The kinetics of the decarboxylation of n‐butylmalonic acid in alkanols and amines

Abstract

Rate constants and activation parameters are reported for the decarboxylation of n-butylmalonic acid in four normal alkanols (hexanol-1, octanol-1, decanol-1, and dodecancil-I) and in five amines (aniline, N-methylaniline, N-ethylaniline, N-n-propylaniline, and A'-n-butylaniline). Both AH^ and AS* of the reaction in both homologous series decrease regularly with increasing length of the hydrocarbon chain of solvent. If we compare data for the reaction in alkanol-amine pairs containing the same total number of carbon atoms in the molecule, we find that the AH* values are identical, but that the value of AS$ is 0.8 eu/mole higher for the reaction in the amines as compared with the alcohol. The rate constant, at all temperatures, is 1.5 times as large in the amine as it is in the corresponding alcohol. Empirical equations are deduced relating the parameters A H $ , AS*, AG*, and k of the reaction to the parameters n and T , where n is the total number of carbon atoms in the solvent molecule and T is the absolute temperature. The results reported herein are compared with previously reported data for malonic acid.The ease of decarboxylation of malonic acid and its derivatives when warmed in polar solvents affords a facile means of studying substituent and solvent effects on chemical reactivity. Studies directed toward unraveling these effects were initiated around 1956 when data were reported on the kinetics of the decarboxylation of malonic acid in two polar solvents, glycerol and dimethyl sulfoxide [l]. Since that time a considerable body of data has appeared on the kinetics of the decarboxylation of malonic acid and its derivatives in numerous polar solvents. Recently results were reported on the decarboxylation of malonic acid in several normal alkanols [Z]. It was shown that the enthalpy of activation of the reaction was a linear function of the number of methylene groups in the alcohol. A decrease in AH* of 600 cal/mole was noted for each additional methylene group in the hydrocarbon chain of the solvent. It had been noted previously that in the case of the decarboxylation of malonic acid in aniline and its N-alkyl derivatives AH* decreased about 300 cal/mole for each additional methylene group on the alkyl moiety [3].Kinetic data have been reported previously on the decarboxylation of nbutylmalonic acid in the molten state, in phenol, and in o-, m-, and p-cresol [4], as wsell as in hexanoic acid and octanoic acid [5]. In order to compare with the 80 7

The decarboxylation of methylmalonic acid and n‐octadecylmalonic acid in normal alkanols

“…The decarboxylation of n-butylmalonic acid in normal alkanols, previously reported, yielded similar results [2].…”

“…The decarboxylation of malonic acid in normal alkanols, previously reported, likewise was found to constitute a reaction series with an isokinetic temperature equal to the melting point of the substrate (135°C) in which the free energy of activation (AGfo) was likewise equal to 29.0 kcal/mole [2]. The decarboxylation of n-butylmalonic acid in normal alkanols, previously reported, yielded similar results [2].…”

“…I t is interesting to compare the data for the decarboxylation of methylmalonic acid in normal alkanols with those previously reported for the decarboxylation of [2] (see Table V). I n both cases it will be observed that each additional methylene group added to the hydrocarbon chain of the solvent lowers AHt in the amount of 0.6 kcal/mole.…”

“…Using the results of the present investigation as well as previously published data for the decarboxylation of malonic acid [2] and of n-butylmalonic acid [2] in normal alkanols, the author has developed an empirical technique for predicting rate constants in the case of the decarboxylation of malonic acid and its n-alkyl derivatives in this type of solvent. The rationale of the various steps involved in this technique for predicting rate constants is outlined below.…”

“…Experimental rate constants and activation parameters have already been reported on the decarboxylation of malonic acid [2] and n-butylmalonic acid [2] in a series of normal alkanols. I n these investigations two interesting results were noted, (a) The enthalpies of activation of the reactions appeared to be a linear function of the number of carbon atoms or methylene groups in the hydrocarbon chain of the solvent, and (b) The free energy of activation at the melting point of the substrate appeared to be a constant, the same for both substrates.…”