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Concept
Dehydration reaction
Summary
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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Ontological neighbourhood
Related concepts (16)
Acetic acid
Acetic acid əˈsiːtᵻk, systematically named ethanoic acid ˌɛθəˈnoʊᵻk, is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water and other trace elements. Acetic acid is the second simplest carboxylic acid (after formic acid). It is an important chemical reagent and industrial chemical, used primarily in the production of cellulose acetate for photographic film, polyvinyl acetate for wood glue, and synthetic fibres and fabrics.
Fischer–Speier esterification
Fischer esterification or Fischer–Speier esterification is a special type of esterification by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst. The reaction was first described by Emil Fischer and Arthur Speier in 1895. Most carboxylic acids are suitable for the reaction, but the alcohol should generally be primary or secondary. Tertiary alcohols are prone to elimination. Contrary to common misconception found in organic chemistry textbooks, phenols can also be esterified to give good to near quantitative yield of products.
Chemical equation
A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas. The reactant entities are given on the left-hand side and the product entities are on the right-hand side with a plus sign between the entities in both the reactants and the products, and an arrow that points towards the products to show the direction of the reaction. The chemical formulas may be symbolic, structural (pictorial diagrams), or intermixed.