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In , an ester is a compound derived from an (either organic or inorganic) in which the atom (H) of at least one group () of that acid is replaced by an group (R). These compounds contain a distinctive functional group. Analogues derived from replaced by other belong to the ester category as well. According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well (e.g. ), but not according to the .

are fatty acid esters of ; they are important in biology, being one of the main classes of and comprising the bulk of and . are cyclic carboxylic esters; naturally occurring lactones are mainly 5- and 6-membered ring lactones. Lactones contribute to the aroma of fruits, butter, cheese, like and other foods.

Esters can be formed from (e.g. esters of , , , , , ), but also from acids that do not contain oxygen (e.g. esters of and ). An example of an ester formation is the substitution reaction between a () and an alcohol (), forming an ester (), where R stands for any group (typically hydrogen or organyl) and R stands for organyl group.

Organyl esters of carboxylic acids typically have a pleasant smell; those of low molecular weight are commonly used as fragrances and are found in and . They perform as high-grade for a broad array of , , , and ,

(1986). 9780969054542, The Group. .
and are one of the largest classes of synthetic on the commercial market.
(1993). 9781420050455, CRC. .
are important plastics, with linked by ester moieties. form the backbone of molecules. , such as , are known for their explosive properties.

There are compounds in which an acidic hydrogen of acids mentioned in this article are not replaced by an organyl, but by some other group. According to some authors, those compounds are esters as well, especially when the first carbon atom of the organyl group replacing acidic hydrogen, is replaced by another atom from the group 14 elements (, , , ); for example, according to them, trimethylstannyl acetate (or trimethyltin acetate) is a ester of , and dibutyltin dilaurate is a ester of , and the Phillips catalyst is a trimethoxysilyl ester of ().

Nomenclature
Etymology
The word ester was coined in 1848 by a German chemist ,Leopold Gmelin, Handbuch der Chemie, vol. 4: Handbuch der organischen Chemie (vol. 1) (Heidelberg, Baden (Germany): Karl Winter, 1848), page 182.
Original text:
b. Ester oder sauerstoffsäure Aetherarten.
Ethers du troisième genre.

Viele mineralische und organische Sauerstoffsäuren treten mit einer Alkohol-Art unter Ausscheidung von Wasser zu neutralen flüchtigen ätherischen Verbindungen zusammen, welche man als gepaarte Verbindungen von Alkohol und Säuren-Wasser oder, nach der Radicaltheorie, als Salze betrachten kann, in welchen eine Säure mit einem Aether verbunden ist.
Translation:
b. Ester or oxy-acid ethers.
Ethers of the third type.

Many mineral and organic acids containing oxygen combine with an alcohol upon elimination of water to form neutral, volatile ether compounds, which one can view as coupled compounds of alcohol and acid-water, or, according to the theory of radicals, as salts in which an acid is bonded with an ether.
probably as a contraction of the German Essigäther, "".

IUPAC nomenclature
The names of esters that are formed from an alcohol and an acid, are derived from the parent alcohol and the parent acid, where the latter may be organic or inorganic. Esters derived from the simplest are commonly named according to the more traditional, so-called "" 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 , has the formula .

The chemical formulas of organic esters formed from carboxylic acids and alcohols usually take the form or RCOOR', where R and R' are the parts of the carboxylic acid and the alcohol, respectively, and R can be a in the case of esters of . For example, (systematically butyl ethanoate), derived from and (systematically ethanoic acid) would be written . Alternative presentations are common including BuOAc and .

Cyclic esters are called , 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 esters are the . One of them are the esters of orthocarboxylic acids. Those esters have the formula , where R stands for any group (organic or inorganic) and R stands for group. For example, triethyl orthoformate () is derived, in terms of its name (but not its synthesis) from of () with .

Esters of inorganic acids
Esters can also be derived from inorganic acids.

Inorganic acids that exist as form two or more types of esters.

  • Thiosulfuric acid forms two types of esters, e.g. O, O-dimethyl thiosulfate () and O, S-dimethyl thiosulfate ()
  • forms esters, e.g. methyl thiocyanate () (if one classifies thiocyanic acid as an inorganic compound), but forms "esters" as well, e.g. methyl isothiocyanate (), although isothiocyanates are not classified as esters by the
  • forms two types of esters: , e.g. triethyl phosphite (), and , e.g. diethyl phosphonate ()

Some inorganic acids that are unstable or elusive form stable esters.

In principle, a part of metal and metalloid , of which many hundreds are known, could be classified as esters of the corresponding acids (e.g., aluminium triethoxide () could be classified as an ester of aluminic acid which is aluminium hydroxide, tetraethyl orthosilicate () could be classified as an ester of orthosilicic acid, and titanium ethoxide () could be classified as an ester of ).

