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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, properties ...
, an ester is a compound derived from an oxoacid (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 Chemical bond, covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or ...
() is replaced by an alkoxy group (), 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 substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
and an alcohol.
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 esters 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 Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and o ...
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 fats ...
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 Volatility (chemistry), volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetherol ...
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 language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a ...
s for a broad array of plastics,
plasticizer A plasticizer (British English, UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity (physics), plasticity, to decrease its viscosity, and/or to decrease friction during its ...
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 ...
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 c ...
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 polymers, synthetic or semi-synthetic materials that use polymers as a main ingredient. Their Plasticity (physics), plasticity makes it possible for plastics to be Injection moulding, moulded, Extrusion, e ...
s, with
monomer In chemistry, a monomer ( ; ''wikt:mono-, mono-'', "one" + ''wikt:-mer, -mer'', "part") is a molecule that can chemical reaction, react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process ...
s linked by ester moieties. Phosphoesters form the backbone of DNA molecules. Nitrate esters, 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 nitration, n ...
, are known for their explosive properties. ''


Nomenclature


Etymology

The word ''ester'' was coined in 1848 by a German chemist Leopold Gmelin, 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 substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
s are commonly named according to the more traditional, so-called " trivial names" 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 (systematically butyl ethanoate), derived from
butanol Butanol (also called butyl alcohol) is a four-carbon Alcohol (chemistry), alcohol with a Chemical formula, formula of Carbon, C4Hydrogen, H9Oxygen, OHydrogen, H, which occurs in five isomeric structures (four structural isomers), from a straight-ch ...
and
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bond ...
(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.


Orthoesters

An uncommon class of organic esters are the orthoesters, which have the formula . Triethylorthoformate () is derived, in terms of its name (but not its synthesis) from orthoformic acid () 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 esters, e.g. triphenylphosphate *
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen ...
forms sulfate esters, e.g., dimethylsulfate *
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 ni ...
forms nitrate esters, e.g. methyl nitrate * boric acid forms borates, e.g. trimethylborate. * carbonic acid forms carbonate esters, e.g. ethylene carbonate 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 hydr ...
form diverse esters * phosphorous acid forms two kinds of phosphite esters, e.g. triethylphosphite () 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 Chromate and dichromate, dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be ...
, which has never been detected, forms di-tert-butyl chromate * sulfurous acid, which is rare, forms dimethylsulfite In principle, all metal and metalloid alkoxides, of which many hundreds are known, could be classified as esters of the hypothetical acids.


Structure and bonding

Esters contain a carbonyl 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 chemical compound, compound with the general formula , where R, R', and R″ represent organic compound, organic functional group, groups or hydrogen atoms ...
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.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, 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 but less polar than alcohols. They participate in
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen (H) atom which is Covalent bond, covalently bound to a more electronegativity, electronegative "donor" atom or group ( ...
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 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, fats 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 and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
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, durians, pears,
banana A banana is an elongated, edible fruit – botanically a berry (botany), berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa (genus), Musa''. In some countries, Cooking banana, bananas used for ...
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 centurie ...
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 ...
. 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 synthetic fibre, fibres for clothing, packaging, conta ...
, acrylate esters, and cellulose acetate. :


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 Vola ...
and flavor industry. Ester bonds are also found in many
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many Repeat unit, repeating subunits. Due to t ...
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 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 (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen ...
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: * 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 azeotropes with
toluene Toluene (), also known as toluol (), is a substituted 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 met ...
, 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, where the substrates are sensitive to harsh conditions like high heat. DCC ( dicyclohexylcarbodiimide) 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 reaction rate, 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 ...
. : Another method for the dehydration of mixtures of alcohols and carboxylic acids is the Mitsunobu reaction: :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 Separation process, separating and analyzing compounds that can be vaporized without Chemical decomposition, decomposition. Typical uses of GC include tes ...
. 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 resins from
acrylic acid Acrylic acid (IUPAC: propenoic acid) is an organic compound with the formula CH2=CHCOOH. It is the simplest unsaturated compound, unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colo ...
.


Alcoholysis of acyl chlorides and acid anhydrides

Alcohols react with acyl chlorides 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 ...
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 chemical compound, compound with the general formula , where R, R', and R″ represent organic compound, organic functional group, groups or hydrogen atoms ...
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 us ...
s to give esters. In the case that an alkyl chloride is used, an iodide salt can catalyze the reaction ( 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 catalysts or highly polar
aprotic solvent A polar aprotic solvent is a solvent that lacks an acidic proton and is polar. Such solvents lack hydroxyl and amine groups. In contrast to protic solvents, these solvents do not serve as proton donors in hydrogen bonding, although they can be prot ...
s such as DMF.


