chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, a dehydration reaction is a

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

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 a ...

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 bioch ...

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

. Dehydration reactions are common processes, the reverse of a

hydration reaction In chemistry, a hydration reaction is a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, che ...

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

monosaccharide Monosaccharides (from Greek ''monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. They are usually colorless, water-solub ...

s, 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, using ...

and

fructose Fructose, or fruit sugar, is a Ketose, ketonic monosaccharide, 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 galacto ...

, can be joined together (to form

saccharose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refine ...

) using dehydration synthesis. The new molecule, consisting of two monosaccharides, is called a disaccharide.

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 component ...

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 catalyst In acid catalysis and base catalysis, a chemical reaction is catalyzed by an acid or a base. By Brønsted–Lowry acid–base theory, the acid is the proton (hydrogen ion, H+) donor and the base is the proton acceptor. Typical reactions catalyze ...

s such as

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

and certain

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These pos ...

s. Some alcohols are prone to dehydration. 3-Hydroxylcarbonyls, called

aldol In organic chemistry, an aldol describes a structural motif consisting of a 3-hydroxy ketone or 3-hydroxyaldehyde. Aldols are usually the product of aldol addition. When used alone, the term "aldol" may refer to 3-hydroxybutanal. Stereochemistr ...

s, release water upon standing at room temperature: :RC(O)CH₂CH(OH)R' → RC(O)CH=CHR' + H₂O In the

dienol benzene rearrangement 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. Dehydration reactions in organic che ...

, 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 Martin's sulfurane is the organosulfur compound with the formula Ph2S C(CF3)2Phsub>2 (Ph = C6H5). It is a white solid that easily undergoes sublimation. The compound is an example of a hypervalent sulfur compound called a sulfurane. As such, the ...

, 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 known ...

acrolein Acrolein (systematic name: propenal) is the simplest unsaturated aldehyde. It is a colorless liquid with a piercing, acrid smell. The smell of burnt fat (as when cooking oil is heated to its smoke point) is caused by glycerol in the burning fa ...

: :

Dehydration reactions in inorganic chemistry

The formation of the pyrophosphate bond is an important dehydration relevant to bioenergetics. Various construction materials are produced by dehydration. Plaster of Paris 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 drywall. ...

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

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