Structure and bonding
Esters derived from and contain a group C=O, which is a group at atom, which gives rise to C–C–O and O–C–O angles. Unlike , carboxylic acid 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 .March, J. Advanced Organic Chemistry 4th Ed. J. Wiley and Sons, 1992: New York. . The p Ka of the alpha-hydrogens on esters of carboxylic acids is around 25 (alpha-hydrogen is a hydrogen bound to the carbon adjacent to the (C=O) of carboxylate esters).

Many carboxylic acid 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. with small rings are restricted to the s-trans (i.e. E) conformation due to their cyclic structure.

Physical properties and characterization
Esters derived from and alcohols are more polar than but less polar than alcohols. They participate in 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 acids of similar molecular weight.

Characterization and analysis
Esters are generally identified by gas chromatography, taking advantage of their volatility. 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 conjunction 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, are, in general, triesters derived from and .Isolation of triglyceride from nutmeg: G. D. Beal "Trimyristen" Organic Syntheses, Coll. Vol. 1, p.538 (1941). Link Esters are responsible for the aroma of many fruits, including , , , , , and .McGee, Harold. On Food and Cooking. 2003, Scribner, New York. Several billion kilograms of are produced industrially annually, important products being polyethylene terephthalate, , and cellulose acetate.
found in a linseed oil, a triester of (center, black) derived of (bottom right, green), (left, red), and (top right, blue).]]

Preparation
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 pleasant characteristic, fruity odor. This leads to their extensive use in the and industry. Ester bonds are also found in many .

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:
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. is a typical catalyst for this reaction. Many other acids are also used such as . 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 are also effective.
  • Removal of water by physical means such as as a low-boiling with , 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 .

Another method for the dehydration of mixtures of alcohols and carboxylic acids is the Mitsunobu reaction:

Carboxylic acids can be esterified using :

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 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 , giving β-hydroxyesters:
This reaction is employed in the production of vinyl ester resin from .

Alcoholysis of acyl chlorides and acid anhydrides
Alcohols react with and to give esters:

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 are less sensitive because amines are stronger and react more rapidly than does water. This method is employed only for laboratory-scale procedures, as it is expensive.

Alkylation of carboxylic acids and their salts
Trimethyloxonium tetrafluoroborate can be used for of carboxylic acids under conditions where acid-catalyzed reactions are infeasible:
Although rarely employed for esterifications, carboxylate salts (often generated in situ) react with , such as , to give esters. Anion availability can inhibit this reaction, which correspondingly benefits from phase transfer catalysts or such highly polar as DMF. An additional iodide salt may, via the Finkelstein reaction, catalyze the reaction of a recalcitrant alkyl halide. Alternatively, salts of a coordinating metal, such as silver, may improve the reaction rate by easing halide elimination.

Transesterification
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 , 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:

A subset of transesterification is the alcoholysis of . This reaction affords 2-ketoesters.

Carbonylation
Alkenes undergo carboalkoxylation in the presence of catalysts. Esters of are produced commercially by this method:
A preparation of methyl propionate is one illustrative example.

The carbonylation of yields , which is the main commercial source of . The reaction is catalyzed by :

Addition of carboxylic acids to alkenes and alkynes
In hydroesterification, alkenes and alkynes insert into the bond of carboxylic acids. is produced industrially by the addition of acetic acid to in the presence of catalysts:

can also be produced by -catalyzed reaction of ethylene, , and :

Silicotungstic acid is used to manufacture by the of by ethylene:

From aldehydes
The Tishchenko reaction involves disproportionation of an in the presence of an anhydrous base to give an ester. are aluminium alkoxides or sodium alkoxides. reacts with sodium benzyloxide (generated from and ) to generate . The method is used in the production of from .

Other methods
  • Favorskii rearrangement of α-haloketones in presence of base
  • Baeyer–Villiger oxidation of ketones with peroxides
  • of with an alcohol
  • Nucleophilic abstraction of a metal–acyl complex
  • Hydrolysis of in aqueous acid
  • Cellulolysis via esterification
  • of using a work up in the presence of hydrochloric acid and various .
  • Anodic oxidation of leading to methyl esters.
  • Interesterification exchanges the fatty acid groups of different esters.

Reactions
Esters are less reactive than acid halides and anhydrides. As with more reactive acyl derivatives, they can react with and primary and secondary amines to give amides, although this type of reaction is not often used, since acid halides give better yields.

