![Ester-general](https://upload.wikimedia.org/wikipedia/commons/3/34/Ester-general.svg)
In
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 ...
, an ester is a
compound
Compound may refer to:
Architecture and built environments
* Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall
** Compound (fortification), a version of the above fortified with defensive struct ...
derived from an
oxoacid
An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...
(organic or inorganic) in which at least one
hydroxyl group
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy g ...
() is replaced by an
alkoxy group
In chemistry, the alkoxy group is an alkyl group which is singularly bonded to oxygen; thus . The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the organic compound ethyl phenyl ether (, also k ...
(), as in the
substitution reaction
A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions ar ...
of a
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
and an
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
.
Glyceride
Glycerides, more correctly known as acylglycerols, are esters formed from glycerol and fatty acids, and are generally very hydrophobic.
Glycerol has three hydroxyl functional groups, which can be esterified with one, two, or three fatty acids ...
s are
fatty acid ester Fatty acid esters (FAEs) are a type of ester that result from the combination of a fatty acid with an alcohol. When the alcohol component is glycerol, the fatty acid esters produced can be monoglycerides, diglycerides, or triglycerides. Dietary f ...
s 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 ...
; they are important in biology, being one of the main classes of
lipids
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
and comprising the bulk of
animal fat
Animal fats and oils are lipids derived from animals: oils are liquid at room temperature, and fats are solid. Chemically, both fats and oils are composed of triglycerides. Although many animal parts and secretions may yield oil, in commercial p ...
s and
vegetable oil
Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or fat ...
s.
Esters typically have a pleasant smell; those of low molecular weight are commonly used as fragrances and are found in
essential oil
An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
s and
pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s. They perform as high-grade
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
s for a broad array of plastics,
plasticizer
A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture.
Plasticiz ...
s,
resin
In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...
s, and
lacquer
Lacquer is a type of hard and usually shiny coating or finish applied to materials such as wood or metal. It is most often made from resin extracted from trees and waxes and has been in use since antiquity.
Asian lacquerware, which may be ca ...
s,
and are one of the largest classes of synthetic lubricants on the commercial market.
Polyester
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
s are important
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
s, with
monomer
In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
Mo ...
s linked by ester
moieties.
Phosphoester
In chemistry, a phosphodiester bond occurs when exactly two of the hydroxyl groups () in phosphoric acid react with hydroxyl groups on other molecules to form two ester bonds. The "bond" involves this linkage . Discussion of phosphodiesters is ...
s form the backbone of
DNA molecules.
Nitrate ester
In organic chemistry, a nitrate ester is an organic functional group with the formula , where R stands for any organic residue. They are the esters of nitric acid and alcohols. A well-known example is nitroglycerin, which is not a ''nitro'' co ...
s, such as
nitroglycerin
Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating g ...
, are known for their explosive properties.
''
Nomenclature
Etymology
The word ''ester'' was coined in 1848 by a German chemist
Leopold Gmelin
Leopold Gmelin (2 August 1788 – 13 April 1853) was a German chemist. Gmelin was a professor at the University of Heidelberg He worked on the red prussiate and created Gmelin's test, and wrote his ''Handbook of Chemistry'', which over successiv ...
, probably as a contraction of the German , "
acetic ether".
IUPAC nomenclature
The names of esters are derived from the parent alcohol and the parent acid, where the latter may be organic or inorganic. Esters derived from the simplest
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s are commonly named according to the more traditional, so-called "
trivial names
In chemistry, a trivial name is a nonsystematic name for a chemical substance. That is, the name is not recognized according to the rules of any formal system of chemical nomenclature such as IUPAC inorganic or IUPAC organic nomenclature. A tr ...
" e.g. as formate, acetate, propionate, and butyrate, as opposed to the IUPAC nomenclature methanoate, ethanoate, propanoate, and butanoate. Esters derived from more complex carboxylic acids are, on the other hand, more frequently named using the systematic IUPAC name, based on the name for the acid followed by the suffix ''-oate''. For example, the ester hexyl octanoate, also known under the trivial name hexyl
caprylate, has the formula .
