Glycols
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−(CH2)4−OH and bisphenol A, and propylene-1,3-diol, or beta propylene glycol, HO−CH2−CH2−CH2−OH.
A geminal diol has two hydroxyl groups bonded to the same atom. These species arise by hydration of the carbonyl compounds. The hydration is usually unfavorable, but a notable exception is formaldehyde which, in water, exists in equilibrium with methanediol H2C(OH)2. Another example is (F3C)2C(OH)2, the hydrated form of hexafluoroacetone; in this case the hydration is made favorable by the electron-withdrawing trifluoromethyl groups.
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.
On commercial scales, the main route to vicinal diols is the hydrolysis of epoxides. The epoxides are prepared by oxidation with hydrogen peroxide of the alkene. An example in the synthesis of trans-cyclohexanediol or by microreactor:
For academic research and pharmaceutical areas, vicinal diols are often produced from the oxidation of alkenes, usually with dilute acidic potassium permanganate. 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. The 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.
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