A diol is the chemical compound containing two hydroxyl groups −OH groups. An aliphatic diol is also called the glycol. This pairing of functional groups is pervasive, in addition to many subcategories shit been identified.
The almost common industrial diol is ethylene glycol. Examples of diols in which the hydroxyl functional groups are more widely separated put 1,4-butanediol HO−CH24−OH in addition to 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 variety arise by hydration of the carbonyl compounds. The hydration is normally unfavorable, but a notable exception is formaldehyde which, in water, exists in equilibrium with methanediol H2COH2. Another example is F3C2COH2, the hydrated earn believe of hexafluoroacetone. many gem-diols undergo further condensation to supply dimeric and oligomeric derivatives. This reaction applies to glyoxal and related aldehydes.
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−CH22−OH, a common bit of antifreeze products. Another example is propane-1,2-diol, or alpha propylene glycol, HO−CH2−CHOH−CH3, used in the food and medicine industry, as alive 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 epoxidation of the alkene. An example in the synthesis of trans-cyclohexanediol or by microreactor:
For academic research and pharmaceutical areas, vicinal diols are often featured from the oxidation of alkenes, commonly with dilute acidic potassium permanganate. Using alkaline potassium manganateVII produces a colour modify from realize deep purple to clear green; acidic potassium manganateVII 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.
Other routes to vic-diols are the hydrogenation of acyloins and the pinacol coupling reaction.
1,3-Diols are often prepared industrially by aldol condensation of ketones with formaldehyde. The resulting carbonyl is reduced using the Cannizzaro reaction or by catalytic hydrogenation:
2,2-Disubstituted propane-1,3-diols are prepared in this way. Examples include 2-methyl-2-propyl-1,3-propanediol and neopentyl glycol.
1,3-Diols can be prepared by hydration of α,β-unsaturated ketones and aldehydes. The resulting keto-alcohol is hydrogenated. Another route involves the hydroformylation of epoxides followed by hydrogenation of the aldehyde. This method has been used for 1,3-propanediol from ethylene oxide.
More specialized routes to 1,3-diols involves the reaction between an alkene and formaldehyde, the Prins reaction. 1,3-diols can be featured diastereoselectively from the corresponding β-hydroxy ketones using the Evans–Saksena, Narasaka–Prasad or Evans–Tishchenko reduction protocols.
1,3-Diols are pointed as syn or anti depending on the relative stereochemistries of the carbon atoms bearing the hydroxyl functional groups. Zincophorin is a natural product that contains both syn and anti 1,3-diols.
Diols where the hydroxyl groups are separated by several carbon centers are broadly prepared by hydrogenation of diesters of the corresponding dicarboxylic acids:
1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,10-decanediol are important precursors to polyurethanes.