The chemical formulas of organic esters usually take the form , where R and R′ are the hydrocarbon parts of the carboxylic acid and the alcohol, respectively. For example,
butyl acetate
''n''-Butyl acetate is an organic compound with the formula . A colorless, flammable liquid, it is the ester derived from n-butanol and acetic acid. It is found in many types of fruit, where it imparts characteristic flavors and has a sweet sme ...
(systematically butyl ethanoate), derived from
butanol
Butanol (also called butyl alcohol) is a four-carbon alcohol with a formula of C4 H9 O H, which occurs in five isomeric structures (four structural isomers), from a straight-chain primary alcohol to a branched-chain tertiary alcohol; all are a b ...
and
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 ...
(systematically ethanoic acid) would be written . Alternative presentations are common including BuOAc and .
Cyclic esters are called
lactone
Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure (), or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring.
Lactones are formed by intramolecular esterification of the co ...
s, regardless of whether they are derived from an organic or inorganic acid. One example of an organic lactone is
γ-valerolactone
γ-Valerolactone (GVL) is an organic compound with the chemical formula, formula C5H8O2. This colourless liquid is one of the more common lactones. GVL is chiral but is usually used as the racemate. It is readily obtained from cellulosic biomass ...
.
Orthoesters
An uncommon class of organic esters are the
orthoester
In organic chemistry, an ortho ester is a functional group containing three alkoxy groups attached to one carbon atom, i.e. with the general formula . Orthoesters may be considered as products of exhaustive alkylation of unstable orthocarboxylic ...
s, which have the formula .
Triethylorthoformate () is derived, in terms of its name (but not its synthesis) from
orthoformic acid
Orthoformic acid or methanetriol is a hypothetical chemical compound with the formula . In this molecule, the central carbon atom is bound to one hydrogen and three hydroxyl groups.
Orthoformic acid has not been isolated to date, and is believe ...
() and
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
.
Inorganic esters
Esters can also be derived from inorganic acids.
*
Phosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
forms
phosphate ester
In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or Aryl, aromatic substituents. They can be conside ...
s, e.g.
triphenylphosphate
*
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 ...
forms
sulfate ester
Organosulfates are a class of organic compounds sharing a common functional group with the structure R-O-SO3−. The SO4 core is a sulfate group and the R group is any organic residue. All organosulfates are formally esters derived from alcohols ...
s, e.g.,
dimethylsulfate
Dimethyl sulfate (DMS) is a chemical compound with chemical formula, formula (CH3O)2SO2. As the ester, diester of methanol and sulfuric acid, its formula is often written as (Methyl, CH3)2Sulfate, SO4 or Me2SO4, where CH3 or Me is methyl. Me2SO ...
*
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
forms
nitrate esters
In organic chemistry, a nitrate ester is an organic functional group with the formula , where R stands for any organic residue. They are the esters of nitric acid and alcohols. A well-known example is nitroglycerin, which is not a ''nitro'' com ...
, e.g.
methyl nitrate
Methyl nitrate is the methyl ester of nitric acid and has the chemical formula CH3NO3. It is a colourless explosive volatile liquid.
Synthesis
It can be produced by the condensation of nitric acid and methanol:
:CH3OH + HNO3 → CH3NO3 + H2O
A ...
*
boric acid
Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolves ...
forms
borate
A borate is any of several boron oxyanions, negative ions consisting of boron and oxygen, such as orthoborate , metaborate , or tetraborate ; or any salt with such anions, such as sodium metaborate, and disodium tetraborate . The name also refe ...
s, e.g.
trimethylborate
Trimethyl borate is the organoboron compound with the formula B(OCH3)3. It is a colourless liquid that burns with a green flame. It is an intermediate in the preparation of sodium borohydride and is a popular reagent in organic chemistry. It is a ...
.
