Geminal diols , 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 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 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 of
alkenes, 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 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. : 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: : 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 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
diastereoselectively from the corresponding β-hydroxy
ketones 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, and are important precursors to
polyurethanes.
Bouveault-Blanc reduction of the diesters of long chain dicarboxylic acids affords the diterminal diols HOCH2(CH2)nCH2OH for heptamethylene glycol (n = 5), nonamethylene glycol (n = 7),
decamethylene glycol (n = 8), tridecamethylene glycol (n = 11),
tetradecamethylene glycol (n = 12),
octadecamethylene glycol (n = 16). == Reactions ==