Editing Hydration reaction (section)

==Organic chemistry==
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]]:
: C<sub>2</sub>H<sub>4</sub>O + H<sub>2</sub>O → HO–CH<sub>2</sub>CH<sub>2</sub>–OH

Acid catalysts are typically used.<ref>{{Ullmann | author1 = Siegfried Rebsdat | author2 = Dieter Mayer | title = Ethylene Glycol | doi = 10.1002/14356007.a10_101}}</ref>

=== Alkenes===
The general [[chemical equation]] for the hydration of alkenes is the following:
:RRC=CH<sub>2</sub>  + H<sub>2</sub>O → RRC(OH)-CH<sub>3</sub>
A [[hydroxyl]] group (OH<sup>&minus;</sup>) attaches to one carbon of the double bond, and a [[proton]] (H<sup>+</sup>) 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 H<sub>2</sub>O [[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 ion|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 [[Sulfate ester|sulphate esters]]. In the case of ethanol production, this step can be written:
:H<sub>2</sub>SO<sub>4</sub> + C<sub>2</sub>H<sub>4</sub> → C<sub>2</sub>H<sub>5</sub>-O-SO<sub>3</sub>H
Subsequently, this sulphate ester is hydrolyzed to regenerate sulphuric acid and release ethanol:
:C<sub>2</sub>H<sub>5</sub>-O-SO<sub>3</sub>H + H<sub>2</sub>O → H<sub>2</sub>SO<sub>4</sub> + C<sub>2</sub>H<sub>5</sub>OH 
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 acid]]s.<ref name="Ullmann1"/>
Here an example reaction mechanism of the hydration of 1-methylcyclohexene to 1-methylcyclohexanol:
[[File:Alkene hydration reaction.svg|center|450px|Hydration reaction mechanism from 1-methylcyclohexene to 1-methylcyclohexanol.]]
Many alternative routes are available for producing alcohols, including the [[hydroboration–oxidation reaction]], the [[oxymercuration reaction#Oxymercuration–reduction|oxymercuration–reduction reaction]], the [[Mukaiyama hydration]], the reduction of ketones and aldehydes and as a biological method [[fermentation]].

===Alkynes===
Acetylene hydrates to give acetaldehyde:<ref>Marc Eckert, Gerald Fleischmann, Reinhard Jira, Hermann M. Bolt, Klaus Golka "Acetaldehyde" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2006, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a01_031.pub2}}.</ref> The process typically relies on mercury catalysts and has been discontinued in the West but is still practiced in China. The Hg<sup>2+</sup> center binds to a C≡C bond, which is then attacked by water. The reaction is
:H<sub>2</sub>O + C<sub>2</sub>H<sub>2</sub> → CH<sub>3</sub>CHO

===Aldehydes and ketones===
Aldehydes and to some extent even ketones, hydrate to [[geminal diol]]s. 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:
{{chem2|RCN  +  H2O  ->  RC(O)NH2}}
This reaction requires catalysts.  Enzymes are used for the commercial production of [[acrylamide]] from [[acrylonitrile]].{{needs source|date=March 2025}}