*
carbonic acid forms
carbonate ester
In organic chemistry, a carbonate ester (organic carbonate or organocarbonate) is an ester of carbonic acid. This functional group consists of a carbonyl group flanked by two alkoxy groups. The general structure of these carbonates is and they ...
s, e.g.
ethylene carbonate
Ethylene carbonate (sometimes abbreviated EC) is the organic compound with the formula (CH2O)2CO. It is classified as the Cyclic compound, cyclic carbonate ester of ethylene glycol and carbonic acid. At room temperature (25 °C) ethylene car ...
Inorganic acids that exist as
tautomers
Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.
The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hyd ...
form diverse esters
*
phosphorous acid
Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...
forms two kinds of
phosphite ester
The general structure of a phosphite ester showing the lone pairs on the P
In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)3. They can be considered as esters of a ...
s, e.g.
triethylphosphite
Triethyl phosphite is an organophosphorus compound with the formula P(OCH2CH3)3, often abbreviated P(OEt)3. It is a colorless, malodorous liquid. It is used as a ligand in organometallic chemistry and as a reagent in organic synthesis
The molec ...
() and
diethylphosphite ().
Inorganic acids that are unstable or elusive form stable esters.
*
chromic acid
The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixt ...
, which has never been detected, forms
di-tert-butyl chromate
Di-''tert''-butyl chromate is an alkoxide with the formula CrO2(OC(CH3)3)2. It is prepared by treatment of ''t''-butanol with chromic anhydride. It forms red crystals at temperatures below –5 °C, above which it melts to give a red oil. ...
*
sulfurous acid
Sulfurous acid (also sulfuric(IV) acid, sulphurous acid (UK), sulphuric(IV) acid (UK)) is the chemical compound with the formula . There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase. ...
, which is rare, forms dimethylsulfite
In principle, all metal and metalloid
alkoxide
In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...
s, of which many hundreds are known, could be classified as esters of the hypothetical acids.
Structure and bonding
Esters contain a
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
center, which gives rise to 120° C–C–O and O–C–O angles. Unlike
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
s, esters are structurally flexible functional groups because rotation about the C–O–C bonds has a low barrier. Their flexibility and low polarity is manifested in their physical properties; they tend to be less rigid (lower melting point) and more volatile (lower boiling point) than the corresponding
amides
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
.
[March, J. ''Advanced Organic Chemistry'' 4th Ed. J. Wiley and Sons, 1992: New York. .] The
p''K''a of the alpha-hydrogens on esters is around 25.
Many esters have the potential for
conformational isomerism
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation). While any two arrangements of atoms in a mole ...
, but they tend to adopt an ''s''-cis (or Z) conformation rather than the ''s''-trans (or E) alternative, due to a combination of
hyperconjugation and dipole minimization effects. The preference for the Z conformation is influenced by the nature of the substituents and solvent, if present.
Lactone
Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure (), or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring.
Lactones are formed by intramolecular esterification of the co ...
s with small rings are restricted to the ''s''-trans (i.e. E) conformation due to their cyclic structure.
Physical properties and characterization
Esters are more polar than
ethers
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
but less polar than alcohols. They participate in
hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s as hydrogen-bond acceptors, but cannot act as hydrogen-bond donors, unlike their parent alcohols. This ability to participate in hydrogen bonding confers some water-solubility. Because of their lack of hydrogen-bond-donating ability, esters do not self-associate. Consequently, esters are more volatile than
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s of similar molecular weight.
[
]
Characterization and analysis
Esters are generally identified by gas chromatography, taking advantage of their volatility. IR spectra for esters feature an intense sharp band in the range 1730–1750 cm−1 assigned to ''ν''C=O. This peak changes depending on the functional groups attached to the carbonyl. For example, a benzene ring or double bond in conjugation with the carbonyl will bring the wavenumber down about 30 cm−1.
