In organic chemistry, a hydration reaction is a chemical reaction in which water is added to an unsaturated substrate. Usually, the substrate is an alkene or an alkyne. This type of reaction is employed industrially to produce ethanol, isopropanol, and 2-butanol.1
- C2H4O + H2O → HO–CH2CH2–OH
Acid catalysts are typically used.2
For the hydration of alkenes, the general chemical equation of the reaction is the following:
- RRC=CH2 in H2O/acid → RRC(-OH)-CH3
A hydroxyl group (OH−) attaches to one carbon of the double bond, and a proton (H+) adds to the second carbon center. 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 sulfate esters. In the case of ethanol production, this step can be written:
- H2SO4 + C2H4 → C2H5-O-SO3H
Subsequently this sulfate ester is hydrolyzed to regenerate sulfuric 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.1 Here an example reaction mechanism of the hydration of 1-methylcyclohexene to 1-methylcyclohexanol.
Many alternative routes are available for producing alcohols, including fermentation and hydrogenation of ketones and aldehydes.
Any unsaturated organic compound is susceptible to hydration. Acetylene hydrates to give acetaldehyde:1 The process typically relies on mercury catalysts and has been discontinued in the West but still practice in China. The Hg2+ center binds to C-C triple bond, which is then attacked by water. The reaction is:
- H2O + C2H2 → CH3CHO
Nitriles undergo hydration to give amides:
- H2O + RCN → RC(O)NH2
This reaction is employed in the production of acrylamide.
Aldehydes and to some extent even ketones, hydrate to for 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.
Hydration is an important process in many other applications, perhaps the largest being the production of Portland cement by the crosslinking of calcium oxides and silicates that is induced by water. Hydration of course is the process by which desiccants function.
- Falbe, Jürgen; Bahrmann, Helmut; Lipps, Wolfgang; Mayer, Dieter (2005), "Alcohols, Aliphatic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a01_279.
- Siegfried Rebsdat; Dieter Mayer (2005), "Ethylene Glycol", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a10_101