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{{short description|Chemical compound containing two hydroxyl (–OH) groups}}
[[Image:Ethylene glycol.svg|thumb|right|[[Ethylene glycol]], a common diol]]

A '''diol''' is a [[chemical compound]] containing two [[hydroxyl group]]s ({{chem2|\sOH}} groups).<ref>{{JerryMarch}}.</ref>  An [[Aliphatic compound|aliphatic]] diol may also be called a '''glycol'''.<ref>{{GoldBookRef|title=diols |file=D01748}}.</ref> This pairing of [[functional group]]s is pervasive, and many subcategories have been identified. They are used as [[protecting group]]s of [[carbonyl group]]s, making them essential in synthesis of organic chemistry.<ref>{{Cite web |title=Carbonyl Protecting Groups - Stability |url=https://www.organic-chemistry.org/protectivegroups/carbonyl.shtm |access-date=2024-04-15 |website=www.organic-chemistry.org}}</ref>

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 {{chem2|HO\s(CH2)4\sOH}} and [[1,3-Propanediol|propylene-1,3-diol]], or beta propylene glycol, {{chem2|HO\sCH2\sCH2\sCH2\sOH}}.

== Synthesis of classes of diols ==
=== Geminal diols ===
[[File:Geminal Diols.png|thumb|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]] H<sub>2</sub>C(OH)<sub>2</sub>.<ref>{{Cite web |last=Gevorg |first=Dr S. |date=2021-11-22 |title=Diols: Nomenclature, Preparation, and Reactions |url=https://www.chemistrysteps.com/diols-nomenclature-preparation-and-reactions/ |access-date=2024-04-15 |website=Chemistry Steps |language=en-US}}</ref> Another example is (F<sub>3</sub>C)<sub>2</sub>C(OH)<sub>2</sub>, the hydrated form of [[hexafluoroacetone]].  Many gem-diols undergo further condensation to give dimeric and oligomeric derivatives.  This reaction applies to [[glyoxal]] and related [[aldehyde]]s.

=== Vicinal diols ===
{{See also|Dihydroxylation}}
In a vicinal diol, the two hydroxyl groups occupy [[vicinal (chemistry)|vicinal]] positions, that is, they are attached to adjacent atoms. These compounds are called glycols<ref>{{Cite web |title=Illustrated Glossary of Organic Chemistry - Glycol |url=http://www.chem.ucla.edu/~harding/IGOC/G/glycol.html |access-date=2024-04-15 |website=www.chem.ucla.edu}}</ref> (though the term can be used more widely).  Examples include ethane-1,2-diol or [[Ethylene_glycol | ethylene glycol]] HO−(CH<sub>2</sub>)<sub>2</sub>−OH, a common ingredient of [[antifreeze]] products. Another example is [[propane-1,2-diol]], or alpha propylene glycol, HO−CH<sub>2</sub>−CH(OH)−CH<sub>3</sub>, 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<ref>trans-cyclohexanediol [[Organic Syntheses]], Coll. Vol. 3, p. 217 (1955); Vol. 28, p.35 ('''1948''') http://www.orgsynth.org/orgsyn/pdfs/CV3P0217.pdf.</ref> or by [[microreactor]]:<ref>''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}}.</ref>
:[[File:Synthesizing trans-1,2-diols.png|thumb|center|627x627px|A route to synthesizing trans-1,2-diols|class=skin-invert-image]]

For academic research and pharmaceutical areas, vicinal diols are often produced from the [[oxidation]] of [[Alkene|alkenes]], usually with dilute [[acidic]] [[potassium permanganate]] or Osmium tetroxide.<ref>{{Cite book |last=McMurry |first=John |title=Organic Chemistry: A Tenth Edition |publisher=Rice University |date=September 20, 2023 |isbn=978-1-951693-98-5 |edition=1st |pages=259–260}}</ref> [[Osmium tetroxide]] can similarly be used to oxidize alkenes to vicinal diols. The chemical reaction called [[Sharpless asymmetric dihydroxylation]] can be used to produce [[Chirality (chemistry)|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.
:[[File:Synthesizing Cis-1,2-Diols.png|thumb|center|632x632px|A route to synthesizing cis-1,2-diols using osmium tetraoxide|class=skin-invert-image]][[File:PrevostReaction.svg|thumb|center|938x938px|An example of Prévost reaction used to synthesize anti diol|class=skin-invert-image]]

Other routes to vic-diols are the hydrogenation of [[acyloin]]s<ref>{{cite journal|last1=Blomquist|first1=A. T.|last2=Goldstein|first2=Albert|title=1,2-Cyclodecanediol|journal=Organic Syntheses|date=1956|volume=36|page=12|doi =10.15227/orgsyn.036.0012}}</ref> 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 materials to produce syn- or anti-1,3-diols.<ref>{{Cite journal |last1=Bode |first1=Silke E. |last2=Wolberg |first2=Michael |last3=Müller |first3=Michael |date=2006 |title=Stereoselective Synthesis of 1,3-Diols |url=http://www.thieme-connect.de/DOI/DOI?10.1055/s-2006-926315 |journal=Synthesis |language=de |volume=2006 |issue=4 |pages=557–588 |doi=10.1055/s-2006-926315 |issn=0039-7881|url-access=subscription }}</ref> The resulting carbonyl is reduced using the [[Cannizzaro reaction]] or by catalytic [[hydrogenation]]:
: RC(O)CH<sub>3</sub>  +  CH<sub>2</sub>O   →   RC(O)CH<sub>2</sub>CH<sub>2</sub>OH
: RC(O)CH<sub>2</sub>CH<sub>2</sub>OH  +  H<sub>2</sub>   →   RCH(OH)CH<sub>2</sub>CH<sub>2</sub>OH  
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 [[diastereoselectivity|diastereoselectively]] from the corresponding β-hydroxy [[ketone]]s using the [[Evans–Saksena reduction|Evans–Saksena]], [[Narasaka–Prasad reduction|Narasaka–Prasad]] or [[Evans–Tishchenko reaction|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.
:[[File:Zincophorin showing syn and anti 1,3-diols.svg|thumb|500px|Zincophorin depicting syn and anti-1,3-diol configurations|none]]

