Ageminal diol (orgem-diol for short) is anyorganic compound having twohydroxylfunctional groups (−OH)bound to the samecarbon atom. Geminal diols are a subclass of thediols, which in turn are a special class ofalcohols. Most of the geminal diols are considered unstable.
The simplest geminal diol ismethanediolCH4O2 orH2C(OH)2. Other examples are:
Geminal diols can be viewed asketone (oraldehyde) hydrates. The two hydroxyl groups in a geminal diol are easily converted to acarbonyl 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, theequilibrium constant for the conversion ofacetone(H3C)2C=O topropane-2,2-diol(H3C)2C(OH)2 is about 10−3,[1] while that offormaldehydeH2C=O to methanediolH2C(OH)2 is 103.[2]
For conversion ofhexafluoroacetone(F3C)2C=O to the diol(F3C)2C(OH)2, the constant is about 10+6, due to the electron withdrawing effect of thetrifluoromethyl groups. Similarly, the conversion ofchloral(Cl3C)HC=O to chloral hydrate is strongly favored by influence of thetrichloromethyl group.
In some cases, such asdecahydroxycyclopentane anddodecahydroxycyclohexane, the geminal diol is stable while the corresponding ketone is not.
Geminal diols can also be viewed as extreme cases ofhemiacetals, formed by reaction of carbonyl compounds with water, instead of with an alcohol.