Diols
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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 diol may also be called a glycol. This pairing of
functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
s 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. 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 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 which, in water, exists in equilibrium with methanediol H2C(OH)2. 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 (though the term can be used more widely). Examples include ethane-1,2-diol 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 for cold environments. Common antifreezes also increase the boiling point of the liquid, allow ...
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: : For academic research and pharmaceutical areas, vicinal diols are often produced from the
oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
of
alkenes In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...
, usually with dilute acidic potassium permanganate or Osmium tetroxide. 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 Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...
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 increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
. 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. : Other routes to vic-diols are the hydrogenation of acyloins 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. 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 redox, reduce or Saturated ...
: : 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 In organic chemistry, 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 () and a hydrogen ...
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, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...
and formaldehyde, the Prins reaction. 1,3-diols can be produced diastereoselectively from the corresponding β-hydroxy
ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
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 A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...
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, and are important precursors to
polyurethane Polyurethane (; often abbreviated PUR and PU) is 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 term ...
s.


Reactions

From the industrial perspective, the dominant reactions of the diols is in the production of
polyurethane Polyurethane (; often abbreviated PUR and PU) is 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 term ...
s and alkyd resins.


General diols

Diols react as
alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...
s, by esterification and
ether In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R†...
formation. Diols such as ethylene glycol are used as co-
monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...
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 fo ...
reactions forming
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
s including some
polyester Polyester is a category of polymers that contain one or two ester linkages 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 some natura ...
s and
polyurethane Polyurethane (; often abbreviated PUR and PU) is 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 term ...
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 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. Protecting groups are used so that the functional group does not react to future reactions. Benzylidene groups are used to protect 1,3-diols. There are extremely useful in biochemistry as shown below of a carbohydrate derivative being protected. Diols can also be used to protect carbonyl groups. 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. 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

*
Alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...
s, chemical compounds with at least one hydroxyl group * Triols, chemical compounds with three hydroxyl groups * Polyols, chemical compounds with multiple hydroxyl groups * Ethylene glycol * Glycol nucleic acid (GNA)


References

{{Authority control Functional groups