Hydration equilibrium Geminal diols can be viewed as
ketone (or
aldehyde) hydrates. The two hydroxyl groups in a geminal diol are easily converted to a
carbonyl or keto group C=O by loss of one water molecule. Conversely, a keto group can combine with water to form the geminal hydroxyl groups. The equilibrium in water solution may be shifted towards either compound. For example, the
equilibrium constant for the conversion of
acetone to
propane-2,2-diol is about 10−3, while that of
formaldehyde to methanediol is 103. For conversion of
hexafluoroacetone to the diol , the constant is about 10+6, due to the electron withdrawing effect of the
trifluoromethyl groups. Similarly, the conversion of
chloral to chloral hydrate is strongly favored by influence of the
trichloromethyl group. In some cases, such as
decahydroxycyclopentane and
dodecahydroxycyclohexane, the geminal diol is stable while the corresponding ketone is not. Geminal diols can also be viewed as extreme cases of
hemiacetals, formed by reaction of carbonyl compounds with water, instead of with an alcohol. ==See also==