In chemistry, a dehydration reaction 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.
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
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.
|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|
|Conversion of an alcohol to alkene (elimination)||R–CH2−CHOH–R → R–CH=CH–R + H2O||for example the conversion of glycerol to acrolein:|
or the dehydration of 2-methyl-1-cyclohexanol to (mainly) 1-methylcyclohexene 
|Conversion of two carboxylic acids an acid anhydride||2 RCOOH → (RCO)2O + H2O|
|Conversion of an amide to a nitrile||RCONH2 → R–CN + H2O|
Other examples of dehydration synthesis reactions are the formation of triglycerides from fatty acids and the formation of glycosidic bonds between carbohydrate molecules, such as the formation of maltose from two glucose molecules.
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