Diol

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Ethylene glycol, a common diol Ethylene glycol.svg
Ethylene glycol, a common diol

A diol is a chemical compound containing two 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]

Contents

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 Geminal Diols.png
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

In a vicinal diol, the two hydroxyl groups occupy vicinal positions, that is, they are attached to adjacent atoms. These compounds are called glycols. [5] Examples include ethane-1,2-diol 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 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 Synthesizing trans-1,2-diols.png
A route to synthesizing trans-1,2-diols

For academic research and pharmaceutical areas, vicinal diols are often produced from the oxidation of alkenes, usually with dilute acidic potassium permanganate or Osmium tetroxide. [8] 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), which both use iodine and the silver salt of a carboxylic acid.

A route to synthesizing cis-1,2-diols using osmium tetraoxide Synthesizing Cis-1,2-Diols.png
A route to synthesizing cis-1,2-diols using osmium tetraoxide
An example of Prevost reaction used to synthesize anti diol PrevostReaction.svg
An example of Prévost reaction used to synthesize anti diol

Other routes to vic-diols are the hydrogenation of acyloins [9] and the 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 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.

Zincophorin depicting syn and anti-1,3-diol configurations Zincophorin showing syn and anti 1,3-diols.jpg
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 polyurethanes and alkyd resins. [11]

General diols

Diols react as alcohols, by 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 Pentanediol cyclization.svg
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. G198-benzylprtct-Image001.jpg
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]

Carbonyl Protection.jpg

Diols can also be converted to lactones 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.

See also

Related Research Articles

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In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl functional group bound to a saturated carbon atom. Alcohols range from the simple, like methanol and ethanol, to complex, like sugar alcohols and cholesterol. The presence of an OH group strongly modifies the properties of hydrocarbons, conferring hydrophilic (water-loving) properties. The OH group provides a site at which many reactions can occur.

<span class="mw-page-title-main">Alkene</span> Hydrocarbon compound containing one or more C=C bonds

In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or in the terminal position. Terminal alkenes are also known as α-olefins.

<span class="mw-page-title-main">Ketone</span> Organic compounds of the form >C=O

In organic chemistry, a ketone is an organic compound with the structure R−C(=O)−R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group −C(=O)−. The simplest ketone is acetone, with the formula (CH3)2CO. Many ketones are of great importance in biology and in industry. Examples include many sugars (ketoses), many steroids, and the solvent acetone.

<span class="mw-page-title-main">Aldehyde</span> Organic compound containing the functional group R−CH=O

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<span class="mw-page-title-main">Acetal</span> Organic compound with the structure >C(O–)2

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<span class="mw-page-title-main">Epoxide</span> Organic compounds with a carbon-carbon-oxygen ring

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<span class="mw-page-title-main">Dicarbonyl</span> Molecule containing two adjacent C=O groups

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Glycol cleavage is a specific type of organic chemistry oxidation. The carbon–carbon bond in a vicinal diol (glycol) is cleaved and instead the two oxygen atoms become double-bonded to their respective carbon atoms. Depending on the substitution pattern in the diol, these carbonyls will be ketones and/or aldehydes.

<span class="mw-page-title-main">Prins reaction</span> Chemical reaction involving organic compounds

The Prins reaction is an organic reaction consisting of an electrophilic addition of an aldehyde or ketone to an alkene or alkyne followed by capture of a nucleophile or elimination of an H+ ion. The outcome of the reaction depends on reaction conditions. With water and a protic acid such as sulfuric acid as the reaction medium and formaldehyde the reaction product is a 1,3-diol (3). When water is absent, the cationic intermediate loses a proton to give an allylic alcohol (4). With an excess of formaldehyde and a low reaction temperature the reaction product is a dioxane (5). When water is replaced by acetic acid the corresponding esters are formed.

Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters where the carbon carries a higher oxidation state. The reaction mainly applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.

The Criegee oxidation is a glycol cleavage reaction in which vicinal diols are oxidized to form ketones and aldehydes using lead tetraacetate. It is analogous to the use of periodate but uses a milder oxidant. This oxidation was discovered by Rudolf Criegee and coworkers and first reported in 1931 using ethylene glycol as the substrate.

References

  1. ↑ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 3rd edition, New York: Wiley, ISBN   9780471854722, OCLC   642506595 .
  2. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " diols ". 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. ↑ Advantages of Synthesizing trans-1,2-Cyclohexanediol in a Continuous Flow Microreactor over a Standard Glass Apparatus Andreas Hartung, Mark A. Keane, and Arno Kraft J. Org. Chem. 2007, 72, 10235–10238 doi : 10.1021/jo701758p.
  8. ↑ McMurry, John (September 20, 2023). Organic Chemistry: A Tenth Edition (1st ed.). Rice University. pp. 259–260. ISBN   978-1-951693-98-5.
  9. ↑ Blomquist, A. T.; Goldstein, Albert (1956). "1,2-Cyclodecanediol". Organic Syntheses. 36: 12. doi:10.15227/orgsyn.036.0012.
  10. ↑ Bode, Silke E.; Wolberg, Michael; Müller, Michael (2006). "Stereoselective Synthesis of 1,3-Diols". Synthesis (in German). 2006 (04): 557–588. doi:10.1055/s-2006-926315. ISSN   0039-7881.
  11. 1 2 Werle, Peter; Morawietz, Marcus; Lundmark, Stefan; Sörensen, Kent; Karvinen, Esko; Lehtonen, Juha (2008). "Alcohols, Polyhydric". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a01_305.pub2. ISBN   978-3527306732.
  12. 1 2 Gevorg, Dr S. (2021-11-22). "Diols: Nomenclature, Preparation, and Reactions". Chemistry Steps. Retrieved 2024-04-15.
  13. 1 2 Manabe, Shino (2021), Nishihara, Shoko; Angata, Kiyohiko; Aoki-Kinoshita, Kiyoko F.; Hirabayashi, Jun (eds.), "Benzylidene protection of diol", Glycoscience Protocols (GlycoPODv2), Saitama (JP): Japan Consortium for Glycobiology and Glycotechnology, 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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