Hydration Promoted by a Methylene Group: A Volumetric Study on Alkynes in Water

Hydrocarbons including a methylene group are generally considered a hydrophobic building block, in the sense that the density of their hydration water is lower than that of bulk water. However, is the methylene group always hydrophobic? In this study, we experimentally determined the partial molar volume of a methylene group in water as 14.01 ± 0.46 cm³ mol^-1 for 1-alkyne, 9.83 ± 0.35 cm³ mol^-1 for 2-alkyne, and 11.39 ± 0.55 cm³ mol^-1 for 3-alkyne. These values are all unusually small compared to the ∼16 cm³ mol^-1 for model compounds from the literature. The subsequent volumetric analysis on the basis of the Kirkwood-Buff parameter indicates that the hydration water is enriched by the addition of a methylene group for 2-alkyne, while it is depleted for the reported model compounds that contain hydrophilic functional groups, 1-alkyne, and 3-alkyne. Our findings suggest that the triple bonded carbons in 2-alkyne that reduce hydration water act as a hydrophobic group in 2-alkyne. Thus, the methylene group should be called "hydrophilic" in this case because it actually recovers the hydration water when placed next to more hydrophobic groups. Therefore, we conclude that the hydrophobicity of a methylene group varies depending on its hydration environment due to other functional groups in the solute.