Any unsaturated organic compound is susceptible to hydration.
Epoxides to glycol Several million tons of
ethylene glycol are produced annually by the hydration of
oxirane, a cyclic compound also known as
ethylene oxide: : C2H4O + H2O → HO–CH2CH2–OH Acid catalysts are typically used.
Alkenes The general
chemical equation for the hydration of alkenes is the following: :RRC=CH2 + H2O → RRC(OH)-CH3 A
hydroxyl group (OH−) attaches to one carbon of the double bond, and a
proton (H+) adds to the other. The reaction is highly exothermic. In the first step, the alkene acts as a nucleophile and attacks the proton, following
Markovnikov's rule. In the second step an H2O
molecule bonds to the other, more highly substituted carbon. The oxygen atom at this point has three bonds and carries a positive charge (i.e., the molecule is an
oxonium). Another water molecule comes along and takes up the extra proton. This reaction tends to yield many undesirable side products, (for example diethyl ether in the process of creating
ethanol) and in its simple form described here is not considered very useful for the production of alcohol. Two approaches are taken. Traditionally the alkene is treated with
sulfuric acid to give alkyl
sulphate esters. In the case of ethanol production, this step can be written: :H2SO4 + C2H4 → C2H5-O-SO3H Subsequently, this sulphate ester is hydrolyzed to regenerate sulphuric acid and release ethanol: :C2H5-O-SO3H + H2O → H2SO4 + C2H5OH This two step route is called the "indirect process". In the "direct process," the acid protonates the alkene, and water reacts with this incipient carbocation to give the alcohol. The direct process is more popular because it is simpler. The acid catalysts include
phosphoric acid and several
solid acids. The process typically relies on mercury catalysts and has been discontinued in the West but is still practiced in China. The Hg2+ center binds to a C≡C bond, which is then attacked by water. The reaction is :H2O + C2H2 → CH3CHO
Aldehydes and ketones Aldehydes and to some extent even ketones, hydrate to
geminal diols. The reaction is especially dominant for
formaldehyde, which, in the presence of water, exists significantly as dihydroxymethane. Conceptually similar reactions include
hydroamination and
hydroalkoxylation, which involve adding amines and alcohols to alkenes.
Nitriles Nitriles are susceptible to hydration to amides: This reaction requires catalysts. Enzymes are used for the commercial production of
acrylamide from
acrylonitrile. ==Inorganic and materials chemistry==