Applications and occurrence
Esters are widespread in nature and are widely used in industry. In nature, fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
s are, in general, triesters derived from 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 ...
and fatty acid
In chemistry, particularly 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, fr ...
s. Esters are responsible for the aroma of many fruits, including apple
An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple fruit tree, trees are agriculture, cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, wh ...
s, durian
The durian (, ) is the edible fruit of several tree species belonging to the genus ''Durio''. There are 30 recognised ''Durio'' species, at least nine of which produce edible fruit. ''Durio zibethinus'', native to Borneo and Sumatra, is the onl ...
s, pear
Pears are fruits produced and consumed around the world, growing on a tree and harvested in the Northern Hemisphere in late summer into October. The pear tree and shrub are a species of genus ''Pyrus'' , in the family Rosaceae, bearing the p ...
s, banana
A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa''. In some countries, bananas used for cooking may be called "plantains", distinguis ...
s, pineapple
The pineapple (''Ananas comosus'') is a tropical plant with an edible fruit; it is the most economically significant plant in the family Bromeliaceae. The pineapple is indigenous to South America, where it has been cultivated for many centuri ...
s, and strawberries
The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown hybrid species of the genus '' Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The fruit is widely ap ...
. Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate
Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and foods ...
, acrylate esters, and cellulose acetate
In biochemistry, cellulose acetate refers to any acetate ester of cellulose, usually cellulose diacetate. It was first prepared in 1865. A bioplastic, cellulose acetate is used as a film base in photography, as a component in some coatings, and ...
.
:
Preparation
Esterification is the general name for 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 ...
in which two reactants (typically an alcohol and an acid) form an ester as the reaction product. Esters are common in organic chemistry and biological materials, and often have a pleasant characteristic, fruity odor. This leads to their extensive use in the fragrance
An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently vol ...
and flavor
Flavor or flavour is either the sensory perception of taste or smell, or a flavoring in food that produces such perception.
Flavor or flavour may also refer to:
Science
*Flavors (programming language), an early object-oriented extension to Lis ...
industry. Ester bonds are also found in many polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s.
Esterification of carboxylic acids with alcohols
The classic synthesis is the Fischer esterification, which involves treating a carboxylic acid with an alcohol in the presence of a dehydrating
In physiology, dehydration is a lack of total body water, with an accompanying disruption of Metabolism, metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental tempe ...
agent:
:RCO2H + R'OH <=> RCO2R' + H2O
The equilibrium constant for such reactions is about 5 for typical esters, e.g., ethyl acetate. The reaction is slow in the absence of a catalyst. 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 ...
is a typical catalyst for this reaction. Many other acids are also used such as polymeric sulfonic acids. Since esterification is highly reversible, the yield of the ester can be improved using Le Chatelier's principle
Le Chatelier's principle (pronounced or ), also called Chatelier's principle (or the Equilibrium Law), is a principle of chemistry used to predict the effect of a change in conditions on chemical equilibria. The principle is named after French c ...
:
* Using the alcohol in large excess (i.e., as a solvent).
* Using a dehydrating agent: sulfuric acid not only catalyzes the reaction but sequesters water (a reaction product). Other drying agents such as molecular sieves are also effective.
* Removal of water by physical means such as distillation
Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distilla ...
as a low-boiling azeotrope
An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This ...
s with toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) at ...
, in conjunction with a Dean-Stark apparatus.
Reagents are known that drive the dehydration of mixtures of alcohols and carboxylic acids. One example is the Steglich esterification, which is a method of forming esters under mild conditions. The method is popular in peptide synthesis
In organic chemistry, peptide synthesis is the production of peptides, compounds where multiple amino acids are linked via amide bonds, also known as peptide bonds. Peptides are chemically synthesized by the condensation reaction of the carboxyl ...
, where the substrates are sensitive to harsh conditions like high heat. DCC (dicyclohexylcarbodiimide
''N'',''N''′-Dicyclohexylcarbodiimide (DCC or DCCD) is an organic compound with the chemical formula (C6H11N)2C. It is a waxy white solid with a sweet odor. Its primary use is to couple amino acids during artificial peptide synthesis. The low ...
) is used to activate the carboxylic acid to further reaction. 4-Dimethylaminopyridine (DMAP) is used as an acyl-transfer catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
.
