Dehydration reaction
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2008-06-30T04:32:17Z
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{{otheruses4|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 usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of [[elimination reaction]]s. Because the hydroxyl group (-OH) is a poor leaving group, having an [[Brønsted acid]] catalyst often helps by protonating the hydroxyl group to give the better leaving group, -OH<sub>2</sub><sup>+</sup>.
In [[organic synthesis]], there are many examples of dehydration reactions:
*Conversion of [[alcohol]]s to [[ether]]s
::2 R-OH → R-O-R + H<sub>2</sub>O
*Conversion of alcohols to [[alkene]]s
::R-CH<sub>2</sub>-CHOH-R → R-CH=CH-R + H<sub>2</sub>O
*Conversion of [[carboxylic acid]]s to [[acid anhydride]]s
::2 RCO<sub>2</sub>H → (RCO)<sub>2</sub>O + H<sub>2</sub>O
*Conversion of [[amide]]s to [[nitrile]]s
::RCONH<sub>2</sub> → R-CN + H<sub>2</sub>O
* in this [[rearrangement reaction]] called the '''dienol benzene rearrangement''' <ref>{{cite journal | title = The Dienol-Benzene Rearrangement. Some Chemistry of 1,4-Androstadiene-3,17-dione | author = Margaret Jevnik Gentles, Jane B. Moss, Hershel L. Herzog, and E. B. Hershberg | journal = [[J. Am. Chem. Soc.]] | year = 1958 | volume = 80 | issue = 14 | pages = 3702–3705 | doi = 10.1021/ja01547a058}}</ref>:
:[[Image:Dienol benzene rearrangement.png|400px|Dienol benzene rearrangement]]
Some dehydration reactions can be mechanistically complex, for instance the reaction of a sugar with concentrated sulphuric acid [http://www.madphysics.com/exp/dehydrating_sugar_with_acid.htm (experiment with video)] to form carbon involves formation of carbon carbon bonds.[http://www.exo.net/~pauld/activities/astronomy/transitvenus/sugarsulfuricacid.htm]
*Sugar (sucrose) is dehydrated[http://chemlearn.chem.indiana.edu/demos/16-1%20The%20Dehydration%20of%20Sugar%20by%20Sulfuric%20Acid.htm]:
:C<sub>12</sub>H<sub>22</sub>O<sub>11</sub> + 98% Sulfuric acid → 12 C <sub>(graphitic foam)</sub> + 11 H<sub>2</sub>O <sub>steam</sub> + Sulfuric acid/water mixture
The reaction is driven by the strongly exothermic reaction sulfuric acid has with water. (Beware that this reaction produces dangerous sulfuric-acid containing steam, and should only be performed in a fume-hood or well ventilated area.)
Common dehydrating agents; concentrated [[sulfuric acid]], concentrated [[phosphoric acid]], hot [[aluminium oxide]], hot ceramic.
==See also==
*[[Hydration reaction]]
==References==
{{reflist}}
[[Category:Elimination reactions]]
[[de:Dehydrierung]]
[[et:Dehüdraatimine]]
[[es:Reacción de deshidratación]]
[[id:Reaksi dehidrasi]]
[[nl:Dehydratie (chemie)]]
[[ja:脱水反応]]
[[pt:Desidratação (química)]]
[[ru:Дегидратация]]
[[tr:Dehidrasyon]]
[[zh:失水反应]]