Transesterification
Esters can be converted to other esters in a process known as transesterification. Transesterification can be either acid- or base-catalyzed, and involves the reaction of an ester with an alcohol. Unfortunately, because the leaving group is also an alcohol, the forward and reverse reactions will often occur at similar rates. Using a large excess of the alcohol or removing the leaving group alcohol (e.g. via ) will drive the forward reaction towards completion, in accordance with Le Chatelier's principle.Wade 2010, pp. 1005–1009.

Hydrolysis and saponification
Acid-catalyzed hydrolysis of esters is also an equilibrium process – essentially the reverse of the Fischer esterification reaction. Because an alcohol (which acts as the leaving group) and water (which acts as the nucleophile) have similar p Ka values, the forward and reverse reactions compete with each other. As in transesterification, using a large excess of reactant (water) or removing one of the products (the alcohol) can promote the forward reaction.

Basic hydrolysis of esters, known as , is not an equilibrium process; a full equivalent of base is consumed in the reaction, which produces one equivalent of alcohol and one equivalent of a carboxylate salt. The saponification of esters of is an industrially important process, used in the production of soap.

Esterification is a reversible reaction. Esters undergo under acidic and basic conditions. Under acidic conditions, the reaction is the reverse reaction of the Fischer esterification. Under basic conditions, acts as a nucleophile, while an alkoxide is the leaving group. This reaction, , is the basis of soap making.

The alkoxide group may also be displaced by stronger nucleophiles such as or primary or secondary to give (ammonolysis reaction):

This reaction is not usually reversible. Hydrazines and hydroxylamine can be used in place of amines. Esters can be converted to through intermediate in the Lossen rearrangement.

Sources of carbon nucleophiles, e.g., 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 .
A typical catalyst is . 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. reduces esters to aldehydes.

Direct reduction to give the corresponding is difficult as the intermediate 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 with a variety of Lewis acids.

Claisen condensation and related reactions
Esters can undergo a variety of reactions with carbon nucleophiles. They react with an excess of a to give tertiary alcohols. Esters also react readily with . In the Claisen condensation, an enolate of one ester ( 1) will attack the carbonyl group of another ester ( 2) to give tetrahedral intermediate 3. The intermediate collapses, forcing out an alkoxide (R'O−) and producing β-keto ester 4.

Crossed Claisen condensations, in which the enolate and nucleophile are different esters, are also possible. An intramolecular Claisen condensation is called a Dieckmann condensation or Dieckmann cyclization, since it can be used to form rings. Esters can also undergo condensations with ketone and aldehyde enolates to give β-dicarbonyl compounds.Carey 2006, pp. 919–924. A specific example of this is the Baker–Venkataraman rearrangement, in which an aromatic ortho-acyloxy ketone undergoes an intramolecular nucleophilic acyl substitution and subsequent rearrangement to form an aromatic β-diketone.Kürti and Czakó 2005, p. 30. The Chan rearrangement is another example of a rearrangement resulting from an intramolecular nucleophilic acyl substitution reaction.

Other ester reactivities
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 .

As for , 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 . Deprotonation gives a nucleophilic , 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 reacts with an electrophile (e.g., ), and is subsequently decarboxylated. Another variation is the Fráter–Seebach alkylation.

Other reactions
  • Esters can be directly converted to .
  • Methyl esters are often susceptible to decarboxylation in the Krapcho decarboxylation.
  • 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 .
  • Pairs of esters are coupled to give α-hydroxyketones in the acyloin condensation.

Protecting groups
As a class, esters serve as for . Protecting a carboxylic acid is useful in peptide synthesis, to prevent self-reactions of the bifunctional . 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 , 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.
(2025). 9783527306732

nail polish remover, , ,
Isopropyl acetate fruity
,
, ,
(pentyl acetate) ,
, (main component of banana essence) (flavoring in )
pear-like
2-Hexenyl acetate fruity, both cis and trans are used, sometimes individually
3,5,5-Trimethylhexyl acetate woody
fruity-orange
, ,
(see also isobornyl acetate)
peppermint
,
,
,
, ,

pleasant, , , sweet
, , rare example of a propionate odorant
, ,
, , , perfumes
Propyl isobutyrate
, honey
banana
Ethyl isobutyrate blueberries, used in alcoholic drinks
Terpinyl butyrate

Methyl pentanoate (methyl valerate)
Ethyl isovalerate , used in alcoholic drinks
Geranyl pentanoate
Pentyl pentanoate (amyl valerate)
,
, , , , used in alcoholic drinks
(amyl caproate) ,
, waxy-green banana
orange
, , , medicinal, ,
Methyl phenylacetate
Methyl salicylate (oil of wintergreen) Modern , , and ointments (UK)

See also

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