:
Another method for the dehydration of mixtures of alcohols and carboxylic acids is the Mitsunobu reaction
The Mitsunobu reaction is an organic reaction that converts an alcohol into a variety of functional groups, such as an ester, using triphenylphosphine and an azodicarboxylate such as diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylat ...
:
:RCO2H + R'OH + P(C6H5)3 + R2N2 -> RCO2R' + OP(C6H5)3 + R2N2H2
Carboxylic acids can be esterified using diazomethane
Diazomethane is the chemical compound CH2N2, discovered by German chemist Hans von Pechmann in 1894. It is the simplest diazo compound. In the pure form at room temperature, it is an extremely sensitive explosive yellow gas; thus, it is almost u ...
:
:RCO2H + CH2N2 -> RCO2CH3 + N2
Using this diazomethane, mixtures of carboxylic acids can be converted to their methyl esters in near quantitative yields, e.g., for analysis by gas chromatography
Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...
. The method is useful in specialized organic synthetic operations but is considered too hazardous and expensive for large-scale applications.
Esterification of carboxylic acids with epoxides
Carboxylic acids are esterified by treatment with epoxides, giving β-hydroxyesters:
:RCO2H + RCHCH2O -> RCO2CH2CH(OH)R
This reaction is employed in the production of vinyl ester resin Vinyl ester resin, or often just vinyl ester, is a resin produced by the esterification of an epoxy resin with acrylic or methacrylic acids. The "vinyl" groups refer to these ester substituents, which are prone to polymerize and thus an inhibitor is ...
resins from acrylic acid
Acrylic acid (IUPAC: propenoic acid) is an organic compound with the formula CH2=CHCOOH. It is the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a ...
.
Alcoholysis of acyl chlorides and acid anhydrides
Alcohols react with acyl chloride
In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...
s and acid anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid.
In organic chemistry, organic acid anhydrides contain the functional group R(CO)O(CO)R'. Organic acid anhydrides often form when one equivalent ...
s to give esters:
:RCOCl + R'OH -> RCO2R' + HCl
: (RCO)2O + R'OH -> RCO2R' + RCO2H
The reactions are irreversible simplifying work-up. Since acyl chlorides and acid anhydrides also react with water, anhydrous conditions are preferred. The analogous acylations of amines to give amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
s are less sensitive because amines are stronger nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
s and react more rapidly than does water. This method is employed only for laboratory-scale procedures, as it is expensive.
Alkylation of carboxylate salts
Although not widely employed for esterifications, salts of carboxylate anions can be alkylating agent with alkyl halide
The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely ...
s to give esters.[ In the case that an alkyl chloride is used, an iodide salt can catalyze the reaction (]Finkelstein reaction
The Finkelstein reaction named after the German chemist Hans Finkelstein, is an SN2 reaction (Substitution Nucleophilic Bimolecular reaction) that involves the exchange of one halogen atom for another. It is an equilibrium reaction, but the reac ...
). The carboxylate salt is often generated ''in situ''. In difficult cases, the silver carboxylate may be used, since the silver ion coordinates to the halide aiding its departure and improving the reaction rate. This reaction can suffer from anion availability problems and, therefore, can benefit from the addition of phase transfer catalyst
In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic reac ...
s or highly polar aprotic solvents such as DMF.
Transesterification
Transesterification
In organic chemistry, transesterification is the process of exchanging the organic group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. The reaction can ...
, which involves changing one ester into another one, is widely practiced:
: RCO2R' + CH3OH -> RCO2CH3 + R'OH
Like the hydrolysation, transesterification is catalysed by acids and bases. The reaction is widely used for degrading triglyceride
A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''tri-'' and ''glyceride'').
