Dehydration reaction

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In chemistry, a dehydration reaction (a.k.a. condensation reaction[1]), also known as Zimmer's Hydrogenesis, is a conversion that involves the loss of water from the reacting molecule or ion. Dehydration reactions are common processes, the reverse of a hydration reaction. Common dehydrating agents used in organic synthesis include sulfuric acid and alumina. Often dehydration reactions are effected with heating.

Dehydration reactions[edit]

The classic example of a dehydration reaction is the Fischer esterification, which involves treating a carboxylic acid with an alcohol in the presence of a dehydrating agent:

RCO2H + R′OH ⇌ RCO2R′ + H2O

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.

The process of hydrolysis is the reverse reaction, meaning that the water is recombined with the two hydroxyl groups and the disaccharide reverts to being monosaccharides.

In the related condensation reaction water is released from two different reactants.

In organic synthesis, there are many examples of dehydration reaction, for example dehydration of alcohols or sugars.

Dehydration reactions
Reaction General equation Examples
Conversion of two alcohols to an ether (substitution) 2 R–OH → R–O–R + H2O
Conversion of an acid and an alcohol to an ester (Fischer–Speier esterification) R−COOH + R'−OH → R−COO−R' + H2O
Fischer–Speier esterification diagram
Conversion of an alcohol to alkene (elimination) R–CH2−CHOH–R → R–CH=CH–R + H2O for example the conversion of glycerol to acrolein:[2]
elimination reaction glycerol to acrolein

or the dehydration of 2-methyl-cyclohexan-1-ol to (mainly) 1-methylcyclohexene,[3] using Martin's sulfurane[4]

Dehydration 1-methyl-2-cyclohexanol

Conversion of ethanol to ethene[5]

   CH3CH2OH → H2C=CH2 + H2O
Conversion of two carboxylic acids an acyl anhydride 2 RCOOH → (RCO)2O + H2O
Conversion of an amide to a nitrile RCONH2 → R–CN + H2O
Dienol–benzene rearrangement Dienol benzene rearrangement[6][7]

Other examples of dehydration synthesis reactions are the formation of triglycerides from fatty acids and the formation of glycosidic bonds between saccharide molecules, such as the formation of maltose from two glucose molecules.

See also[edit]

References[edit]

  1. ^ oktatási hivatal, OKTATÁSI ÉS KULTURÁLIS MINISZTÉRIUM (2008). BIOLÓGIA ANGOL NYELVEN EMELT SZINTŰ ÍRÁSBELI VIZSGA 2008. május 16. 8:00 (PDF). Hungary: OKTATÁSI ÉS KULTURÁLIS MINISZTÉRIUM. pp. 3 / 20 I. The importance of some inorganic molecules in the living world 10.
  2. ^ H. Adkins; W. H. Hartung (1926). "Acrolein". Organic Syntheses. 6: 1. doi:10.15227/orgsyn.006.0001.; Collective Volume, 1, p. 15
  3. ^ 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.
  4. ^ 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.
  5. ^ 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.
  6. ^ H. Plieninger; Gunda Keilich (1956). "Die Dienol-Benzol-Umlagerung" [The dienol-benzene rearrangement]. Angew. Chem. (in German). 68 (19): 618. doi:10.1002/ange.19560681914.
  7. ^ Margaret Jevnik Gentles; Jane B. Moss; Hershel L. Herzog; E. B. Hershberg (1958). "The Dienol-Benzene Rearrangement. Some Chemistry of 1,4-Androstadiene-3,17-dione". J. Am. Chem. Soc. 80 (14): 3702–3705. doi:10.1021/ja01547a058.