Hydration Reaction
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In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, a hydration reaction is a
chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...
in which a substance combines with
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
. In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
, water is added to an unsaturated substrate, which is usually an
alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...
or an
alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
. This type of reaction is employed industrially to produce
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
,
isopropanol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable, organic compound with a pungent alcoholic odor. Isopropyl alcohol, an organic polar molecule, is miscible in water, ethanol, an ...
, and butan-2-ol..


Organic chemistry

Any unsaturated organic compound is susceptible to hydration.


Epoxides to glycol

Several million tons of
ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes: as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
are produced annually by the hydration of oxirane, a cyclic compound also known as
ethylene oxide Ethylene oxide is an organic compound with the chemical formula, formula . It is a cyclic ether and the simplest epoxide: a three-membered ring (chemistry), ring consisting of one oxygen atom and two carbon atoms. Ethylene oxide is a colorless ...
: : C2H4O + H2O → HO–CH2CH2–OH Acid catalysts are typically used.


Alkenes

The general
chemical equation A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas. The reactant entities are given on the left-hand side and the Product (chemistry), product entities are on the right-hand side ...
for the hydration of alkenes is the following: :RRC=CH2 + H2O → RRC(OH)-CH3 A
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
group (OH) attaches to one carbon of the double bond, and a
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
(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 organic chemistry, Markovnikov's rule or Markownikoff's rule describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870. Explanation The rule states that with the addition of a ...
. In the second step an H2O
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
) 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 Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
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 Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...
and several
solid acid Solid acids are acids that are insoluble in the reaction medium. They are often used as heterogeneous catalysts. Many solid acids are zeolites. A variety of techniques are used to quantify the strength of solid acids. Examples Examples of ino ...
s. 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 Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
.


Alkynes

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. . 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 diol A geminal diol (or gem-diol for short) is any organic compound having two hydroxyl functional groups () bound to the same carbon atom. Geminal diols are a subclass of the diols, which in turn are a special class of alcohols. Most of the geminal ...
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: This reaction requires catalysts. Enzymes are used for the commercial production of acrylamide from
acrylonitrile Acrylonitrile is an organic compound with the formula and the structure . It is a colorless, volatile liquid. It has a pungent odor of garlic or onions. Its molecular structure consists of a vinyl group () linked to a nitrile (). It is an im ...
.{{needs source, date=March 2025


Inorganic and materials chemistry

Hydration is an important process in many other applications; one example is the production of
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...
by the crosslinking of calcium oxides and silicates that is induced by water. Hydration is the process by which desiccants function.


See also

* Aquation


References

Addition reactions General chemistry