Triglycerides are the main constituents of body fat in humans and other vertebrates, as w ...
s, e.g. in the production of fatty acid esters and alcohols. Poly(ethylene terephthalate)
Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and foods, ...
is produced by the transesterification of dimethyl terephthalate
Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terepht ...
and ethylene glycol:[
: (C6H4)(CO2CH3)2 + 2 C2H4(OH)2 -> \frac C6H4)(CO2)2(C2H4)\mathit + 2 CH3OH
A subset of transesterification is the alcoholysis of ]diketene
Diketene is an organic compound with the molecular formula , and which is sometimes written as . It is formed by dimerization of ketene, . Diketene is a member of the oxetane family. It is used as a reagent in organic chemistry. It is a colorles ...
. This reaction affords 2-ketoesters.[
:(CH2CO)2 + ROH -> CH3C(O)CH2CO2R]
Carbonylation
Alkenes undergo "hydroesterification In industrial chemistry, carboalkoxylation is a process for converting alkenes to esters. This reaction is a form of carbonylation. A closely related reaction is hydrocarboxylation, which employs water in place of alcohols
A commercial applicat ...
" in the presence of metal carbonyl
Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe ch ...
catalysts. Esters of propanoic acid
Propionic acid (, from the Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is a liqu ...
are produced commercially by this method:
:C2H4 + ROH + CO -> C2H5CO2R
A preparaton of methyl propionate
Methyl propionate, also known as methyl propanoate, is an organic compound with the molecular formula CH3CH2CO2CH3. It is a colorless liquid with a fruity, rum-like odor.
Preparation
Methyl propionate can be prepared by esterification of propion ...
is one illustrative example.
:C2H4 + CO + MeOH -> MeO2CCH2CH3
The carbonylation of methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
yields methyl formate
Methyl formate, also called methyl methanoate, is the methyl ester of formic acid. The simplest example of an ester, it is a colorless liquid with an ethereal odour, high vapor pressure, and low surface tension. It is a precursor to many other co ...
, which is the main commercial source of formic acid
Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Es ...
. The reaction is catalyzed by sodium methoxide
Sodium methoxide is the simplest sodium alkoxide. With the formula , it is a white solid, which is formed by the deprotonation of methanol. Itis a widely used reagent in industry and the laboratory. It is also a dangerously caustic base.
P ...
:
:CH3OH + CO -> CH3O2CH
Addition of carboxylic acids to alkenes and alkynes
In hydroesterification In industrial chemistry, carboalkoxylation is a process for converting alkenes to esters. This reaction is a form of carbonylation. A closely related reaction is hydrocarboxylation, which employs water in place of alcohols
A commercial applicat ...
, alkenes and alkynes insert into the H-O bond of carboxylic acids. Vinyl acetate
Vinyl acetate is an organic compound with the formula CH3CO2CH=CH2. This colorless liquid is the precursor to polyvinyl acetate and ethene-vinyl acetate copolymers, important industrial polymers.
Production
The worldwide production capacity of v ...
is produced industrially by the addition of acetic acid to acetylene
Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
in the presence of zinc acetate
Zinc acetate is a salt with the formula Zn(CH3CO2)2, which commonly occurs as the dihydrate Zn(CH3CO2)2·2H2O. Both the hydrate and the anhydrous forms are colorless solids that are used as dietary supplements. When used as a food additive, it has ...
catalysts: Presently, zinc acetate
Zinc acetate is a salt with the formula Zn(CH3CO2)2, which commonly occurs as the dihydrate Zn(CH3CO2)2·2H2O. Both the hydrate and the anhydrous forms are colorless solids that are used as dietary supplements. When used as a food additive, it has ...
is used as the catalyst:
:CH3CO2H + C2H2 -> CH3CO2CHCH2
Vinyl acetate
Vinyl acetate is an organic compound with the formula CH3CO2CH=CH2. This colorless liquid is the precursor to polyvinyl acetate and ethene-vinyl acetate copolymers, important industrial polymers.
