In chemistry, esters are chemical compounds derived from an acid (organic or inorganic) in which at least one –OH (hydroxyl) group is replaced by an –O–alkyl (alkoxy) group. Usually, esters are derived from a carboxylic acid and an alcohol. Glycerides, which are fatty acid esters of glycerol, are important esters in biology, being one of the main classes of lipids, and making up the bulk of animal fats and vegetable oils. Esters with low molecular weight are commonly used as fragrances and found in essential oils and pheromones. Phosphoesters form the backbone of DNA molecules. Nitrate esters, such as nitroglycerin, are known for their explosive properties, while polyesters are important plastics, with monomers linked by ester moieties.



The word 'ester' was coined in 1848 by German chemist Leopold Gmelin, probably as a contraction of the German Essigäther, "acetic ether".

IUPAC nomenclature

Ester names are derived from the parent alcohol and the parent acid, where the latter might be organic or inorganic. Esters derived from the simplest carboxylic acids 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 additional 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, additionally known under the trivial name hexyl caprylate, has the formula CH3(CH2)6CO2(CH2)5CH3.

Ethyl acetate derived from an alcohol (blue) and an acyl group (yellow) derived from a carboxylic acid.

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 (systematically butyl ethanoate), derived from butanol and acetic acid (systematically ethanoic acid) would be written CH3CO2C4H9. Alternative presentations are common including BuOAc and CH3COOC4H9.

Cyclic esters are called lactones, regardless of whether they're derived from an organic or an inorganic acid. One example of a (organic) lactone is γ-valerolactone.


An uncommon class of organic esters are the orthoesters, which have the formula RC(OR′)3. Triethylorthoformate (HC(OC2H5)3) is derived, in terms of its name (but not its synthesis) from orthoformic acid (HC(OH)3) and ethanol.

Inorganic esters

A phosphoric acid ester

Esters can additionally be derived from an inorganic acid and an alcohol. Thus, the nomenclature extends to inorganic oxo acids and their corresponding esters: phosphoric acid and phosphate esters/organophosphates, sulfuric acid and sulphate esters/organosulfates, nitric acid and nitrate, and boric acid and borates. For example, triphenyl phosphate is the ester derived from phosphoric acid and phenol. Organic carbonates are derived from carbonic acid; for example, ethylene carbonate is derived from carbonic acid and ethylene glycol.

So far an alcohol and inorganic acid are linked via oxygen atoms. The definition of inorganic acid ester that feature inorganic chemical elements links between alcohols and the inorganic acid – the phosphorus atom linking to three alkoxy functional groups in organophosphate – can be extended to the same elements in various combinations of covalent bonds between carbons and the central inorganic atom and carbon–oxygen bonds to central inorganic atoms. For example, phosphorus features three carbon–oxygen–phosphorus bondings and one phosphorus–oxygen double bond in organophosphates,

structure of a generic organophosphate

three carbon–oxygen–phosphorus bondings and no phosphorus–oxygen double bonds in phosphite esters or organophosphites,

structure of a generic phosphite ester showing the lone pairs on the P

two carbon–oxygen–phosphorus bondings, no phosphorus–oxygen double bonds but one phosphorus–carbon bond in phosphonites,

structure of a generic phosphonite – ester of phosphonous acid

one carbon–oxygen–phosphorus bondings, no phosphorus–oxygen double bonds but two phosphorus–carbon bonds in phosphinites.

structure of a generic phosphinite.

In corollary, boron features borinic esters (n = 2), boronic esters (n = 1), and borates (n = 0).

As oxygen is a group 16 chemical element, sulphur atoms can replace a few oxygen atoms in carbon–oxygen–central inorganic atom covalent bonds of an ester. As a result, thiosulfinates' and thiosulfonates, with a central inorganic sulphur atom, demonstrate clearly the assortment of sulfur esters, that additionally includes sulfates, sulfites, sulfonates, sulfinates, sulfenates esters.

Structure and bonding

Esters contain a carbonyl center, which gives rise to 120 ° C–C–O and O–C–O angles. Unlike amides, 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. The pKa 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. Lactones 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 bonds as hydrogen-bond acceptors, but can't act as hydrogen-bond donors, unlike their parent alcohols. This ability to participate in hydrogen bonding confers a few water-solubility. Because of their lack of hydrogen-bond-donating ability, esters don't self-associate. Consequently, esters are more volatile than carboxylic acids 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 νC=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 and fatty acids. Esters are responsible for the aroma of a large number of fruits, including apples, durians, pears, bananas, pineapples, and strawberries. Several billion kilogrammes of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.

Representative triglyceride found in a linseed oil, a triester (triglyceride) derived of linoleic acid, alpha-linolenic acid, and oleic acid.


Esterification is the general name for a 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 characteristic pleasant, fruity odor. This leads to their extensive use in the fragrance and flavor industry. Ester bonds are additionally found in a large number of polymers.

