Definitions

-diol

Diol

[dahy-awl, -ol]
A diol or glycol is a chemical compound containing two hydroxyl groups (-OH groups) Vicinal diols have hydroxyl groups attached to adjacent atoms. Examples of vicinal diol compounds are ethylene glycol and propylene glycol. Geminal diols have hydroxyl groups bonded to the same atom. 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.

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

Synthesis of diols

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 or by microreactor :

Reactions

See also

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

References

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