Re-evaluation of solution enthalpies of aliphatic hydrocarbons functional derivatives and terpenes in n-heptane and their validation by thermochemical consistency

Abstract

The aim of this study was to investigate the influence of the position of the functional group and the choice of solvent on the relationships between the enthalpies of solution and the alkyl chain length of aliphatic hydrocarbons functional derivatives. The results show that the selection of an appropriate solvent for functional derivatives, such as methanol for alcohols and acetonitrile for alkyl nitriles, is not a viable option because the enthalpies of solution increase non-linearly with increasing alkyl chain length, leading to significant errors in the prediction of enthalpies of solution.On the other hand, the enthalpies of solution of aliphatic hydrocarbons functional derivatives in n-heptane decrease non-linearly with increasing number of methylene fragments in the chain. In this work, complementary measurements of the enthalpies of solution of homologous series of dialkyl ethers, alkyl-amines and nitroalkanes in n-heptane were carried out. These new data were used to develop an approach to estimate the enthalpies of solution of dialkyl ethers, alkyl-nitriles and alkyl-amines in n-heptane. The absolute deviations between predicted and experimental solution enthalpies for most of the investigated compounds do not exceed 0.5 kJ mol−1. The proposed approach to predict the solution enthalpies of aliphatic hydrocarbon derivatives in n-heptane was applied to estimate the enthalpies of fusion of 11 aliphatic hydrocarbon derivatives, which agreed well with the values obtained by differential scanning calorimetry. Complementary vapour pressure measurements were carried out on cyclic aliphatic hydrocarbons from the terpene family using a transpiration method to derive their enthalpies of vaporisation. The latter were used for independent validation of the re-evaluated solution enthalpies.