Geminal diol
A compound with two hydroxyl groups bonded to the same carbon atom. Geminal diols commonly form when water adds to a carbonyl group.
Carbonyl group: A functional group containing a carbon atom double-bonded to oxygen. Geminal diols form when water adds across this bond.
Hydroxyl group: The functional group consisting of an oxygen atom bonded to hydrogen, written −OH. A geminal diol contains two hydroxyl groups attached to one carbon.
Formaldehyde: The simplest aldehyde, with formula CH₂O, also called methanal. In water, formaldehyde is present predominantly as its geminal diol, methanediol.
Carbonyl compound: An organic compound containing a carbon–oxygen double bond, as in an aldehyde or ketone. A carbonyl compound is the dehydrated counterpart of its geminal diol.
Nucleophilic addition: A reaction in which a nucleophile bonds to an electrophilic atom, adding across a multiple bond. Water acts as the nucleophile that converts a carbonyl group into a geminal diol.
Geminal substituent: A substituent described as geminal when it is attached to the same atom as another substituent of interest. The two hydroxyl groups are geminal because they share a carbon atom.
Chloral hydrate: The crystalline geminal diol formed by hydration of chloral, or trichloroacetaldehyde. It is a stable, isolable hydrate of a carbonyl compound.
Hemiacetal: A compound whose carbon bears one hydroxyl group and one alkoxy group, along with two other substituents. A hemiacetal has one hydroxyl and one ether group where a geminal diol has two hydroxyls.
Hydration reaction: A chemical reaction that adds the elements of water to a molecule. Carbonyl hydration is the direct route from a carbonyl compound to its geminal diol.
Tetrahedral molecular geometry: A molecular geometry in which a central atom has four bonds directed toward the corners of a tetrahedron. Hydration changes the carbonyl carbon from trigonal planar to tetrahedral.