphate buffers. The selectivity of azo coupling at the 3-position of the azo component almost does not depend on the nature of electron-donor substituent. Electron-acceptor groups considerably reduce the selectivity of formation of the 3-isomer. Previous conclusions, according to which the rate-determining stages are different depending on the site of azo coupling of diazotized orthanilic acid with 7-acetylamino-4-hydroxynaphthalene-2-sulfonic acid, were found to be also valid for some 5-substituted orthanilic acid derivatives. The rate-determining stage in the azo coupling at the 3-position can change upon introduction of strong electron-acceptor substituents capable of interacting with the diazo group. The contribution of the multicenter mechanism of deprotonation of the σ-complex with participation of water molecules was presumed to increase in going to the diazo components having electron-donor substituents.