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An ester is a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, m ...
derived from an
acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special case of , proton donors form the H3O+ and are ...
(organic or inorganic) in which at least one –OH
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the ...

hydroxyl
group is replaced by an –O–
alkyl In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pr ...

alkyl
(
alkoxy In chemistry, the alkoxy group is an alkyl (carbon and hydrogen chain) group singularly bonded to oxygen; thus R–O. The range of alkoxy group is vast, the simplest being methoxy (CH3O–). An ethoxy group (CH3CH2O–) is found in the organic c ...
) group, as in the substitution reaction of a
carboxylic acid A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
and an
alcohol In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...

alcohol
.
Glyceride , the simplest possible fat after Triformin 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 este ...
s are
fatty acid ester Fatty acid esters (FAEs) are a type of ester An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a and an . s are s of ; they are impo ...
s of
glycerol Glycerol (; also called glycerine in British English and glycerin in American English) is a simple polyol 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

lipids
and comprising the bulk of animal fats and vegetable oils. 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 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 matte ...
s and
pheromone A pheromone (from Ancient Greek ' "to bear" and hormone) 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 s ...

pheromone
s. They perform as high-grade solvents 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 Plasticity may refer to: Science * Plasticity (physics), in engineering and physics, the propensity of ...
s,
resin In polymer chemistry and materials science, resin is a solid or highly Viscosity, viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focus ...

resin
s, and
lacquer Lacquer is a type of hard and potentially shiny coating A coating is a covering that is applied to the surface of an object, usually referred to as the substrate Substrate may refer to: Physical layers *Substrate (biology), the natural e ...

lacquer
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 syntheticA synthetic is an artificial material produced by organic chemistry, organic chemical synthesis. Synthetic may also refer to: In the sense of both "combination" and "artificial" * Synthetic chemical or s ...

plastic
s, with
monomer In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

monomer
s linked by ester moieties.
Phosphoester A phosphodiester bond occurs when exactly two of the hydroxyl groups A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen atom Chemical bond , covalently bonded to one hydrogen atom. In or ...
s form the backbone of
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
molecules.
Nitrate ester A nitrate ester is the organic functional group with the formula RONO2, where R stands for any organic residue. They are the esters An ester is a chemical compound A chemical compound is a chemical substance composed of many identical molecul ...
s, such as
nitroglycerin Nitroglycerin (NG), also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive An explosive (or explosive material) is a reactive substance that contains a grea ...

nitroglycerin
, 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 A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classical language ...

Leopold Gmelin
, probably as a contraction of the German ''Essigäther'', "
acetic ether
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 acids are commonly named according to the more traditional, so-called "
trivial names In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they und ...
" 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
caprylate
, has the formula CH3(CH2)6CO2(CH2)5CH3. The chemical formulas of organic esters usually take the form RCO2R′, where R and R′ are the hydrocarbon parts of the carboxylic acid and the alcohol, respectively. For example,
butyl acetate ''n''-Butyl acetate, also known as butyl ethanoate, is an ester An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl (alkoxy) group, as in the subs ...

butyl acetate
(systematically butyl ethanoate), derived from
butanol Butanol (also called butyl alcohol) is a four-carbon alcohol In , alcohol is an that carries at least one (−OH) bound to a atom. The term alcohol originally referred to the primary alcohol (ethyl alcohol), which is and is the main ...
and
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
(systematically ethanoic acid) would be written CH3CO2C4H9. Alternative presentations are common including BuOAc and CH3COOC4H9. Cyclic esters are called
lactoneLactones 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 ...

lactone
s, regardless of whether they are derived from an organic or inorganic acid. One example of an organic lactone is
γ-valerolactone
γ-valerolactone
.


Orthoesters

An uncommon class of organic esters are the
orthoester In organic chemistry, an orthoester is a functional group containing three alkoxy groups attached to one carbon atom, i.e. with the general formula RC(OR′)3. Orthoesters may be considered as products of exhaustive alkylation Alkylation is the tr ...
s, which have the formula RC(OR′)3.
Triethylorthoformate
Triethylorthoformate
(HC(OC2H5)3) is derived, in terms of its name (but not its synthesis) from orthoformic acid (HC(OH)3) and
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
.


Inorganic esters

Esters can also be derived from inorganic acids. *
Phosphoric acid Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a weak acid with the chemical formula . The pure compound is a colorless solid. All three hydrogens are acidic to varying degrees and can be lost from the molecule a ...

