Chemical equilibrium of formaldehyde and methanediol in hot water: Free-energy analysis of the solvent effect

The chemical equilibrium between formaldehyde (HCHO) and its hydrated form, methanediol (CH2(OH)2), is computationally investigated in solvent water. Using the method of energy representation, the solvation free energy of methanediol is evaluated as a function of (solvent) density and temperature, and the solvent effect is discussed over a wide range of thermodynamic conditions. It is shown, in agreement with the previously reported tendency, that the solvation free energy of methanediol depends strongly on the temperature and is not monotonic against the density variation at a fixed supercritical temperature. In the hydration reaction of formaldehyde, the solvent water favors methanediol more at a lower temperature. The formation of methanediol is unfavored in the absence of solvent, and formaldehyde in hot water above ∼ 200 °C is predominantly present in the unhydrated form.