Diol

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Glycols
)
Ethylene glycol, a common diol

A diol is a

hydroxyl groups (−OH groups).[1] An aliphatic diol is also called a glycol.[2] This pairing of functional groups is pervasive, and many subcategories have been identified. They are used as protecting groups of carbonyl groups, making them essential in synthesis of organic chemistry.[3]

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

Propane-2,2-diol, an example of a geminal diol

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.[4] 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

See also: Dihydroxylation

In a vicinal diol, the two hydroxyl groups occupy

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 epoxidation of the alkene. An example in the synthesis of trans-cyclohexanediol[6] or by microreactor:[7]

A route to synthesizing trans-1,2-diols

For academic research and pharmaceutical areas, vicinal diols are often produced from the

catalyst. Another method is the Woodward cis-hydroxylation (cis diol) and the related Prévost reaction
(anti diol), which both use iodine and the silver salt of a carboxylic acid.

A route to synthesizing cis-1,2-diols using osmium tetraoxide
An example of Prévost reaction used to synthesize anti diol

Other routes to vic-diols are the hydrogenation of

pinacol coupling
reaction.

1,3-Diols

1,3-Diols are often prepared industrially by aldol condensation of ketones with formaldehyde. You can use many different starting material to produce syn- or anti-1,3-diols.[10] 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

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.

Zincophorin depicting syn and anti-1,3-diol configurations

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 polyurethanes.[11]

Reactions

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

alkyd resins.[11]

General diols

Diols react as

esterification and ether formation.[12]

Diols such as ethylene glycol are used as co-monomers in polymerization reactions forming polymers including some polyesters and polyurethanes.[12] 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.

A common diol reaction to produce a cyclic ether




1,2-diols and 1,3-diols can be protected using a protecting group.[13] Protecting groups are used so that the functional group does not react to future reactions. Benzylidene groups are used to protect 1,3-diols.[13] There are extremely useful in biochemistry as shown below of a carbohydrate derivative being protected.

A reaction that protects 1,3-diol using a benzylidene group.

Diols can also be used to protect carbonyl groups.[14] They are commonly used and are quite efficient at synthesizing cyclic acetals. These protect the carbonyl groups from reacting from any further synthesis until it is necessary to remove them. The reaction below depicts a diol being used to protect a carbonyl using zirconium tetrachloride.[15]

Diols can also be converted to lactones employing the Fétizon oxidation reaction.

Vicinal diols

In

Diol oxidation
.

Geminal diols

In general, organic geminal diols readily dehydrate to form a carbonyl group.

See also

Wikimedia Commons has media related to Diols.

References

  1. OCLC 642506595
    .
  2. doi:10.1351/goldbook.D01748
    .
  3. ^ "Carbonyl Protecting Groups - Stability". www.organic-chemistry.org. Retrieved 2024-04-15.
  4. ^ Gevorg, Dr S. (2021-11-22). "Diols: Nomenclature, Preparation, and Reactions". Chemistry Steps. Retrieved 2024-04-15.
  5. ^ "Illustrated Glossary of Organic Chemistry - Glycol". www.chem.ucla.edu. Retrieved 2024-04-15.
  6. ^ trans-cyclohexanediol Organic Syntheses, Coll. Vol. 3, p. 217 (1955); Vol. 28, p.35 (1948) http://www.orgsynth.org/orgsyn/pdfs/CV3P0217.pdf.
  7. doi:10.1021/jo701758p
    .
  8. ISBN 978-1-951693-98-5
    .
  9. doi:10.15227/orgsyn.036.0012
    .
  10. ISSN 0039-7881
    .
  11. ^
    ISBN 978-3527306732
    .
  12. ^ a b Gevorg, Dr S. (2021-11-22). "Diols: Nomenclature, Preparation, and Reactions". Chemistry Steps. Retrieved 2024-04-15.
  13. ^
    PMID 37590710
    , retrieved 2024-04-14
  14. ^ Angewandte Chemie International Edition in English. Wiley.
  15. ^ "Zirconium Tetrachloride (ZrCl4) Catalyzed Highly Chemoselective and Efficient Acetalization of Carbonyl Compounds". www.organic-chemistry.org. Retrieved 2024-04-14.
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