Esterification of carboxylic acids

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 is a typical catalyst for this reaction. Many additional acids are additionally 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 catalyses the reaction but sequesters water (a reaction product). Other drying agents such as molecular sieves are additionally effective.
  • Removal of water by physical means such as distillation as a low-boiling azeotropes with toluene, 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. DMAP (4-dimethylaminopyridine) is used as an acyl-transfer catalyst.

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:

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. The method is useful in specialised organic synthetic operations but is considered too hazardous and expensive for large scale applications.

Alcoholysis of acyl chlorides and acid anhydrides

Alcohols react with acyl chlorides and acid anhydrides 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 additionally react with water, anhydrous conditions are preferred. The analogous acylations of amines to give amides are less sensitive because amines are stronger nucleophiles 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 halides to give esters. In the case that an alkyl chloride is used, an iodide salt can catalyse the reaction (Finkelstein reaction). The carboxylate salt is often generated in situ. In difficult cases, the silver carboxylate might be used, after 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 solvents such as DMF.


Transesterification, which involves changing one ester into another one, is widely practiced:


Like the hydrolysation, transesterification is catalysed by acids and bases. The reaction is widely used for degrading triglycerides, 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)21n {(C6H4)(CO2)2(C2H4)}n + 2 CH3OH


Alkenes undergo "hydroesterification" in the presence of metal carbonyl catalysts. Esters of propionic acid are produced commercially by this method:

C2H4 + ROH + CO → C2H5CO2R

The carbonylation of methanol yields methyl formate, which is the main commercial source of formic acid. The reaction is catalysed by sodium methoxide:


Addition of carboxylic acids to alkenes

In the presence of palladium-based catalysts, ethylene, acetic acid, and oxygen react to give vinyl acetate:

C2H4 + CH3CO2H + 12 O2 → C2H3O2CCH3 + H2O

Direct routes to this same ester aren't possible because vinyl alcohol is unstable.

Other methods


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 is weakly basic (less so than in amides) but forms adducts.

Addition of nucleophiles at carbonyl

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. 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 might additionally be displaced by stronger nucleophiles such as ammonia or primary or secondary amines to give amides: (ammonolysis reaction)


This reaction isn't 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.


Compared to ketones and aldehydes, esters are relatively resistant to reduction. The introduction of catalytic hydrogenation in the early part of the twentieth 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.

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), and is subsequently decarboxylated. An Additional variation is the Fráter–Seebach alkylation.

Other reactions

Protecting groups

As a class, esters serve as protecting groups for carboxylic acids. 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 a large number of amino acids; the t-butyl ester tends to be more expensive. Notwithstanding 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 a large number of occur naturally in the essential oils of plants. This has additionally led to their commonplace use in artificial flavourings and fragrances when those odours aim to be mimicked.

Ester nameFormulaOdor or occurrence
Allyl hexanoatepineapple
Benzyl acetatepear, strawberry, jasmine
Bornyl acetatepine
Butyl acetateapple, honey
Butyl butyratepineapple
Butyl propanoatepear drops
Ethyl acetatenail polish remover, model paint, model airplane glue
Ethyl benzoatesweet, wintergreen, fruity, medicinal, cherry, grape
Ethyl butyratebanana, pineapple, strawberry
Ethyl hexanoatepineapple, waxy-green banana
Ethyl cinnamatecinnamon
Ethyl formatelemon, rum, strawberry
Ethyl heptanoateapricot, cherry, grape, raspberry
Ethyl isovalerateapple
Ethyl lactatebutter, cream
Ethyl nonanoategrape
Ethyl pentanoateapple
Geranyl acetategeranium
Geranyl butyratecherry
Geranyl pentanoateapple
Isobutyl acetatecherry, raspberry, strawberry
Isobutyl formateraspberry
Isoamyl acetatepear, banana (flavoring in Pear drops)
Isopropyl acetatefruity
Linalyl acetatelavender, sage
Linalyl butyratepeach
Linalyl formateapple, peach
Methyl acetateglue
Methyl anthranilategrape, jasmine
Methyl benzoatefruity, ylang ylang, feijoa
Methyl butyrate (methyl butanoate)pineapple, apple, strawberry
Methyl cinnamatestrawberry
Methyl pentanoate (methyl valerate)flowery
Methyl phenylacetatehoney
Methyl salicylate (oil of wintergreen)Modern root beer, wintergreen, Germolene and Ralgex ointments (UK)
Nonyl caprylateorange
Octyl acetatefruity-orange
Octyl butyrateparsnip
Amyl acetate (pentyl acetate)apple, banana
Pentyl butyrate (amyl butyrate)apricot, pear, pineapple
Pentyl hexanoate (amyl caproate)apple, pineapple
Pentyl pentanoate (amyl valerate)apple
Propyl acetatepear
Propyl hexanoateblackberry, pineapple, cheese, wine
Propyl isobutyraterum
Terpenyl butyratecherry