Inchemistry, ahydration reaction is achemical reaction in which asubstance combines withwater. Inorganic chemistry, water is added to an unsaturated substrate, which is usually analkene or analkyne. This type of reaction is employed industrially to produceethanol,isopropanol, andbutan-2-ol.[1]
Any unsaturated organic compound is susceptible to hydration.
Several million tons ofethylene glycol are produced annually by the hydration ofoxirane, a cyclic compound also known asethylene oxide:
Acid catalysts are typically used.[2]
The generalchemical equation for the hydration of alkenes is the following:
Ahydroxyl group (OH−) attaches to one carbon of the double bond, and aproton (H+) adds to the other. The reaction is highly exothermic. In the first step, the alkene acts as a nucleophile and attacks the proton, followingMarkovnikov's rule. In the second step an H2Omolecule 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 anoxonium). 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 creatingethanol) 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 withsulfuric acid to give alkylsulphate esters. In the case of ethanol production, this step can be written:
Subsequently, this sulphate ester is hydrolyzed to regenerate sulphuric acid and release ethanol:
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 includephosphoric acid and severalsolid 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 thehydroboration–oxidation reaction, theoxymercuration–reduction reaction, theMukaiyama hydration, the reduction of ketones and aldehydes and as a biological methodfermentation.
Acetylene hydrates to give acetaldehyde:[3] 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
Aldehydes and to some extent even ketones, hydrate togeminal diols. The reaction is especially dominant for formaldehyde, which, in the presence of water, exists significantly as dihydroxymethane.
Conceptually similar reactions includehydroamination andhydroalkoxylation, which involve adding amines and alcohols to alkenes.
Nitriles are susceptible to hydration to amides:RCN + H2O → RC(O)NH2This reaction requires catalysts. Enzymes are used for the commercial production ofacrylamide fromacrylonitrile.[citation needed]
Hydration is an important process in many other applications; one example is the production ofPortland cement by the crosslinking of calcium oxides and silicates that is induced by water. Hydration is the process by which desiccants function.
