A diol is a chemical compound containing two hydroxyl groups (−OH groups). This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is ethylene glycol.Examples of diols in which the hydroxyl functional groups are more widely separated include 1,4-butanediol HO−(CH 2) 4 −OH and bisphenol A, and propylene-1,3-diol, or beta propylene glycol, HO−CH 2 −CH 2 −CH 2 −OH.n general, organic geminal diols readily dehydrate to form a carbonyl group. For example, carbonic acid ((HO) 2 C=O) is unstable and has a tendency to convert to carbon dioxide (CO 2) and water (H 2 O). Nevertheless, in rare situations the chemical equilibrium is in favor of the geminal diol.