Hydration reaction

In

chemistry

, a hydration reaction is a

chemical reaction

in which a substance combines with water. In organic chemistry, water is added to an unsaturated substrate, which is usually an

alkene

or an

alkyne

. This type of reaction is employed industrially to produce

ethanol

, isopropanol, and

butan-2-ol

..

Organic chemistry

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

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 other carbon of the double bond. 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.
Here an example reaction mechanism of the hydration of 1-methylcyclohexene to 1-methylcyclohexanol:
:
Many alternative routes are available for producing alcohols, including the hydroboration–oxidation reaction, the oxymercuration–reduction reaction, the

Mukaiyama hydration

, the reduction of ketones and aldehydes and as a biological method

fermentation

.

Hydration of other substrates

Any unsaturated organic compound is susceptible to hydration. Acetylene hydrates to give acetaldehyde: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. 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 C≡C 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 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.

Inorganic and materials chemistry

Hydration is an important process in many other applications; one example is the production of

Portland cement

by the crosslinking of calcium oxides and silicates that is induced by water. Hydration is the process by which desiccants function.

References

Addition reactions
General chemistry