In chemistry, a dehydration reaction is a

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...

that involves the loss of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

from the reacting

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

or ion. Dehydration reactions are common processes, the reverse of a hydration reaction.

Dehydration reactions in organic chemistry

Esterification

The classic example of a dehydration reaction is the

Fischer esterification Fischer is a German occupational surname, meaning fisherman. The name Fischer is the fourth most common German surname. The English version is Fisher. People with the surname A * Abraham Fischer (1850–1913) South African public official * Ad ...

, 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.

Etherification

Two monosaccharides, such as

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, usi ...

and

fructose Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorb ...

, can be joined together (to form saccharose) using dehydration synthesis. The new molecule, consisting of two monosaccharides, is called a

disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, ...

Nitrile formation

Nitriles are often prepared by dehydration of primary amides. :RC(O)NH₂ → RCN + H₂O

Ketene formation

Ketene is produced by heating

acetic acid Acetic acid , systematically named ethanoic acid , 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 componen ...

and trapping the product: :CH₃CO₂H → CH₂=C=O + H₂O

Alkene formation

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 an 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. : Dehydration 1-methyl-2-cyclohexanol

Double dehydration is illustrated by the conversion of

glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids know ...

to acrolein: : EliminationReactionGlycerol2Acrolein2

Dehydration reactions in inorganic chemistry

The formation of the

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a P–O–P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate (Na2H2P2O7) and tetrasodium pyrophosphate (Na4P2O7), among o ...

bond is an important dehydration relevant to bioenergetics. Various construction materials are produced by dehydration.

Plaster of Paris Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of buildings, while "r ...

is produced by dehydration of

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and dr ...

in a kiln:Franz Wirsching "Calcium Sulfate" in Ullmann's Encyclopedia of Industrial Chemistry, 2012 Wiley-VCH, Weinheim. :CaSO4.2H2O + ''heat'' -> CaSO4.1/2H2O + 1 1/2H2O ''(released as steam).'' The resulting dry powder is ready to be mixed with water to form a stiff but workable paste that hardens.

References

{{Authority control Elimination reactions