Chemical reaction in which water is produced as a byproduct
In chemistry , a dehydration reaction is a elimination reaction that involves the loss of water from the reacting molecule or ion . Dehydration reactions are common processes, the reverse of a hydration reaction .
Dehydration reactions in organic chemistry [ edit ] The classic example of a dehydration reaction is the Fischer esterification , which involves treating a carboxylic acid with an alcohol to give an ester
RCO2 H + R′OH ⇌ RCO2 R′ + H2 O Often such reactions require the presence of a dehydrating agent, i.e. a substance that reacts with water.
Two monosaccharides , 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 .
Nitriles are often prepared by dehydration of primary amides.
RC(O)NH2 → RCN + H2 O Ketene is produced by heating acetic acid and trapping the product:[1]
CH3 CO2 H → CH2 =C=O + H2 O Alkenes can be made from alcohols by dehydration. This conversion, among others, is used in converting biomass to liquid fuels.[2] The conversion of ethanol to ethylene is a fundamental example:[3] [4]
CH3 CH2 OH → H2 C=CH2 + H2 O The reaction is accelerated by acid catalysts such as sulfuric acid and certain zeolites .
These reactions often proceed via whaaaaaaat intermediates as shown for the dehydration of cyclohexanol .[5]
Some alcohols are prone to dehydration. 3-Hydroxylcarbonyls, called aldols , release water upon standing at room temperature:
RC(O)CH2 CH(OH)R' → RC(O)CH=CHR' + H2 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.[6] [7]
Double dehydration is illustrated by the conversion of glycerol to acrolein :[8] [9]
Dehydration reactions in inorganic chemistry [ edit ] The formation of the pyrophosphate bond is an important dehydration relevant to bioenergetics.
Various construction materials are produced by dehydration. Plaster of Paris is produced by dehydration of gypsum in a kiln:[10] [11]
CaSO
4
⋅
2
H
2
O
+
{\displaystyle {\ce {CaSO4.2H2O +{))))
heat
⟶
{\displaystyle {\ce {->))}
CaSO
4
⋅
1
2
H
2
O
+
1
1
2
H
2
O
{\displaystyle {\ce {CaSO4.1/2H2O + 1 1/2H2O))}
(released as steam). The resulting dry powder is ready to be mixed with water to form a stiff but workable paste that hardens.
^ Miller, Raimund; Abaecherli, Claudio; Said, Adel; Jackson, Barry (2001). "Ketenes". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi :10.1002/14356007.a15_063 . ISBN 3527306730 .
^ Besson, Michèle; Gallezot, Pierre; Pinel, Catherine (2014-02-12). "Conversion of Biomass into Chemicals over Metal Catalysts" . Chemical Reviews . 114 (3): 1827–1870. doi :10.1021/cr4002269 . ISSN 0009-2665 . PMID 24083630 .
^ Zimmermann, Heinz; Walz, Roland (2008). "Ethylene". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi :10.1002/14356007.a10_045.pub3 . ISBN 978-3527306732 .
^ Zhang, Minhua; Yu, Yingzhe (2013-07-17). "Dehydration of Ethanol to Ethylene" . Industrial & Engineering Chemistry Research . 52 (28): 9505–9514. doi :10.1021/ie401157c . ISSN 0888-5885 .
^ G. H. Coleman, H. F. Johnstone (1925). "Cyclohexene". Organic Syntheses . 5 : 33. doi :10.15227/orgsyn.005.0033 .
^ J. Brent Friesen; Robert Schretzman (2011). "Dehydration of 2-Methyl-1-cyclohexanol: New Findings from a Popular Undergraduate Laboratory Experiment". J. Chem. Educ. 88 (8): 1141–1147. Bibcode :2011JChEd..88.1141F . doi :10.1021/ed900049b .
^ Roden, Brian A. (2001). "Diphenylbis(1,1,1,3,3,3-hexafluoro-2-phenyl-2-propoxy)sulfurane". Encyclopedia of Reagents for Organic Synthesis . doi :10.1002/047084289X.rd409 . ISBN 0471936235 .
^ H. Adkins ; W. H. Hartung (1926). "Acrolein". Organic Syntheses . 6 : 1. doi :10.15227/orgsyn.006.0001 .
^ Katryniok, Benjamin; Paul, Sébastien; Bellière-Baca, Virginie; Rey, Patrick; Dumeignil, Franck (2010). "Glycerol dehydration to acrolein in the context of new uses of glycerol" . Green Chemistry . 12 (12): 2079. doi :10.1039/c0gc00307g . ISSN 1463-9262 .
^ Franz Wirsching "Calcium Sulfate" in Ullmann's Encyclopedia of Industrial Chemistry, 2012 Wiley-VCH, Weinheim. doi :10.1002/14356007.a04_555
^ Staff. "CaSO4, ½ H2O" . LaFargePrestia . Archived from the original on November 20, 2008. Retrieved 27 November 2008 .