Dediazoniation of Mono-substituted Benzenediazonium Tetrafluoroborate in Aqueous Solution at pH 1-7

Abstract

The dediazoniation of substituted (m-or p-) benzenediazonium tetrafluoroborate (R-C₆H₄-N₂BF₄, R=H, Me, MeO, Cl, Br, NO₂, COOR' and OPh) in aqueous solutions (pH 1-7) was studied kinetically. The concentrations of diazonium salts were determined by measuring the concentration of the azo compound formed by NW acid colorimetrically. The main products of the dediazoniation were phenols and phenylazophenols. The decomposition rate is of first order with respect to the concentration of the diazonium salt and the rate constants ( k ) are markedly dependent on the pH values of the medium: at lower pH the values of k are constant, but above a certain value of pH it increases with pH linerly. These results were explained in terms of the equilibriums between diazonium ion and diazotate, ArN₂⁺ ↔ ArN₂OH ↔ArN₂O^- and it was concluded that the values of observed k at lower pH is the rate const ant of the dediazoniation of diazonium ion. The values of k depend also on the substituents. Hammett's relation between log k and σ is set up linerly for meta and several para (CN, NO₂and COOEt) substituents whereas for other para substituents some scatter from the linearity is found. The scatter was reduced by using σ₁+1.8|σ_R| instead of σ in plotting. This deviation was explained in terms of the resonance electron donability of the substituent and the stabiliza tion of the initial state.