FURTHER STUDIES ON DECARBOXYLATION OF N-HEXYLMALONIC ACID IN POLAR SOLVENTS

Rate constants and activation parameters are reported for the decarboxylation of n-hexylmalonic acid in 12 polar liquids - 1,3-propanediol, 1,2-propanediol, glycerol, 2,3-butanediol, 1-decanol, 1-octanol, 1-hexanol, N,N-dimethylaniline, o-toluidine, N-methylaniline, o-ethylaniline, and aniline. It is observed that the inductive effect of the n-hexyl group of the substrate causes an inversion of the order of increasing activation energy with increasing molecular weight of the members of the various homologous groups (glycols, alcohols, and amines). The average change in the enthalpy of activation per methylene group for the decarboxylation of n-hexylmalonic acid in the amines is compared with corresponding values previously reported for five other acids - malonic acid, oxanilic acid, oxalic acid, cinnamalmalonic acid, and benzylmalonic acid. The values obtained for all the compounds turn out to be exact multiples of the absolute magnitude of the value for malonic acid. A plot of ΔH‡ vs. ΔS‡ for the decarboxylation of n-hexylmalonic acid in each homologous group is linear, each line having a slope of 378°K or 105°C - the same temperature as the melting point of n-hexylmalonic acid. An interesting comparison is made between the data for the decarboxylation of n-hexylmalonic acid in alcohols and that for the solvolysis of methyl p-toluenesulfonate in alcohols reported by Hyne and Robertson. The melting point of p-toluenesulfonic acid is 105° - the same as the melting point of n-hexylmalonic acid. The isokinetic temperature for the solvolysis of the ester in alcohols is also 105° - the same as that for the decarboxylation of n-hexylmalonic acid in alcohols. Finally, the average change of the enthalpy of activation per methylene group on going from one alcohol to another is exactly the same for both reactions, namely, 947 cal.