Thermodynamics of solvation of some linear and branched aliphatic aldehydes in water and heptane

The enthalpies of dissolution in water at infinite dilution (i.d.), ΔHtot∞, of propanal, butanal, 2-methylpropanal, pentanal, and 2,2-dimethylpropanal were measured by isothermal calorimetry at T = 298.15 K. The enthalpies of solution, ΔsolHm∞, in water were derived by subtracting the enthalpies of the hydration reaction at 298.15 K from the measured enthalpies of dissolution. The values obtained were combined with the standard enthalpies of vaporisation, ΔvapHm∘, to obtain the corresponding enthalpies of solvation at i.d., ΔsolvHm∞. The CH2 group contribution to ΔsolvHm∞ in water for linear aliphatic aldehydes was (−3.4 ± 0.5) kJ · mol−1 in good agreement with the average value obtained for a large number of organic compounds containing linear alkyl chains. The simple additive method adopted to estimate the molar enthalpy of solvation at i.d in water for the CHO functional group and its internal CO group gave (−28.1 ± 1.5) kJ · mol−1 and (−22.5 ± 1.5) kJ · mol−1, respectively. The enthalpies of solvation in heptane of linear and branched aldehydes with 3 to 11 C atoms were obtained using our earlier enthalpies of solution at i.d. and T = 298.15 K and recent experimental enthalpies of vaporisation from literature. Methylene group contribution to ΔsolvHm∞ for linear aliphatic aldehydes in heptane was (−4.8 ± 0.1) kJ · mol−1. Enthalpy of transfer from heptane to water for CH2 group was (2.1 ± 0.4) kJ · mol−1.