Production
The worldwide production capacity of v ...
can also be produced by palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
-catalyzed reaction of ethylene, 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 oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
:
:C2H4 + CH3CO2H + 1/2 O2 -> C2H3O2CCH3 + H2O
Silicotungstic acid is used to manufacture ethyl acetate
Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...
by the alkylation
Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting ...
of 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 ...
by ethylene:
:C2H4 + CH3CO2H -> CH3CO2C2H5
From aldehydes
The Tishchenko reaction The Tishchenko reaction is an organic chemical reaction that involves disproportionation of an aldehyde in the presence of an alkoxide. The reaction is named after Russian organic chemist Vyacheslav Tishchenko, who discovered that aluminium alko ...
involve disproportionation
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...
of an aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
in the presence of an anhydrous base to give an ester. Catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s are aluminium alkoxides or sodium alkoxides. Benzaldehyde
Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful.
It is a colorless liquid with a characteristic almond-like odor. ...
reacts with sodium benzyloxide (generated from sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
and benzyl alcohol
Benzyl alcohol is an aromatic alcohol with the formula C6H5CH2OH. The benzyl group is often abbreviated "Bn" (not to be confused with "Bz" which is used for benzoyl), thus benzyl alcohol is denoted as BnOH. Benzyl alcohol is a colorless liquid w ...
) to generate benzyl benzoate
Benzyl benzoate is an organic compound which is used as a medication and insect repellent. As a medication it is used to treat scabies and lice. For scabies either permethrin or malathion is typically preferred. It is applied to the skin as a ...
. The method is used in the production of ethyl acetate
Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...
from acetaldehyde
Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the mos ...
.[
]
Other methods
* Favorskii rearrangement
The Favorskii rearrangement is principally a rearrangement of cyclopropanones and α-halo ketones that leads to carboxylic acid derivatives. In the case of cyclic α-halo ketones, the Favorskii rearrangement constitutes a ring contraction. This ...
of α-haloketones in presence of base
* Baeyer–Villiger oxidation
The Baeyer–Villiger oxidation is an organic reaction that forms an ester from a ketone or a lactone from a cyclic ketone, using peroxyacids or peroxides as the oxidant. The reaction is named after Adolf von Baeyer and Victor Villiger who ...
of ketones with peroxides
* Pinner reaction
The Pinner reaction refers to the acid catalysed reaction of a nitrile with an alcohol to form an imino ester salt (alkyl imidate salt); this is sometimes referred to as a Pinner salt.
The reaction is named after Adolf Pinner, who first described ...
of nitrile
In organic chemistry, a nitrile is any organic compound that has a functional group. The prefix ''cyano-'' is used interchangeably with the term ''nitrile'' in industrial literature. Nitriles are found in many useful compounds, including met ...
s with an alcohol
* Nucleophilic abstraction
Nucleophilic abstraction is a type of an organometallic reaction which can be defined as a nucleophilic attack on a ligand which causes part or all of the original ligand to be removed from the metal along with the nucleophile.Spessard, Gary; Mie ...
of a metal–acyl complex
*Hydrolysis of orthoesters
In organic chemistry, an ortho ester is a functional group containing three alkoxy groups attached to one carbon atom, i.e. with the general formula . Orthoesters may be considered as products of exhaustive alkylation of unstable orthocarboxylic ...
in aqueous acid
*Cellulolysis via esterification
* Ozonolysis
In organic chemistry, ozonolysis is an organic reaction where the unsaturated bonds of alkenes (), alkynes (), or azo compounds () are cleaved with ozone (). Alkenes and alkynes form organic compounds in which the multiple carbon–carbon b ...
of alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s using a Work-up (chemistry), work up in the presence of hydrochloric acid and various alcohols.
* Electrosynthesis#Anodic oxidations, Anodic oxidation of Methyl group, methyl ketones leading to methyl esters.
* Fat interesterification, Interesterification exchanges the fatty acid groups of different esters.
Reactions
Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic but is attacked by strong nucleophiles (amines, alkoxides, hydride sources, organolithium compounds, etc.). The C–H bonds adjacent to the carbonyl are weakly acidic but undergo deprotonation with strong bases. This process is the one that usually initiates condensation reactions. The carbonyl oxygen in esters is weakly basic, less so than the carbonyl oxygen in amides due to resonance donation of an electron pair from nitrogen in amides, but forms adducts.
