Ester


In chemistry, an ester is a chemical compound derived from an acid in which at least one –OH group is replaced by an –O–alkyl group. Usually, esters are derived from substitution reaction of 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. Esters usually have a sweet smell and are considered high-quality solvents for a broad array of plastics, plasticizers, resins, and lacquers. They are also one of the largest classes of synthetic lubricants on the commercial market.
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Nomenclature

Etymology

The word ester was coined in 1848 by a 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 may 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 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 CH36CO25CH3.
group 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, derived from butanol and acetic acid would be written CH3CO2C4H9. Alternative presentations are common including BuOAc and CH3COOC4H9.
Cyclic esters are called lactones, regardless of whether they are derived from an organic or an 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 RC3. Triethylorthoformate is derived, in terms of its name from orthoformic acid and ethanol.

Inorganic esters

Esters can also be derived from inorganic acids.
Inorganic acids that exist as tautomers form diverse esters
Inorganic acids that are unstable or elusive form stable esters.
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 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 and more volatile 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 conformation rather than the s-trans 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 conformation due to their cyclic structure.
, distances in picometers.

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 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. 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 many fruits, including apples, durians, pears, bananas, pineapples, and strawberries. Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.

Preparation

Esterification is the general name for a chemical reaction in which two reactants 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 and flavor industry. Ester bonds are also found in many polymers.

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. 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:
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 is used to activate the carboxylic acid to further reaction. 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:
Carboxylic acids can be esterified using diazomethane:
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 epoxides, giving β-hydroxyesters:
This reaction is employed in the production of vinyl ester resin resins from acrylic acid.

Alcoholysis of acyl chlorides and acid anhydrides

Alcohols react with acyl chlorides and acid anhydrides 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 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 catalyze the 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 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 is produced by the transesterification of dimethyl terephthalate and ethylene glycol:
A subset of transesterification is the alcoholysis of diketene. This reaction affords 2-ketoesters.

Carbonylation

Alkenes undergo "hydroesterification" in the presence of metal carbonyl catalysts. Esters of propionic acid are produced commercially by this method:
A preparaton of methyl propionate is one illustrative example.
The carbonylation of methanol yields methyl formate, which is the main commercial source of formic acid. The reaction is catalyzed by sodium methoxide:

Addition of carboxylic acids to alkenes and alkynes

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

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 to generate benzyl benzoate. The method is used in the production of ethyl acetate from acetaldehyde.

Other methods

Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic but is attacked by strong nucleophiles. 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. 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 amides:
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.
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. 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 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, and is subsequently decarboxylated. Another variation is the Fráter–Seebach alkylation.

Other reactions

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 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.
Ester nameStructureOdor 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
Isopropyl acetatefruity
Linalyl acetatelavender, sage
Linalyl butyratepeach
Linalyl formateapple, peach
Methyl acetateglue
Methyl anthranilategrape, jasmine
Methyl benzoatefruity, ylang ylang, feijoa
Methyl butyrate pineapple, apple, strawberry
Methyl cinnamatestrawberry
Methyl pentanoate flowery
Methyl phenylacetatehoney
Methyl salicylate Modern root beer, wintergreen, Germolene and Ralgex ointments
Nonyl caprylateorange
Octyl acetatefruity-orange
Octyl butyrateparsnip
Amyl acetate apple, banana
Pentyl butyrate apricot, pear, pineapple
Pentyl hexanoate apple, pineapple
Pentyl pentanoate apple
Propyl acetatepear
Propyl hexanoateblackberry, pineapple, cheese, wine
Propyl isobutyraterum
Terpenyl butyratecherry