Transesterification

Transesterification In organic chemistry, transesterification is the process of exchanging the organic Functional group, group R″ of an ester with the organic group R' of an Alcohol (chemistry), alcohol. These reactions are often catalyst, catalyzed by the additio ...
, 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 ''wikt:tri-#Prefix, tri-'' and ''glyceride''). Triglycerides are the main constituents of body fat in humans and other ...
s, e.g. in the production of fatty acid esters and alcohols. Poly(ethylene terephthalate) is produced by the transesterification of
dimethyl terephthalate Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the ester, diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan " ...
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. 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 Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula CH3OH (a methyl group In organic chemistry, a methyl group is an alkyl derived from m ...
yields methyl formate, which is the main commercial source of
formic acid Formic acid (), Preferred IUPAC name, 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 nota ...
. The reaction is catalyzed by sodium methoxide: :CH3OH + CO -> CH3O2CH


Addition of carboxylic acids to alkenes and alkynes

In hydroesterification, alkenes and alkynes insert into the H-O bond of carboxylic acids. Vinyl acetate is produced industrially by the addition of acetic acid to acetylene in the presence of zinc acetate catalysts: Presently, zinc acetate is used as the catalyst: :CH3CO2H + C2H2 -> CH3CO2CHCH2 Vinyl acetate can also be produced by
palladium Palladium is a chemical element with the Symbol (chemistry), 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 2 Pallas, asteroid ...
-catalyzed reaction of ethylene,
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bond ...
, and
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
: :C2H4 + CH3CO2H + 1/2 O2 -> C2H3O2CCH3 + H2O Silicotungstic acid is used to manufacture ethyl acetate by the alkylation of
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bond ...
by ethylene: :C2H4 + CH3CO2H -> CH3CO2C2H5


From aldehydes

The Tishchenko reaction involve disproportionation 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 group ...
in the presence of an anhydrous base to give an ester.
Catalyst Catalysis () is the process of increasing the reaction rate, 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 ...
s are aluminium alkoxides or sodium alkoxides. Benzaldehyde reacts with sodium benzyloxide (generated from
sodium Sodium is a chemical element with the Symbol (chemistry), 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 element, group 1 of the ...
and
benzyl alcohol Benzyl alcohol is an aromatic Alcohol (chemistry), 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 ...
) 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 lot ...
. The method is used in the production of ethyl acetate from
acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informa ...
.


Other methods

* Favorskii rearrangement of α-haloketones in presence of base * Baeyer–Villiger oxidation of ketones with peroxides * Pinner reaction 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 meth ...
s with an alcohol * Nucleophilic abstraction of a metal–acyl complex *Hydrolysis of orthoesters in aqueous acid *Cellulolysis via esterification * Ozonolysis of
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—its atom making four electro ...
s using a work up in the presence of
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is a component of the gas ...
and various alcohols. * Anodic oxidation of
methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom chemical bond, bonded to three hydrogen atoms, having chemical formula . In chemical formula, formulas, the group is often skeletal formula#Pseudoe ...
ketones leading to methyl esters. * 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
adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a Addition reaction, direct addition of two or more distinct molecules, resulting in a single reaction product containing a ...
s.


Hydrolysis and saponification

Esterification is a reversible reaction. Esters undergo
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
under acidic and basic conditions. Under acidic conditions, the reaction is the reverse reaction of the Fischer esterification. Under basic conditions,
hydroxide Hydroxide is a polyatomic ion, diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually Self-ionization ...
acts as a nucleophile, while an alkoxide is the leaving group. This reaction,
saponification Saponification is a process of converting esters into soap, soaps and Alcohol (chemistry), alcohols by the action of aqueous alkali (for example, aqueous sodium hydroxide solutions). Soaps are salt (chemistry), salts of fatty acids, which in turn a ...
, is the basis of soap making. : The alkoxide group may also be displaced by stronger nucleophiles such as
ammonia Ammonia is an inorganic chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a dis ...
or primary or secondary
amine In chemistry, amines (, ) are chemical compound, compounds and functional groups that contain a base (chemistry), basic nitrogen atom with a lone pair. Amines are formally derivative (chemistry), derivatives of ammonia (), wherein one or mo ...
s to give
amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent organic compound, organic functional group, groups or hydrogen atoms ...
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 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 Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula lithium, Lialuminium, Alhydride, H4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a re ...
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 In organic chemistry, ethers are a class of organic compound, compounds that contain an ether functional group, group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl o ...
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 alkoxides. 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 us ...
), 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 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 acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s. 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 In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent organic compound, organic functional group, groups or hydrogen atoms ...
, 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 Functional group, group R″ of an ester with the organic group R' of an Alcohol (chemistry), alcohol. These reactions are often catalyst, catalyzed by the additio ...
*
Ether lipid In organic chemistry, ethers are a class of organic compound, compounds that contain an ether functional group, group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl o ...


References


External links


An introduction to esters


{{Authority control Functional groups