Diol

A diol is a chemical compound containing two hydroxyl groups (-OH groups).

Vicinal diols have hydroxyl groups bonded to atoms next to each other, i. e. bonded to each other. Examples of vicinal diol compounds are ethylene glycol and propylene glycol.

Geminal diols have hydroxyl groups bonded to the same atom. When the atom is a carbon atom in an organic compound, the diol section of the molecule is unstable and has a tendency to dehydrate forming a carbonyl group. For example, carbonic acid (HO-CO-OH) is unstable and has a tendency to convert to carbon dioxide (CO₂) and water (H₂O). Only when formaldehyde (H-CO-H) is dissolved in water is the chemical equilibrium in favor of the geminal diol version of the compound called formalin [H-C(OH)₂-H].

Synthesis of diols

Diols are produced from the oxidation of alkenes, usually with acidic potassium permanganate (also known as potassium manganate (VII)). Alkenes are produced from the catalytic cracking of longer alkanes found in crude oil. Using alkaline potassium manganate(VII) produces a colour change from clear deep purple to clear colourless or clear green.

Reactions

A diol reacts like an alcohol.