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A diol is a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
containing two
hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
s ( groups). An
aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane ...
diol is also called a glycol. This pairing of
functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...
s is pervasive, and many subcategories have been identified. The most common industrial diol is
ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an o ...
. Examples of diols in which the hydroxyl functional groups are more widely separated include 1,4-butanediol and propylene-1,3-diol, or beta propylene glycol, .


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 Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
which, in water, exists in equilibrium with methanediol H2C(OH)2. Another example is (F3C)2C(OH)2, the hydrated form of
hexafluoroacetone Hexafluoroacetone (HFA) is a chemical compound with the formula (CF3)2CO. It is structurally similar to acetone; however, its reactivity is markedly different. It a colourless, hygroscopic, nonflammable, highly reactive gas characterized by a mus ...
. 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 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 or by
microreactor A microreactor or microstructured reactor or microchannel reactor is a device in which chemical reactions take place in a confinement with typical lateral dimensions below 1 mm; the most typical form of such confinement are microchannels.''R ...
: : For academic research and pharmaceutical areas, vicinal diols are often produced from the
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
of
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
s, usually with dilute acidic 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 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 In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
and a chiral
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
. Another method is the Woodward cis-hydroxylation (cis diol) and the related Prévost reaction (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 acyloins and the
pinacol coupling A pinacol coupling reaction is an organic reaction in which a carbon–carbon bond is formed between the carbonyl groups of an aldehyde or a ketone in presence of an electron donor in a free radical process. The reaction product is a vicinal diol. ...
reaction.


1,3-Diols

1,3-Diols are often prepared industrially by aldol condensation of ketones with
formaldehyde Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
. The resulting carbonyl is reduced using the Cannizzaro reaction or by catalytic
hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic ...
: : 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 (IUPAC name: 2,2-dimethylpropane-1,3-diol) is an organic chemical compound. It is used in the synthesis of polyesters, paints, lubricants, and plasticizers. When used in the manufacture of polyesters, it enhances the stability ...
. 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 ...
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 In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
and
formaldehyde Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
, the
Prins reaction 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 c ...
. 1,3-diols can be produced diastereoselectively from the corresponding β-hydroxy
ketone In organic chemistry, a ketone is a functional group 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)– (which contains a carbon-oxygen double b ...
s 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.


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.


Reactions

From the industrial perspective, the dominant reactions of the diols is in the production of polyurethanes and alkyd resins.


General diols

Diols react as alcohols, by esterification and
ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again ...
formation. Diols such as
ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an o ...
are used as co-
monomer In chemistry, a monomer ( ; '' mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...
s in
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many f ...
reactions forming
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
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 natura ...
s and polyurethanes. 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. Diols can also be converted to lactones employing the
Fétizon oxidation Fétizon oxidation is the oxidation of primary and secondary alcohols utilizing the compound silver(I) carbonate absorbed onto the surface of celite also known as Fétizon's reagent first employed by Marcel Fétizon in 1968. It is a mild reagent, s ...
reaction.


Vicinal diols

In
glycol cleavage 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 substitutio ...
, 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 metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
to form a carbonyl group. For example, carbonic acid ((HO)2C=O) is unstable and has a tendency to convert to
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
(CO2) and water (H2O). Nevertheless, in rare situations the
chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the ...
is in favor of the geminal diol. For example, when
formaldehyde Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
(H2C=O) is dissolved in
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
, the geminal diol (H2C(OH)2, methanediol) is favored. Other examples are the cyclic geminal diols decahydroxycyclopentane (C5(OH)10) and dodecahydroxycyclohexane (C6(OH)12), which are stable, whereas the corresponding oxocarbons (C5O5 and C6O6) do not seem to be.


See also

* Alcohols, chemical compounds with at least one
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...
group * Triols, chemical compounds with three hydroxyl groups * Polyols, chemical compounds with multiple hydroxyl groups *
Ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an o ...
*
Glycol nucleic acid Glycol nucleic acid (GNA), sometimes also referred to as glycerol nucleic acid, is a nucleic acid similar to DNA or RNA but differing in the composition of its sugar-phosphodiester backbone, using propylene glycol in place of ribose or deoxyribo ...
(GNA)


References

{{Authority control Functional groups