R2COH2

R2COH2 denotes a geminal diol, a molecule in which a carbon atom bonded to two R groups also bears two hydroxyl groups. In organic notation it is often viewed as the hydration product of a ketone, R2C=O, by the addition of water: R2C=O + H2O ⇌ R2C(OH)2. The term R2COH2 emphasizes the two hydroxyl substituents on the same carbon as the two organic substituents.

Structurally, R2COH2 features a tetrahedral carbon center carrying two alkyl or aryl groups (the R's) and two OH groups. Because the two OH groups are on the same carbon, geminal diols are typically highly reactive toward dehydration back to the carbonyl compound, especially for simple ketones. The stability of these diols is highly dependent on the R groups; many ketone-derived geminal diols are not isolated under ordinary conditions and exist only in trace amounts in aqueous solution.

Formation and equilibrium conditions vary. Hydration of ketones to form R2C(OH)2 generally proceeds slowly in neutral water and can be catalyzed by acids or bases. The equilibrium favors the carbonyl form for most ketones, making the diol a minor component in solution. Formaldehyde is a notable exception, where the hydrate methanediol is relatively stabilized in water.

Reactivity and relevance. Geminal diols can serve as intermediates in organic reactions and in the mechanism of carbonyl hydration in aqueous media. They are also discussed in the context of acetal chemistry, since further reaction of geminal diols can lead to acetal formation under acidic conditions.

See also: geminal diol, hydration of carbonyls, acetals.