chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, a dehydration reaction is a

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

that involves the loss of an H₂O from the reacting

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

(s) or ion(s). This reaction results in the release of the H₂O as

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

. When the reaction involves the coupling of two molecules into a single molecule it is referred to as a

condensation reaction In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...

. 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 a hydration reaction. The reverse of a condensation reaction yielding water is called

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

Condensation reactions occurring in living organisms

Condensation dehydration reactions are fundamental to the existence of life as this type of reaction produces proteins from

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

and

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

from

nucleotides Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

, fats from

fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

, and

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

(eg. cellulose, starch, sugar, lactose) from monosaccharides (eg. glucose and fructose). The formation of the

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate () and tetrasodium pyrophosphate (), among others. Often pyrophosphates a ...

bond is an important dehydration reaction relevant to bioenergetics.

Phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

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 of ATP and therefore does not result in the release of H₂O. These reactions are all mediated by

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

Condensation 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 * ...

, 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. Chemically, monosaccharides are polyhy ...

s, such as

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

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 gal ...

, 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, ...

Dehydration reactions in organic chemistry resulting in unsaturated bonds

Nitrile formation

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

Ketene formation

Ketene In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary valence (chemistry), monovalent functional group, chemical groups (or two separate Substituent, substitution sites in the same molecule). The na ...

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 compone ...

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

Alkene formation

Alkenes can be made from alcohols by dehydration. This conversion, among others, is used in converting biomass to liquid fuels. The conversion of ethanol to

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

is a fundamental example: : CH₃CH₂OH → H₂C=CH₂ + H₂O The reaction is accelerated by

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 catalyz ...

s such as

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

and certain

zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a meta ...

s. These reactions often proceed via

carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...

intermediates as shown for the dehydration of

cyclohexanol Cyclohexanol is the organic compound with the formula HOCH(CH2)5. The molecule is related to cyclohexane by replacement of one hydrogen atom by a hydroxyl group. This compound exists as a deliquescent colorless solid with a camphor-like odor, whi ...

. :

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 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)2Ph (Ph = C6H5). It is a white solid that easily undergoes Sublimation (phase transition), sublimation. The compound is an example of a hypervalent sulfur compound call ...

, which reacts irreversibly with water. Double dehydration is illustrated by the conversion of

glycerol Glycerol () is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pha ...

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

: :

Dehydration reactions in inorganic chemistry

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 "re ...

is produced by dehydration of

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, 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, drywall and blackboard or sidewalk ...

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