Editing Dehydration reaction

{{short description|Chemical reaction in which water is produced as a byproduct}}
{{About|chemical reactions resulting in the loss of water from a molecule|the removal of water from solvents and reagents|Desiccation}}

In [[chemistry]], a '''dehydration reaction''' is a [[chemical reaction]] that involves the loss H<sub>2</sub>O from the reacting [[molecule]](s) or [[ion]](s). This reaction results in the release of the H<sub>2</sub>O as [[water]]. When the reaction involves the coupling of two molecules into a single molecule it is referred to as a [[condensation reaction]]. Dehydration reactions are common processes in the manufacture of chemical compounds as well as naturally occurring within living organisms.

The reverse of a dehydration reaction is called a [[hydration reaction]]. The reverse of a condensation reaction yielding water is called [[hydrolysis]].

==Condensation reactions occurring in living organisms==
Condensation dehydration reactions are fundamental to the existence of life as this type of reaction produces proteins from [[amino acids]], [[DNA]] and [[RNA]] from [[nucleotides]], [[fat|fats]] from [[fatty acids]], and [[polysaccharides]] (eg. cellulose, starch, sugar, lactose) from monosaccharides (eg. glucose and fructose). 

The formation of the [[pyrophosphate]] bond is an important dehydration reaction relevant to bioenergetics. [[Phosphorylation]] is a type of condensation dehydration reaction that is widely used to catalyze condensation reactions in living organisms. This phosphorylation usually involves the simultaneous dephosphorylation of [[Adenosine triphosphate|ATP]] and therefore does not result in the release of H<sub>2</sub>O. 

These reactions are all mediated by [[enzymes]].

==Condensation dehydration reactions in organic chemistry==

===Esterification===
The classic example of a dehydration reaction is the [[Fischer esterification]], which involves treating a carboxylic acid with an alcohol to give an ester
:RCO<sub>2</sub>H + R′OH {{eqm}} RCO<sub>2</sub>R′ + H<sub>2</sub>O
Often such reactions require the presence of a dehydrating agent, i.e. a substance that reacts with water.

===Etherification===
Two [[monosaccharide]]s, such as [[glucose]] and [[fructose]], can be joined together (to form [[saccharose]]) using dehydration synthesis. The new molecule, consisting of two monosaccharides, is called a [[disaccharide]].

==Dehydration reactions in organic chemistry resulting in unsaturated bonds==

===Nitrile formation===
Nitriles are often prepared by dehydration of primary amides.
:RC(O)NH<sub>2</sub>  →   RCN  + H<sub>2</sub>O

===Ketene formation===
[[Ethenone|Ketene]] is produced by heating [[acetic acid]] and trapping the product:<ref>{{Ullmann |doi=10.1002/14356007.a15_063|title=Ketenes|year=2001|last1=Miller|first1=Raimund|last2=Abaecherli|first2=Claudio|last3=Said|first3=Adel|last4=Jackson|first4=Barry|isbn=3527306730}}</ref>
:CH<sub>3</sub>CO<sub>2</sub>H  →   CH<sub>2</sub>=C=O  + H<sub>2</sub>O

===Alkene formation===
Alkenes can be made from alcohols by dehydration.  This conversion, among others, is used in converting biomass to liquid fuels.<ref>{{Cite journal |last1=Besson |first1=Michèle |last2=Gallezot |first2=Pierre |last3=Pinel |first3=Catherine |date=2014-02-12 |title=Conversion of Biomass into Chemicals over Metal Catalysts |url=https://pubs.acs.org/doi/10.1021/cr4002269 |journal=Chemical Reviews |language=en |volume=114 |issue=3 |pages=1827–1870 |doi=10.1021/cr4002269 |pmid=24083630 |issn=0009-2665|url-access=subscription }}</ref>  The conversion of ethanol to [[ethylene]] is a fundamental example:<ref>{{cite book |first1=Heinz |last1=Zimmermann |first2=Roland |last2=Walz |chapter=Ethylene |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH |location=Weinheim |year=2008 |doi=10.1002/14356007.a10_045.pub3|isbn=978-3527306732}}</ref><ref>{{Cite journal |last1=Zhang |first1=Minhua |last2=Yu |first2=Yingzhe |date=2013-07-17 |title=Dehydration of Ethanol to Ethylene |url=https://pubs.acs.org/doi/10.1021/ie401157c |journal=Industrial & Engineering Chemistry Research |language=en |volume=52 |issue=28 |pages=9505–9514 |doi=10.1021/ie401157c |issn=0888-5885|url-access=subscription }}</ref>
:&nbsp;&nbsp; CH<sub>3</sub>CH<sub>2</sub>OH → H<sub>2</sub>C=CH<sub>2</sub> + H<sub>2</sub>O
The reaction is accelerated by [[acid catalyst]]s such as [[sulfuric acid]] and certain [[zeolite]]s.
These reactions often proceed via [[carbocation]] intermediates as shown for the dehydration of [[cyclohexanol]].<ref>{{cite journal |doi=10.15227/orgsyn.005.0033|title=Cyclohexene|journal=Organic Syntheses|year=1925|volume=5|page=33|author=G. H. Coleman, H. F. Johnstone}}</ref>
:[[File:Cyclohexensynthese1.svg|class=skin-invert-image|350px]]
:[[File:Cyclohexensynthese2.svg|class=skin-invert-image|440px]]

