Diol

Ethylene glycol, a simple diol

A diol or glycol is a chemical compound containing two hydroxyl groups (-OH groups) ^[1]

A vicinal diol is a diol with two hydroxyl groups in vicinal positions, that is, attached to adjacent atoms. Examples include 1,2-ethanediol or ethylene glycol HO-(CH₂)₂-OH, a common ingredient of antifreeze products; and propane-1,2-diol or propylene glycol, HO-CH₂-CH(OH)-CH₃.

A geminal diol has two hydroxyl groups bonded to the same atom. Examples include methanediol H₂C(OH)₂ and 1,1,1,3,3,3-hexafluoropropane-2,2-diol (F₃C)₂C(OH)₂, the hydrated form of hexafluoroacetone.

Examples of diols in which the hydroxyl functional groups are more widely separated include 1,4-butanediol HO-(CH₂)₄-OH and bisphenol A.

Synthesis

Because diols are a common functional group arrangement, numerous methods of preparation have been developed.

• Vicinal diols can be produced from the oxidation of alkenes, usually with dilute acidic potassium permanganate, also 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 vicinal diols.
• hydrogen peroxide reacts with an alkene to the epoxide and then by saponification to the diol for example in the synthesis of trans-cyclohexanediol batch ^[2] or by microreactor ^[3]:

• A chemical reaction called Sharpless asymmetric dihydroxylation can be used to produce chiral diols from alkenes using an osmate reagent and a chiral catalyst.
• Another method is the Woodward cis-hydroxylation (cis diol) and the related Prévost reaction (anti diol), depicted below, which both use iodine and the silver salt of a carboxylic acid.

• In the Prins reaction 1,3-diols can be formed in a reaction between an alkene and formaldehyde.
• Geminal diols can be formed by the hydration of ketones.

Reactions

General diols

A diol reacts like an alcohol, such as esterification and ether formation.

Diols such as ethylene glycol are used as co-monomers in polymerization reactions forming polymers including some polyesters and polyurethanes. A different monomer with two identical functional groups, such as a dioyl dichloride or dioic acid is required to continue the process of polymerization through repeated esterification processes.

A diol can be converted to cyclic ether by using an acid catalyst, this is diol cyclization. Firstly, it involves protonation of the hydroxyl group. Then, followed by intramolecular nucleophilic substitution, the second hydroxyl group attacks the electron deficient carbon. Provided that there are enough carbon atoms that the angle strain is not too much, a cyclic ether can be formed.

Vicinal diols

In glycol cleavage, the C-C bond in a vicinal diol is cleaved with formation of ketone or aldehyde functional groups.

Geminal diols

In general, organic geminal diols readily dehydrate to form a carbonyl group. For example, carbonic acid ((HO)₂C=O) is unstable and has a tendency to convert to carbon dioxide (CO₂) and water (H₂O). Nevertheless, in rare situations the chemical equilibrium is in favor of the geminal diol. For example, when formaldehyde (H₂C=O) is dissolved in water the geminal diol (H₂C(OH)₂), methanediol, is favored. Other examples are the cyclic geminal diols decahydroxycyclopentane (C₅(OH)₁₀) and dodecahydroxycyclohexane (C₆(OH)₁₂), which are stable, whereas the corresponding oxocarbons (C₅O₅ and C₆O₆) do not seem to be.

See also

• Alcohols, chemical compounds with one hydroxyl group
• Triols, chemical compounds with three hydroxyl group
• Polyols, chemical compounds with multiple hydroxyl groups
• Ethylene glycol

References