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A diol is a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, m ...
containing two
hydroxyl group A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group ( ...

hydroxyl group
s (−OH groups). An
aliphatic In organic chemistry, hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
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 Ethylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecula ...

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
propylene-1,3-diol
, or beta propylene glycol, HO−CH2−CH2−CH2−OH.


Synthesis of classes of diols


Geminal diols

A
geminal diol A geminal diol (or gem-diol for short) is any organic compound having two hydroxyl functional groups (-oxygen, Ohydrogen, H) covalent bond, bound to the same carbon atom. Geminal diols are a subclass of the diols, which in turn are a special class ...
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 Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
which, in water, exists in equilibrium with
methanediol Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chem ...
H2C(OH)2. Another example is (F3C)2C(OH)2, the hydrated form of
hexafluoroacetone Hexafluoroacetone (HFA) is a chemical compound with the chemical formula, formula (CF3)2CO. It is structurally similar to acetone; however, its reactivity is markedly different. It a colourless, hygroscopic, nonflammable, highly reactive gas chara ...
. 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 An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression Freezing-point depression is a drop in the temperature at which a substance freezing, free ...

antifreeze
products. Another example is
propane-1,2-diol Propylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecu ...
, 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 An epoxide is a cyclic ether Ethers are a class of organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to ...

epoxide
s. The epoxides are prepared by epoxidation of the alkene. An example in the synthesis of trans-cyclohexanediol or by
microreactor use micromachining techniques to miniaturize the reactor design. Applications include fuel processors for generating hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atom ...
: : For academic research and pharmaceutical areas, vicinal diols are often produced from the
oxidation Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

oxidation
of
alkene In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo dur ...

alkene
s, usually with dilute
acidic An acid is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms ...
potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4 and composed of potassium ion, K+ and permanganate, . It is a purplish-black crystalline salt, that dissolves in water to give intensely pink or purple solutions. P ...

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 Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass ...

Osmium tetroxide
can similarly be used to oxidize alkenes to vicinal diols. The chemical reaction called
Sharpless asymmetric dihydroxylation Sharpless asymmetric dihydroxylation (also called the Sharpless bishydroxylation) is the chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, ch ...
can be used to produce
chiral Chirality is a property of important in several branches of science. The word ''chirality'' is derived from the (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from its ; that is, i ...
diols from alkenes using an osmate
reagent A reagent is a substance or compound added to a system to cause a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...
and a chiral
catalyst that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air. Catalysis () is the process of increasing the reaction rate, rate of a ...

catalyst
. Another method is the Woodward cis-hydroxylation (cis diol) and the related
Prévost reactionThe Prévost reaction is chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of matter having constan ...
(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 Acyloins or α-hydroxy ketones are a class of organic compounds which all possess a hydroxy group adjacent to a ketone group. The named acyloin is derived from the fact that they are formally derived from reductive coupling of carboxylic acyl gro ...

acyloin
s and the
pinacol couplingA pinacol coupling reaction is an organic reactionImage:Claisen Rearrangement Scheme.png, 400px, The Claisen rearrangementOrganic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition re ...
reaction.


1,3-Diols

1,3-Diols are often prepared industrially by
aldol condensation An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl Chemical compound, compound to form a β-hydroxyaldehyde or β-hydroxyketone (an aldol reaction), followed by dehydration ...

aldol condensation
of ketones with
formaldehyde Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
. The resulting carbonyl is reduced using the
Cannizzaro reaction The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base (chemistry), base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carb ...
or by catalytic
hydrogenation Hydrogenation is a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of matter having constant ...

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
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 Hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group (CHO) and a hydrogen atom to a carbon-carbon do ...

hydroformylation
of epoxides followed by hydrogenation of the aldehyde. This method has been used for 1,3-propanediol from
ethylene oxide Ethylene oxide is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation ...

ethylene oxide
. More specialized routes to 1,3-diols involves the reaction between an
alkene In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo dur ...

alkene
and
formaldehyde Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
, the
Prins reaction
Prins reaction
. 1,3-diols can be produced
diastereoselectively
diastereoselectively
from the corresponding β-hydroxy
ketone In chemistry, a ketone is a functional group with the structure R2C=O, where R can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond). The simplest ketone is acetone (R = R' = methyl) ...
s using the Evans–Saksena,
Narasaka–Prasad
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
Zincophorin
is a
natural product A natural product is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together b ...
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 acidA dicarboxylic acid is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecul ...
s: :(CH2)n(CO2R)2 + 4 H2 → (CH2)n(CH2OH)2 + 2 H2O + 2 ROH
1,4-butanediol 1,4-Butanediol, colloquially known as BD, is a primary alcohol A primary alcohol is an alcohol In , alcohol is an that carries at least one (−OH) bound to a atom. The term alcohol originally referred to the primary alcohol (ethy ...

1,4-butanediol
, 1,5-pentanediol,
1,6-hexanediol
1,6-hexanediol
, 1,10-decanediol are important precursors to
polyurethane Polyurethane (often abbreviated PUR and PU) referes to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane ...

polyurethane
s.


Reactions

From the industrial perspective, the dominant reactions of the diols is in the production of
polyurethane Polyurethane (often abbreviated PUR and PU) referes to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane ...

polyurethane
s and
alkyd resin An alkyd is a polyester Polyester is a category of polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecule ...
s.


General diols

Diols react as
alcohol In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...

alcohol
s, by
esterification An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl ( alkoxy) group, as in the substitution reaction of a carboxylic acid and an alcohol. Glycer ...

esterification
and
ether Ethers are a class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reaction ...

ether
formation. Diols such as
ethylene glycol Ethylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecula ...

ethylene glycol
are used as co-
monomer In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

monomer
s in
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks.Clayden, J ...
reactions forming
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s including some
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
s and
polyurethane Polyurethane (often abbreviated PUR and PU) referes to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane ...

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 . 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
lactoneLactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure (), or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring. Lactones are formed by intramolecular esterification of the co ...

lactone
s employing the Fétizon oxidation reaction.


Vicinal diols

In , 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 In physiology, dehydration is a lack of total body water, with an accompanying disruption of Metabolism, metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temper ...
to form a
carbonyl group In organic chemistry Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . S ...

carbonyl group
. For example,
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and s ...

carbonic acid
((HO)2C=O) is unstable and has a tendency to convert to
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
(CO2) and water (H2O). Nevertheless, in rare situations the
chemical equilibrium In a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any sub ...
is in favor of the geminal diol. For example, when
formaldehyde Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
(H2C=O) is dissolved in
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...
, the geminal diol (H2C(OH)2,
methanediol Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chem ...
) is favored. Other examples are the cyclic geminal diols (C5(OH)10) and
dodecahydroxycyclohexane Dodecahydroxycyclohexane is an organic chemistry, organic compound with molecular formula C6O12H12 or C6(OH)12. It is a sixfold geminal diol with a cyclohexane backbone and can be regarded as a sixfold hydrate of cyclohexanehexone (C6O6). Dihydrat ...
(C6(OH)12), which are stable, whereas the corresponding
oxocarbon An oxocarbon or oxide of carbon is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element hel ...
s (C5O5 and C6O6) do not seem to be.


See also

*
Alcohol In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...

Alcohol
s, chemical compounds with at least one
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the ...

hydroxyl
group * s, chemical compounds with three hydroxyl groups *
Polyol A polyol is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and ...
s, chemical compounds with multiple hydroxyl groups *
Ethylene glycol Ethylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecula ...

Ethylene glycol
* Glycol nucleic acid (GNA)


References

{{Authority control Functional groups