Phosphoric acid
forms
phosphate ester Organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compound Organophosphorus compounds are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally a ...
s, e.g.
triphenylphosphate
triphenylphosphate
*
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
forms
sulfate ester Organosulfates are a class of s sharing a common with the structure R-O-SO3−. The SO4 core is a group and the R group is any organic residue. All organosulfates are formally s derived from s and , although many are not prepared in this way. ...

sulfate ester
s, e.g.,
dimethylsulfate Dimethyl sulfate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by c ...

dimethylsulfate
*
nitric acid Nitric acid (), also known as ''aqua fortis'' (Latin for "strong water") and spirit of niter, is a highly corrosive mineral acid. The pure compound is colorless, but older samples tend to acquire a yellow cast due to decomposition into nitroge ...

nitric acid
forms nitrate esters, e.g.
methyl nitrate
methyl nitrate
*
boric acid Boric acid, also called hydrogen borate, boracic acid, and orthoboric acid is a weak, monobasic Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is ...

boric acid
forms
borate Borates are boron-oxygen compounds, which form boron oxyanions. These can be wikt:trigon, trigonal or tetrahedron, tetrahedral in structure, or more loosely can consist of chemical mixtures which contain borate anions of either description. The e ...
s, e.g. trimethylborate. *
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and s ...

carbonic acid
forms
carbonate ester A carbonate ester (organic carbonate or organocarbonate) is an ester An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a and an . s ...

carbonate ester
s, e.g.
ethylene carbonate Ethylene carbonate (sometimes abbreviated EC) is the organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to ...

ethylene carbonate
Inorganic acids that exist as tautomers form diverse esters *
phosphorous acid Phosphorous acid is the Compound (chemistry), compound described by the chemical formula, formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate ...

phosphorous acid
forms two kinds of
phosphite ester The general structure of a phosphite ester showing the lone pairs on the P In 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 an unobserv ...
s, e.g.
triethylphosphite
triethylphosphite
(P(OEt)3) and diethylphosphite (HP(O)(OEt)2). 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 Sulfuric acid ( American spelling) or sulphuric acid ( Commonwealth spelling), also known as oil of vitriol, is a mineral acid composed of the elem ...
, which has never been detected, forms di-tert-butyl chromate *
sulfurous acid Sulfurous acid (also Sulfuric(IV) acid, Sulphuric acid (UK), Sulphuric(IV) acid (UK)) is the chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed ...

sulfurous acid
, which is rare, forms dimethylsulfite In principle, all metal and metalloid
alkoxide 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 RO−, where R is the organic substituent. Alkoxides are strong Base (chemistry), bases ...

alkoxide
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 Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pro ...

carbonyl
center, which gives rise to 120° C–C–O and O–C–O angles. Unlike
amide In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, ...

amide
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 Organic chemistry is a branch of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Che ...
.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.
LactoneLactones 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 ...

Lactone
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 Ethers are a class of organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecul ...
but less polar than alcohols. They participate in
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
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 A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
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 Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

fat
s are in general triesters derived from
glycerol Glycerol (; also called glycerine in British English and glycerin in American English) is a simple polyol 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 Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s. Esters are responsible for the aroma of many fruits, including
apple An apple is an edible fruit In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this fie ...

apple
s,
durian The durian () is the edible fruit of several tree species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biolog ...

durian
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 In biology, a species is the basic unit of biological ...

pear
s,
banana A banana is an elongated, edible fruit In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specia ...

banana
s,
pineapple The pineapple (''Ananas comosus'') is a tropical plant with an edible fruit and is the most economically significant plant in the family Bromeliaceae. The pineapple is indigenous to South America, where it has been cultivated for many centurie ...

pineapple
s, and
strawberries The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown Hybrid (biology), hybrid species of the genus ''Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The f ...

strawberries
. 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 A thermoplastic, or thermosoft plastic, is a plastic polymer A polymer (; Greek ''wikt:poly-, poly-' ...

polyethylene terephthalate
, acrylate esters, and
cellulose acetate Cellulose acetate refers to any acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetal image:Simple Periodic Table Chart-en.svg, upright=1.75, Nonmetals (and metalloids ...

cellulose acetate
. :


Preparation

Esterification is the general name for a
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
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 flavor Flavor, or flavour, is the perceptual Perception (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branc ...

fragrance
and
flavor Flavor, or flavour, is the perceptual Perception (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area arou ...
industry. Ester bonds are also found in many
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s.


Esterification of carboxylic acids with alcohols

The classic synthesis is the
Fischer esterification Fischer is an German surname, derived from the profession of the fisherman. The name Fischer is the fourth most common German surname. The English version is Fisher Fisher is an archaic term for a fisherman, revived as gender-neutral. Fisher, Fi ...
, 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 In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies ...
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 Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

Sulfuric acid
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, is a principle of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mole ...
: * 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 A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecule ...
are also effective. * Removal of water by physical means such as
distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically com ...

distillation
as a low-boiling
azeotrope An azeotrope () or a constant boiling point mixture is a mixture 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 m ...

azeotrope
s with
toluene Toluene (), also known as toluol (), is an aromatic hydrocarbon. It is a colorless, Water (molecule), water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group ( ...

toluene
, in conjunction with a . 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 Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . ...
, where the substrates are sensitive to harsh conditions like high heat. DCC () is used to activate the carboxylic acid to further reaction.
4-Dimethylaminopyridine 4-Dimethylaminopyridine (DMAP) is a derivative of pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula Carbon, C5Hydrogen, H5Nitrogen, N. It is structurally related to b ...
(DMAP) is used as an acyl-transfer
catalyst Catalysis () is the process of increasing the rate of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In cla ...

catalyst
. : 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 azodicarboxylate ...
: :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 PechmannHans von Pechmann (1 April 1850 – 19 April 1902) was a German chemist, renowned for his discovery of diazomethane in 1894. Pechmann condensation and Pech ...

diazomethane
: :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 In chemical analysis, chromatography is a laboratory technique for the Separation process, separation of a mixture into its components. The mixture is dissolved in a fluid solvent (g ...
. 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 resinVinyl 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. The diester product is ...
resins from
acrylic acid Acrylic acid (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science ...

acrylic acid
.


