Rates and pH-dependent product distributions of the CuCl2-catalyzed dediazoniation ofp-nitrobenzenediazonium ­tetrafluoroborate in aqueous acid

Abstract

The rates of formation and yields of products from the dediazoniation of p-nitrobenzenediazonium tetrafluoroborate (PNBD) in aqueous solutions over a range of HCl, NaCl and CuCl 2 concentrations at 60 °C were examined. Two main products were observed: p-nitrophenol (ArOH) and p-nitrochlorobenzene (ArCl). Trace amounts of nitrobenzene (ArH) and p-nitrofluorobenzene (ArF) were detected. Added CuCl 2 speeds the reaction and both the rate of dediazoniation and ArOH yield (unlike ArCl) are very sensitive to pH. The results are completely consistent with the heterolytic dediazoniation mechanism, i.e. rate-determining formation of a highly reactive aryl cation followed by competitive formation of dediazoniation products. PNBD kinetics are first order (with respect to PNBD) in the absence of and presence of CuCl2, except at low acidity and in the presence of low to moderate CuCl2 concentrations. The non-first-order kinetics are attributed to a competing reaction between PNBD and the ArOH product. The results suggest a simple method for preparing halobenzenes in high yield. Copyright © 1999 John Wiley & Sons, Ltd.