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In
chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. T ...

chemistry
, a hydration reaction is a
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
in which a substance combines with water. In
organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and th ...
, water is added to an unsaturated substrate, which is usually an
alkene In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo dur ...

alkene
or an
alkyne In organic chemistry Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . ...

alkyne
. 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 Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
,
isopropanol Isopropyl alcohol (International Union of Pure and Applied Chemistry, IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a Transparency and translucency, colorless, Flammability, flammable chemical compound (chemical formula CH ...
, and
butan-2-ol
butan-2-ol
..


Organic chemistry


Epoxides to glycol

Several million tons of
ethylene glycol Ethylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecula ...

ethylene glycol
are produced annually by the hydration of
oxirane Ethylene oxide is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecule ...

oxirane
, a cyclic compound also known as
ethylene oxide Ethylene oxide is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, beh ...

ethylene oxide
: : C2H4O + H2O → HO–CH2CH2–OH Acid catalysts are typically used.


Alkenes

For the hydration of alkenes, the general
chemical equation A chemical equation is the symbolic representation of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical ...
of the reaction is the following: :RRC=CH2 in H2O/acid → RRC(OH)-CH3 A
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the ...

hydroxyl
group (OH) attaches to one carbon of the double bond, and a
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

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 organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...

Markovnikov's rule
. In the second step an H2O
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

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 Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a weak acid with the chemical formula . The pure compound is a colorless solid. All three hydrogens are acidic to varying degrees and can be lost from the molecule ...

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 , the reduction of ketones and aldehydes and as a biological method
fermentation Fermentation is a metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as Cell signalin ...

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 Acrylamide (or acrylic amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula RC(=O)NR′R″, where R, R', and R″ represent organic compound, orga ...
. Aldehydes and to some extent even ketones, hydrate to geminal
diol A diol is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical ...

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 Portland cement is the most common type of cement A cement is a binder (material), binder, a substance used for construction that solidification, sets, hardens, and adheres to other materials to bind them together. Cement is seldom used ...

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