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 -Carbonyl, COChlorine, Cl. Their formula is usually written RCOCl, where R is a side chain. They are reactive derivatives of carboxylic ac ...
s and
acid anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid. In organic chemistry, organic acid anhydrides contain the functional group R(CO)O(CO)R'. Organic acid anhydrides often form w ...

acid anhydride
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 Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, ...

amide
s are less sensitive because amines are stronger
nucleophile In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

nucleophile
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'' known as halogenoalkanes or alkyl halides) are alkane In , an alkane, or paraffin (a historical that also has ), is an . In other words, an alkane consists of and atoms arranged in a structure in which all the s are . ...
s to give esters. In the case that an
alkyl chloride An organochloride, organochlorine compound, chlorocarbon, or chlorinated hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-h ...
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, SN2 reaction (Substitution Nucleophilic Bimolecular reaction) that involves the exchange of one halogen atom for another. It is an chemical equilibrium, ...

Finkelstein reaction
). 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 catalystIn chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo ...
s or highly polar
aprotic solventA protic solvent is a solvent A solvent (from the Latin language, Latin ''wikt:solvo#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 ...
s such as .


Transesterification

Transesterification In organic chemistry Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . Stu ...

Transesterification
, 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 An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a an ...

triglyceride
s, e.g. in the production of fatty acid esters and alcohols. Poly(ethylene terephthalate) is produced by the transesterification of dimethyl terephthalate and ethylene glycol: : (C6H4)(CO2CH3)2 + 2 C2H4(OH)2 →  ''n'' + 2 CH3OH A subset of transesterification is the alcoholysis of diketene. This reaction affords 2-ketoesters. :(CH2CO)2 + ROH → CH3C(O)CH2CO2R


Carbonylation

Alkenes undergo "hydroesterification" in the presence of metal carbonyl catalysts. Esters of propanoic acid are produced commercially by this method: :C2H4 + ROH + CO → C2H5CO2R A preparaton of methyl propionate is one illustrative example. :C2H4 + CO + MeOH → MeO2CCH2CH3 The carbonylation of methanol yields methyl formate, which is the main commercial source of formic acid. The reaction is catalyzed by sodium methoxide: :CH3OH + CO → CH3O2CH


Addition of carboxylic acids to alkenes and alkynes

In the presence of palladium-based catalysts, ethylene,
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
, and oxygen react to give vinyl acetate: :C2H4 + CH3CO2H +  O2 → C2H3O2CCH3 + H2O Direct routes to this same ester are not possible because vinyl alcohol is unstable. Carboxylic acids also add across alkynes to give the same products. Silicotungstic acid is used to manufacture ethyl acetate by the alkylation of
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
by ethylene: :C2H4 + CH3CO2H → CH3CO2C2H5


From aldehydes

The Tishchenko reaction involve disproportionation of an aldehyde in the presence of an anhydrous base to give an ester. Catalysts are aluminium alkoxides or sodium alkoxides. Benzaldehyde reacts with sodium benzyloxide (generated from sodium and benzyl alcohol) to generate benzyl benzoate. The method is used in the production of ethyl acetate from acetaldehyde.


Other methods

* Favorskii rearrangement of α-haloketones in presence of base * Baeyer–Villiger oxidation of ketones with peroxides * Pinner reaction of nitriles with an alcohol * Nucleophilic abstraction of a metal–acyl complex *Hydrolysis of orthoesters in aqueous acid *Cellulolysis via esterification * Ozonolysis of alkenes 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 acid and basic conditions. Under acidic conditions, the reaction is the reverse reaction of the
Fischer esterification Fischer is an German surname, derived from the profession of the fisherman. The name Fischer is the fourth most common German surname. The English version is Fisher Fisher is an archaic term for a fisherman, revived as gender-neutral. Fisher, Fi ...
. 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 Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, ...

amide
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 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 RO−, where R is the organic substituent. Alkoxides are strong Base (chemistry), bases ...

alkoxide
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'' known as halogenoalkanes or alkyl halides) are alkane In , an alkane, or paraffin (a historical that also has ), is an . In other words, an alkane consists of and atoms arranged in a structure in which all the s are . ...
), 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 A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
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 Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . Stu ...

Transesterification
* Ether lipid


References


External links


An introduction to esters


{{Authority control Functional groups Esters,