Abstract
Abstract The ultrasonic absorptions in n-butyric, isobutyric, n-valeric, isovaleric, and n-caproic acids have been measured in the frequency range from 2.5 to 95 MHz in order to study the relationship between the ultrasonic absorption and the chemical structure of the alkyl group in carboxylic acids. The relaxation absorptions were observed for all the acids with relaxation frequencies in the range of 1.6–8.7 MHz. The kinetic parameters have been calculated on the basis of an assumption that the absorptions are associated with the breakage of one of the two hydrogen bonds in the dimer, as in the case of propionic acid. The values of the heat of reaction, ΔH, were calculated to be 4.7–6.4 kcal per mole of dimer. By an application of the results to Taft’s equation, logK=ρ*σ*+sEs, it is deduced that the change in the ΔH in a series of monocarboxylic acids is due to variations in the inductive effect of each alkyl group. However, the change in the heat of activation, ΔH≠, can not be explained in terms of the inductive effect.
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