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Concept
Réaction de déshydratation
Résumé
In chemistry, a dehydration reaction is a chemical reaction that involves the loss of water from the reacting molecule or ion. Dehydration reactions are common processes, the reverse of a hydration reaction.
The classic example of a dehydration reaction is the Fischer esterification, which involves treating a carboxylic acid with an alcohol to give an ester
RCO2H + R′OH RCO2R′ + H2O
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 + H2O
Ketene is produced by heating acetic acid and trapping the product:
CH3CO2H → CH2=C=O + H2O
Alkenes can be made from alcohols by dehydration. This conversion, among others, is a key reaction in converting biomass to liquid fuels. The conversion of ethanol to ethene is a fundamental example:
CH3CH2OH → H2C=CH2 + H2O
The reaction is slow in the absence of acid catalysts such as sulfuric acid and certain zeolites.
Some alcohols are prone to dehydration. 3-Hydroxylcarbonyls, called aldols, release water upon standing at room temperature:
RC(O)CH2CH(OH)R' → RC(O)CH=CHR' + H2O
In the dienol benzene rearrangement, dehydration leads to aromatization.
Often 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.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.
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