=== 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 acid]]s: 
:(CH<sub>2</sub>)<sub>n</sub>(CO<sub>2</sub>R)<sub>2</sub>  +  4 H<sub>2</sub>   →   (CH<sub>2</sub>)<sub>n</sub>(CH<sub>2</sub>OH)<sub>2</sub>  +  2 H<sub>2</sub>O  +  2 ROH
[[1,4-butanediol]], [[1,5-Pentanediol|1,5-pentanediol]], [[1,6-Hexanediol|1,6-hexanediol]], and {{ill|1,10-decanediol|es|1,10-decanodiol}} are important precursors to [[polyurethane]]s.<ref name=Ull>{{Ullmann |first1=Peter |last1=Werle |first2=Marcus |last2=Morawietz |first3=Stefan |last3=Lundmark |first4=Kent |last4=Sörensen |first5=Esko |last5=Karvinen |first6=Juha |last6=Lehtonen|title=Alcohols, Polyhydric|year=2008|doi=10.1002/14356007.a01_305.pub2}}</ref>

== Reactions ==
From the industrial perspective, the dominant reactions of the diols is in the production of [[polyurethane]]s and [[alkyd resin]]s.<ref name=Ull/>

=== General diols ===
Diols react as [[Alcohol (chemistry)|alcohol]]s, by [[esterification]] and [[ether]] formation.<ref name=":0">{{Cite web |last=Gevorg |first=Dr S. |date=2021-11-22 |title=Diols: Nomenclature, Preparation, and Reactions |url=https://www.chemistrysteps.com/diols-nomenclature-preparation-and-reactions/ |access-date=2024-04-15 |website=Chemistry Steps |language=en-US}}</ref> 
 
Diols such as [[ethylene glycol]] are used as co-[[monomer]]s in [[polymerization]] reactions forming [[polymer]]s including some [[polyester]]s and [[polyurethane]]s.<ref name=":0" /> 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.
[[File:Pentanediol cyclization.svg|thumb|943x943px|A common diol reaction to produce a cyclic ether|none]]


1,2-diols and 1,3-diols can be protected using a protecting group.<ref name=":2">{{Citation |last=Manabe |first=Shino |title=Benzylidene protection of diol |date=2021 |work=Glycoscience Protocols (GlycoPODv2) |editor-last=Nishihara |editor-first=Shoko |url=https://www.ncbi.nlm.nih.gov/books/NBK593981/ |access-date=2024-04-14 |place=Saitama (JP) |publisher=Japan Consortium for Glycobiology and Glycotechnology |pmid=37590710 |editor2-last=Angata |editor2-first=Kiyohiko |editor3-last=Aoki-Kinoshita |editor3-first=Kiyoko F. |editor4-last=Hirabayashi |editor4-first=Jun}}</ref> Protecting groups are used so that the functional group does not react to future reactions. Benzylidene groups are used to protect 1,3-diols.<ref name=":2" /> There are extremely useful in biochemistry as shown below of a carbohydrate derivative being protected.
[[File:G198-benzylprtct-Image001.jpg|center|thumb|501x501px|A reaction that protects 1,3-diol using a benzylidene group.]]
Diols can also be used to protect carbonyl groups.<ref>{{Cite book |url=http://dx.doi.org/10.1002/(issn)1521-3773a |title=Angewandte Chemie International Edition in English |date=2018 |publisher=Wiley|doi=10.1002/(issn)1521-3773a }}</ref> 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.<ref>{{Cite web |title=Zirconium Tetrachloride (ZrCl4) Catalyzed Highly Chemoselective and Efficient Acetalization of Carbonyl Compounds |url=https://www.organic-chemistry.org/abstracts/literature/813.shtm |access-date=2024-04-14 |website=www.organic-chemistry.org}}</ref>
[[File:Carbonyl_Protection.jpg|center|thumb|592x592px]]
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 (chemistry)|vicinal]] diol is cleaved with formation of ketone or aldehyde functional groups. See [[Diol oxidation]].

=== Geminal diols ===
In general, organic geminal diols readily [[dehydration reaction|dehydrate]] to form a [[carbonyl group]].

== See also ==
{{Commons category|Diols}}
* [[Alcohol (chemistry)|Alcohol]]s, chemical compounds with at least one [[hydroxyl]] group
* [[Triol]]s, chemical compounds with three hydroxyl groups
* [[Polyol]]s, chemical compounds with multiple hydroxyl groups
* [[Ethylene glycol]]
* [[Glycol nucleic acid]] (GNA)

== References ==
{{Reflist}}

{{Authority control}}
[[Category:Functional groups]]
[[Category:Diols| ]]