Proton transfer in complexes of water with hydrogen halides in aprotic solvents

Abstract

scopic investigation of complexes of water with strong mineral acids HC1 and HBr in aprotic solvents. The IR spectra of complexes of water with hydrogen halides in methylene chloride and nitromethane were measured on Speeord 75IR and Perkin Elmer 580B spectrophotomete rs in demountable liquid cuvettes with thicknesses 0.054, 0.24, and 1 mm with KBr windows. The method of production of the hydrogen halides, purification of the solvents, and preparation of the solutions was described in [6 ]. In the work we used solutions with a water content of 0.001-0.2 M and hydrogen halides 0.02-0.4 M. The concentration of free acid and water molecules in the solutions was determined according to the intensity of the bands of the stretching vibrations of unperturbed HX and H20 molecules. Figure 1 presentsthe spectra of complexes of water with HC1 (a) and HBr (b) in nitromethane, as well as with HBr (c} in methylene chloride. It can be seen that continuous absorption is observed in the IR spectra of these solutions, extending over the entire spectral range 3600-500 cm -~. In the case of solutions in methylene chloride, on account of the intense bands of the solvent, the absorption spectrum can be reliably recorded only up to 1600 cm -1. We investigated the concentration dependences of the absorption at various frequencies on the number of free water and hydrogen halide molecules. Let us emphasize that the measurement of such principles in this case is a necessary condition for the assignment of the spectrum to a complex of definite composition. This is due to the fact that complexes of strong acids with water are poorly soluble in aprotie solvents and have a tendency to precipitate in the form of a suspension, sometimes poorly visible, during the process of preparation of the solution. Such a suspension was found to have an IR spectrum very close to the " spectrum of a concentrated aqueous solution of the acid. Thus, for example, it was found that the maximum permissible concentration of complexes of H20 with HC1 in carbon tetrachloride is achieved at CH20 < 2" 10 -4 M and CHC 1 < 10 -2 1V[. Reliably measurable concentrations of the complexes in the systems studied could be created for HC1 with H20 in nitromethane, and for HBr with H20 in nitromethane and dichloromethane. For complexes of H20 with HBr, both in nitromethane and in dichloromethane, the absorption intensity in all portions of the spectrum is a quadratic function of the free water concentration and a linear function of the free acid concentration within the limits of accuracy of the measurements (Fig. 2). From this it follows that primarily associates consisting of one acid molecule and two water molecules, i.e., with a 1 : 2 composition, are formed in solution [6]. Such complexes are formed in highly dilute solutions ( CH20 ~ 0.1-0.001 M; CHX -~ 0.1-0.01 M) even when the acid concentration significantly exceeds the water concentration.