Hydrolysis and saponification
Esterification is a reversible reaction. Esters undergo hydrolysis under acidic and basic conditions. Under acidic conditions, the reaction is the reverse reaction of the Fischer esterification. Under basic conditions, hydroxide acts as a nucleophile, while an alkoxide is the leaving group. This reaction, saponification, is the basis of soap making.
:
The alkoxide group may also be displaced by stronger nucleophiles such as ammonia or primary or secondary amines to give amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
s: (ammonolysis reaction)
:RCO2R' + NH2R'' -> RCONHR'' + R'OH
This reaction is not usually reversible. Hydrazines and hydroxylamine can be used in place of amines. Esters can be converted to isocyanates through intermediate hydroxamic acids in the Lossen rearrangement.
Sources of carbon nucleophiles, e.g., Grignard reagents and organolithium compounds, add readily to the carbonyl.
Reduction
Compared to ketones and aldehydes, esters are Carbonyl reduction#Trends in carbonyl reactivity, relatively resistant to reduction. The introduction of catalytic hydrogenation in the early part of the 20th century was a breakthrough; esters of fatty acids are hydrogenated to fatty alcohols.
:RCO2R' + 2 H2 -> RCH2OH + R'OH
A typical catalyst is copper chromite. Prior to the development of catalytic hydrogenation, esters were reduced on a large scale using the Bouveault–Blanc reduction. This method, which is largely obsolete, uses sodium in the presence of proton sources.
Especially for fine chemical syntheses, lithium aluminium hydride is used to reduce esters to two primary alcohols. The related reagent sodium borohydride is slow in this reaction. DIBAH reduces esters to aldehydes.
Direct reduction to give the corresponding ether is difficult as the intermediate hemiacetal tends to decompose to give an alcohol and an aldehyde (which is rapidly reduced to give a second alcohol). The reaction can be achieved using triethylsilane with a variety of Lewis acids.
Claisen condensation and related reactions
As for aldehydes, the hydrogen atoms on the carbon adjacent ("α to") the carboxyl group in esters are sufficiently acidic to undergo deprotonation, which in turn leads to a variety of useful reactions. Deprotonation requires relatively strong bases, such as alkoxide
In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...
s. Deprotonation gives a nucleophilic enolate, which can further react, e.g., the Claisen condensation and its intramolecular equivalent, the Dieckmann condensation. This conversion is exploited in the malonic ester synthesis, wherein the diester of malonic acid reacts with an electrophile (e.g., alkyl halide
The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely ...
), and is subsequently decarboxylated. Another variation is the Fráter–Seebach alkylation.
Other reactions
* Phenyl esters react to hydroxyarylketones in the Fries rearrangement.
* Specific esters are functionalized with an α-hydroxyl group in the Chan rearrangement.
* Esters with β-hydrogen atoms can be converted to alkenes in ester pyrolysis.
*A direct conversion of esters to nitriles.
Protecting groups
As a class, esters serve as protecting groups for carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s. Protecting a carboxylic acid is useful in peptide synthesis, to prevent self-reactions of the bifunctional amino acids. Methyl and ethyl esters are commonly available for many amino acids; the ''t''-butyl ester tends to be more expensive. However, ''t''-butyl esters are particularly useful because, under strongly acidic conditions, the ''t''-butyl esters undergo elimination to give the carboxylic acid and isobutylene, simplifying work-up.
List of ester odorants
Many esters have distinctive fruit-like odors, and many occur naturally in the essential oils of plants. This has also led to their common use in artificial flavorings and fragrances which aim to mimic those odors.
See also
* List of esters
* Amide, an ester analog with oxygen replaced by nitrogen
* Cyanate ester
* Oligoester
* Polyolester
* Thioester, an ester analog with oxygen replaced by sulfur
* Transesterification
In organic chemistry, transesterification is the process of exchanging the organic group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. The reaction can ...
* Ether lipid
References
External links
An introduction to esters
{{Authority control
Functional groups
Esters,