Diol

A diol is a chemical compound containing two

hydroxyl group

s (−OH groups). An aliphatic diol is also called a glycol. 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

propylene-1,3-diol

, or beta propylene glycol, HO−CH₂−CH₂−CH₂−OH.

Synthesis of classes of diols

Geminal diols

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. Many gem-diols undergo further condensation to give dimeric and oligomeric derivatives. This reaction applies to glyoxal and related aldehydes.

Vicinal diols

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

epoxide

s. 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 produced from the

oxidation

of

alkene

s, 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.
:

Other routes to vic-diols are the hydrogenation of

acyloin

s and the pinacol coupling reaction.

1,3-Diols

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

:
: RC(O)CH₃ + CH₂O → RC(O)CH₂CH₂OH
: RC(O)CH₂CH₂OH + H₂ → RCH(OH)CH₂CH₂OH
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 produced

diastereoselectively

from the corresponding β-hydroxy ketones using the Evans–Saksena,

Narasaka–Prasad

or Evans–Tishchenko reduction protocols.

1,3-Diols are described 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.

1,4-, 1,5-, and longer diols

Diols where the hydroxyl groups are separated by several carbon centers are generally prepared by hydrogenation of diesters of the corresponding dicarboxylic acids:
:(CH₂)_n(CO₂R)₂ + 4 H₂ → (CH₂)_n(CH₂OH)₂ + 2 H₂O + 2 ROH

1,4-butanediol

, 1,5-pentanediol,

1,6-hexanediol

, 1,10-decanediol are important precursors to

polyurethane

s.

Reactions

From the industrial perspective, the dominant reactions of the diols is in the production of

polyurethane

s and alkyd resins.

General diols

Diols react as

alcohol

s, by

esterification

and

ether

formation.

Diols such as

ethylene glycol

are used as co-

monomer

s in polymerization reactions forming

polymer

s including some polyesters and

polyurethane

s. 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.

Diols can also be converted to

lactone

s employing the Fétizon oxidation reaction.

Vicinal diols

In

glycol cleavage

, the C−C bond in a vicinal diol is cleaved with formation of ketone or aldehyde functional groups. See Diol oxidation.

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

*

Alcohol

s, chemical compounds with at least one

hydroxyl

group
*

Triol

s, chemical compounds with three hydroxyl groups
* Polyols, chemical compounds with multiple hydroxyl groups
*

Ethylene glycol

* Glycol nucleic acid (GNA)

References

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Functional groups