Adiol is achemical compound containing twohydroxyl groups (−OH groups).[1] Analiphatic diol may also be called aglycol.[2] This pairing offunctional groups is pervasive, and many subcategories have been identified. They are used asprotecting groups ofcarbonyl groups, making them essential in synthesis of organic chemistry.[3]
The most common industrial diol isethylene glycol. Examples of diols in which the hydroxyl functional groups are more widely separated include 1,4-butanediolHO−(CH2)4−OH andpropylene-1,3-diol, or beta propylene glycol,HO−CH2−CH2−CH2−OH.
Ageminal 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 isformaldehyde which, in water, exists in equilibrium withmethanediol H2C(OH)2.[4] Another example is (F3C)2C(OH)2, the hydrated form ofhexafluoroacetone. Many gem-diols undergo further condensation to give dimeric and oligomeric derivatives. This reaction applies toglyoxal and relatedaldehydes.
In a vicinal diol, the two hydroxyl groups occupyvicinal positions, that is, they are attached to adjacent atoms. These compounds are called glycols[5] (though the term can be used more widely). Examples include ethane-1,2-diol or ethylene glycol HO−(CH2)2−OH, a common ingredient ofantifreeze products. Another example ispropane-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 ofepoxides. The epoxides are prepared by epoxidation of the alkene. An example in the synthesis of trans-cyclohexanediol[6] or bymicroreactor:[7]
For academic research and pharmaceutical areas, vicinal diols are often produced from theoxidation ofalkenes, usually with diluteacidicpotassium permanganate or Osmium tetroxide.[8]Osmium tetroxide can similarly be used to oxidize alkenes to vicinal diols. The chemical reaction calledSharpless asymmetric dihydroxylation can be used to producechiral diols from alkenes using an osmatereagent and a chiralcatalyst. Another method is theWoodward cis-hydroxylation (cis diol) and the relatedPrévost reaction (anti diol), which both use iodine and the silver salt of a carboxylic acid.
Other routes to vic-diols are the hydrogenation ofacyloins[9] and thepinacol coupling reaction.
1,3-Diols are often prepared industrially byaldol condensation of ketones withformaldehyde. You can use many different starting materials to produce syn- or anti-1,3-diols.[10] The resulting carbonyl is reduced using theCannizzaro reaction or by catalytichydrogenation:
2,2-Disubstituted propane-1,3-diols are prepared in this way. Examples include 2-methyl-2-propyl-1,3-propanediol andneopentyl 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 thehydroformylation of epoxides followed by hydrogenation of the aldehyde. This method has been used for 1,3-propanediol fromethylene oxide.
More specialized routes to 1,3-diols involves the reaction between analkene andformaldehyde, thePrins reaction. 1,3-diols can be produceddiastereoselectively from the corresponding β-hydroxyketones using theEvans–Saksena,Narasaka–Prasad orEvans–Tishchenko reduction protocols.
1,3-Diols are described assyn oranti depending on the relative stereochemistries of the carbon atoms bearing the hydroxyl functional groups.Zincophorin is anatural product that contains bothsyn andanti 1,3-diols.
Diols where the hydroxyl groups are separated by several carbon centers are generally prepared by hydrogenation of diesters of the correspondingdicarboxylic acids:
1,4-butanediol,1,5-pentanediol,1,6-hexanediol, and1,10-decanediol [es] are important precursors topolyurethanes.[11]
From the industrial perspective, the dominant reactions of the diols is in the production ofpolyurethanes andalkyd resins.[11]
Diols react asalcohols, byesterification andether formation.[12]
Diols such asethylene glycol are used as co-monomers inpolymerization reactions formingpolymers including somepolyesters andpolyurethanes.[12] A different monomer with two identical functional groups, such as adioyl 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 isdiol 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, acyclic ether can be formed.
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.
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]
Diols can also be converted tolactones employing theFétizon oxidation reaction.
Inglycol cleavage, the C−C bond in avicinal diol is cleaved with formation of ketone or aldehyde functional groups. SeeDiol oxidation.
In general, organic geminal diols readilydehydrate to form acarbonyl group.
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