Inchemistry, adehydration reaction is achemical reaction that involves the loss H₂O from the reactingmolecule(s) orion(s). This reaction results in the release of the H₂O aswater. When the reaction involves the coupling of two molecules into a single molecule it is referred to as acondensation reaction. Dehydration reactions are common processes in the manufacture of chemical compounds as well as naturally occurring within living organisms.

The reverse of a dehydration reaction is called ahydration reaction. The reverse of a condensation reaction yielding water is calledhydrolysis.

Condensation dehydration reactions are fundamental to the existence of life as this type of reaction produces proteins fromamino acids,DNA andRNA fromnucleotides,fats fromfatty acids, andpolysaccharides (eg. cellulose, starch, sugar, lactose) from monosaccharides (eg. glucose and fructose).

The formation of thepyrophosphate bond is an important dehydration reaction relevant to bioenergetics.Phosphorylation is a type of condensation dehydration reaction that is widely used to catalyze condensation reactions in living organisms. This phosphorylation usually involves the simultaneous dephosphorylation ofATP and therefore does not result in the release of H₂O.

These reactions are all mediated byenzymes.

The classic example of a dehydration reaction is theFischer 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.

Twomonosaccharides, such asglucose andfructose, can be joined together (to formsaccharose) using dehydration synthesis. The new molecule, consisting of two monosaccharides, is called adisaccharide.

Nitriles are often prepared by dehydration of primary amides.

RC(O)NH₂ → RCN + H₂O

Ketene is produced by heatingacetic acid and trapping the product:^[1]

CH₃CO₂H → CH₂=C=O + H₂O

Alkenes can be made from alcohols by dehydration. This conversion, among others, is used in converting biomass to liquid fuels.^[2] The conversion of ethanol toethylene is a fundamental example:^[3][4]

   CH₃CH₂OH → H₂C=CH₂ + H₂O

The reaction is accelerated byacid catalysts such assulfuric acid and certainzeolites.These reactions often proceed viacarbocation intermediates as shown for the dehydration ofcyclohexanol.^[5]

Some alcohols are prone to dehydration. 3-Hydroxylcarbonyls, calledaldols, release water upon standing at room temperature:

RC(O)CH₂CH(OH)R' → RC(O)CH=CHR' + H₂O

The reaction is induced by dehydrating reagents. For example, 2-methyl-cyclohexan-1-ol dehydrates to 1-methylcyclohexene in the presence ofMartin's sulfurane, which reacts irreversibly with water.^[6][7]

Double dehydration is illustrated by the conversion ofglycerol toacrolein:^[8][9]

Various construction materials are produced by dehydration.Plaster of Paris is produced by dehydration ofgypsum in a kiln:^[10][11]

${\displaystyle {\text{CaSO}}_4^{}\cdot 2{\text{H}}_2^{}\text{O}+}$ heat${\displaystyle ⟶}$ ${\displaystyle {\text{CaSO}}_4^{}\cdot \frac{1}{2}{\text{H}}_2^{}\text{O}+1\frac{1}{2}{\text{H}}_2^{}\text{O}}$ (released as steam).

The resulting dry powder is ready to be mixed with water to form a stiff but workable paste that hardens.