Diol
A diol is a
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.[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
In a vicinal diol, the two hydroxyl groups occupy
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]
For academic research and pharmaceutical areas, vicinal diols are often produced from the
Other routes to vic-diols are the hydrogenation of
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
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
Reactions
From the industrial perspective, the dominant reactions of the diols is in the production of
General diols
Diols react as
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.
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.
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
Geminal diols
In general, organic geminal diols readily dehydrate to form a carbonyl group.
See also
- hydroxylgroup
- Triols, chemical compounds with three hydroxyl groups
- Polyols, chemical compounds with multiple hydroxyl groups
- Ethylene glycol
- Glycol nucleic acid (GNA)
References
- OCLC 642506595.
- .
- ^ "Carbonyl Protecting Groups - Stability". www.organic-chemistry.org. Retrieved 2024-04-15.
- ^ Gevorg, Dr S. (2021-11-22). "Diols: Nomenclature, Preparation, and Reactions". Chemistry Steps. Retrieved 2024-04-15.
- ^ "Illustrated Glossary of Organic Chemistry - Glycol". www.chem.ucla.edu. Retrieved 2024-04-15.
- ^ trans-cyclohexanediol Organic Syntheses, Coll. Vol. 3, p. 217 (1955); Vol. 28, p.35 (1948) http://www.orgsynth.org/orgsyn/pdfs/CV3P0217.pdf.
- .
- ISBN 978-1-951693-98-5.
- .
- ISSN 0039-7881.
- ^ ISBN 978-3527306732.
- ^ a b Gevorg, Dr S. (2021-11-22). "Diols: Nomenclature, Preparation, and Reactions". Chemistry Steps. Retrieved 2024-04-15.
- ^ PMID 37590710, retrieved 2024-04-14
- ^ Angewandte Chemie International Edition in English. Wiley.
- ^ "Zirconium Tetrachloride (ZrCl4) Catalyzed Highly Chemoselective and Efficient Acetalization of Carbonyl Compounds". www.organic-chemistry.org. Retrieved 2024-04-14.