Protonation of carboxylic acids by hydrogen bromide in dibromodifluoromethane: rates of proton exchange, relative basicities, and the acidity of HBr–CBr2F2

Abstract

Ethanoic, propanoic, butanoic, 2,2-dimethylpropanoic, phenylethanoic, and benzoic acids are fully protonated by HBr–CBr₂F₂, and at low temperatures (<168 K) separate ¹H n.m.r. signals for HBr and RCO₂H₂⁺ are observed. If the temperature is reduced even further then resolution of the two acid proton signals of the latter may be observed. Dichloroethanoic and trichloroethanoic acids are not protonated by this acid system. 2-Chloropropanoic acid may be partly protonated. Line-shape analysis of the n.m.r. spectra at low temperatures has been used to determine the rates of proton exchange between HBr and RCO₂H₂⁺, and in this way an assessment of the relative basicities of the carboxylic acids is obtained. The behaviour of HBr–CBr₂F₂ is compared with superacid systems.