Nuclear magnetic resonance (n.m.r.) measurements on proton-exchange kinetics in aliphatic alcohol -water solutions have been reported by several authors (1-5). The protolysis reaction is found to be both acid and base catalyzed. Recently, Fukumi et al. (5) have reported specific rate constants for different processes in the water-2-propanol system. They calculated a value of 2.0 1 mole-' s-' at 28 "C for the rate constant, k, of proton exchange between 2-propanol and water with no externally added acid or base 111 ROH + *HOH + ROH* + HOH where R = i-C,H,. This value is to be compared with Ic = 0.35 f 0.06 1 mole-' s-I at 42O, reported by Paterson (3b). In view of the wide disagreement, we report here the specific rate constants for reaction [I], determined at four different temperatures, as well as the energy of activation. In addition, the effects of the presence of several electron donors on the kinetics of reaction [ I ] are reported. Experimental MaterialsAll reagents used were Fisher reagent grade, dried and doubly distilled by the usual methods. De-ionized, doubly distilled water was prepared; its p H varied between 6.7 and 7.0. The alcohol-water solutions were prepared by weight and the concentrations in moles/l calculated from the known density data at various temperatures (6). Since the solutions slowly leach impurities from the glass (3), a given set of n.m.r. measurements were done on the same day that the materials were distilled. Nuclear Magnetic Resonance MeasurementsSpectra were obtained with a Varian A-60 spectrometer. Care was taken to keep the radiofrequency power level well below saturation and the field homogeneity such that a resolution of 0.3 Hz or better was attained. During the recording of the spectra the temperature remained constant to -I lo. The precision in the n.m.r. measurements varied from 0.02 to 0.05 Hz, depending on signal shape. All measurements were an average of at least six determinations. Results and DiscussionIn neat 2-propanol at 31°, the -OH signal was a sharp doublet with spacing 4.22 Hz. In the alcohol-water solutions used in the present study, the -OH signals in 2-propanol and water are quite well separated; thus at 31°, for [H20] = 7.2 M, [2-propanol] = 11.2 M, the two -OH signals are separated by 44.6 Hz. The linewidth of the water signal, however, could not be measured accurately because of interference from the methine proton signal, and this has also been reported by Paterson (3b).The specific rate constant, k, for reaction [l] was calculated from the relation (1, 3) where zR0, is the mean lifetime of a hydroxyl proton on a 2-propanol molecule ROH before exchange with a proton on a water molecule.In order to evaluate successfully the mean lifetimes, it is necessary to know two experimentally determinable parameters: ( I ) the transverse relaxation time, T2, in the absence of exchange and (2) the peak separation of the O H doublet in neat 2-propanol, which is due to spin-spin interaction, Sm. Transverse relaxation time, T2, was considered to be governed chiefly b...
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