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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 *.
Synthesis of diols
Diols can be produced from the oxidation of alkenes, usually with dilute acidic potassium permanganate known as potassium manganate(VII). Using alkaline potassium manganate(VII) produces a colour change from clear deep purple to clear green; acidic potassium manganate(VII) turns clear colourless. Osmium tetroxide can similarly be used to oxidize alkenes to diols. A chemical reaction called Sharpless bishydroxylation can be used to yield chiral diols from alkenes using an osmate reagent and a chiral catalyst.
In the Prins reaction 1,3-diols can be formed in a reaction beteen an alkene and formaldehyde.
Reactions
A diol reacts like an alcohol. Diols such as ethylene glycol are used as co-monomers in some polymerization reactions forming polymers such as certain polyesters and polyurethanes.
See also glycol cleavage.
Examples
See also
"Diol"