Some alcohols are prone to dehydration.  3-Hydroxylcarbonyls, called [[aldol]]s, release water upon standing at room temperature:
:RC(O)CH<sub>2</sub>CH(OH)R'  →  RC(O)CH=CHR'  +  H<sub>2</sub>O

The reaction is induced by dehydrating reagents.  For example, 2-methyl-cyclohexan-1-ol dehydrates to 1-methylcyclohexene in the presence of [[Martin's sulfurane]], which reacts irreversibly with water.<ref>{{cite journal | title = Dehydration of 2-Methyl-1-cyclohexanol: New Findings from a Popular Undergraduate Laboratory Experiment | author1 = J. Brent Friesen | author2 = Robert Schretzman | journal = [[J. Chem. Educ.]] | year = 2011 | volume = 88 | issue = 8 | pages = 1141–1147 | doi = 10.1021/ed900049b| bibcode = 2011JChEd..88.1141F }}</ref><ref>{{cite journal|first1=Brian A.|last1= Roden|title=Diphenylbis(1,1,1,3,3,3-hexafluoro-2-phenyl-2-propoxy)sulfurane|year=2001|journal=Encyclopedia of Reagents for Organic Synthesis|doi=10.1002/047084289X.rd409|isbn= 0471936235}}</ref>

Double dehydration is illustrated by the conversion of [[glycerol]] to [[acrolein]]:<ref>{{OrgSynth | year = 1926|title = Acrolein | author1 = H. Adkins | authorlink1 = Homer Burton Adkins | author2 = W. H. Hartung | doi = 10.15227/orgsyn.006.0001 | volume = 6| page = 1}}</ref><ref>{{Cite journal |last1=Katryniok |first1=Benjamin |last2=Paul |first2=Sébastien |last3=Bellière-Baca |first3=Virginie |last4=Rey |first4=Patrick |last5=Dumeignil |first5=Franck |date=2010 |title=Glycerol dehydration to acrolein in the context of new uses of glycerol |url=http://xlink.rsc.org/?DOI=c0gc00307g |journal=Green Chemistry |language=en |volume=12 |issue=12 |pages=2079 |doi=10.1039/c0gc00307g |issn=1463-9262|url-access=subscription }}</ref>
:[[File:EliminationReactionGlycerol2Acrolein2.svg|class=skin-invert-image|elimination reaction glycerol to acrolein|alt=Elimination reaction of glycerol to acrolein]]

==Dehydration reactions in inorganic chemistry==

Various construction materials are produced by dehydration. [[Plaster of Paris]] is produced by dehydration of [[gypsum]] in a kiln:<ref name=Ullmanns>Franz Wirsching "Calcium Sulfate" in Ullmann's Encyclopedia of Industrial Chemistry, 2012 Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a04_555}}</ref><ref name=lafarge>{{cite web|url=http://www.lafargeprestia.com/caso4___h2o.html |title=CaSO4, ½ H2O |last=Staff |publisher=[[Lafarge (company)|LaFargePrestia]] |access-date=27 November 2008 |url-status=dead |archive-url=https://web.archive.org/web/20081120163316/http://www.lafargeprestia.com/caso4___h2o.html |archive-date=November 20, 2008 }}</ref>
:<chem>CaSO4.2H2O +{}</chem> ''heat'' <chem>-></chem> <chem>CaSO4.1/2H2O + 1 1/2H2O</chem> ''(released as steam).''

The resulting dry powder is ready to be mixed with water to form a stiff but workable paste that hardens.


==References==
{{Reflist}}
{{Organic reactions}}
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[[Category:Elimination reactions]]