A diol is a chemical compound containing two hydroxyl groups (—OH groups).[1] This pairing of functional groups is pervasive and many subcategories have been identified. The most common diols in nature are sugars and their polymers, cellulose. The most common industrial diol is ethyleneglycol. Examples of diols in which the hydroxyl functional groups are more widely separated include 1,4-butanediol HO—(CH2)4—OH and bisphenol A, and propylene-1,3-diol or beta propylene glycol, HO-CH2-CH2-CH2-OH.
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A geminal diol has two hydroxyl groups bonded to the same atom. Examples include methanediol H2C(OH)2, the hydrated form of formaldehyde. Another example is (F3C)2C(OH)2, the hydrated form of hexafluoroacetone.
In a vicinal diol, the two hydroxyl groups occupy vicinal positions, that is, they are attached to adjacent atoms. These compounds are called glycols. Examples include 1,2-ethanediol or ethylene glycol HO—(CH2)2—OH, a common ingredient of antifreeze products. Another example is propane-1,2-diol or alpha propylene glycol, HO—CH2—CH(OH)—CH3 used in the food and medicine industry as well as a relatively non-poisonous antifreeze product.
Bisphenol A is an important compound that contains two phenol groups. Phenols are aromatic alcohol.
| Linearity of the diol | Hydroxyls on adjacent carbons (vicinal diols) | Hydroxyls on non-adjacent carbons |
|---|---|---|
| Linear | Ethylene glycol | 1,3-Propanediol, 1,4-Butanediol, 1,5-Pentanediol, 1,8-octanediol, |
| Branched | 1,2-Propanediol, 1,2-Butanediol, 2,3-Butanediol | 1,3-Butanediol, 1,2-pentanediol, Etohexadiol, p-Menthane-3,8-diol, 2-Methyl-2,4-pentanediol |
Because diols are a common functional group arrangement, numerous methods of preparation have been developed.
Diols react as alcohols, by 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.
In glycol cleavage, the C-C bond in a vicinal diol is cleaved with formation of ketone or aldehyde functional groups. See Diol oxidation.
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.