In chemistry, an ester is a chemical compound derived from an acid
(organic or inorganic) in which at least one –OH (hydroxyl) group is
replaced by an –O–alkyl (alkoxy) group. 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
1.1 Etymology 1.2 IUPAC nomenclature 1.3 Orthoesters 1.4 Inorganic esters
2 Structure and bonding 3 Physical properties and characterization
3.1 Characterization and analysis
4 Applications and occurrence 5 Preparation
5.1 Esterification of carboxylic acids 5.2 Alcoholysis of acyl chlorides and acid anhydrides 5.3 Alkylation of carboxylate salts 5.4 Transesterification 5.5 Carbonylation 5.6 Addition of carboxylic acids to alkenes 5.7 Other methods
6.1 Addition of nucleophiles at carbonyl
7 List of ester odorants 8 See also 9 References 10 External links
The word 'ester' was coined in 1848 by a German chemist Leopold
Gmelin, probably as a contraction of the German Essigäther,
IUPAC nomenclature of organic chemistry
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 are derived from an organic or an inorganic acid. One example of a (organic) lactone is γ-valerolactone. Orthoesters 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 also 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 sulfate 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, sulfur atoms can replace some oxygen atoms in carbon–oxygen–central inorganic atom covalent bonds of an ester. As a result, thiosulfinates' and thiosulfonates, with a central inorganic sulfur atom, demonstrate clearly the assortment of sulfur esters, that also 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 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.
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
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.
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 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.
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 specialized organic
synthetic operations but is considered too hazardous and expensive for
Alcoholysis of acyl chlorides and acid anhydrides
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 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 (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 solvents such as DMF. Transesterification Transesterification, which involves changing one ester into another one, is widely practiced:
RCO2R′ + CH3OH → RCO2CH3 + R′OH
Like the hydrolysation, transesterification is catalysed by acids and bases. The reaction is widely used for degrading 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)2 → 1⁄n (C6H4)(CO2)2(C2H4) n + 2 CH3OH
Carbonylation 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 catalyzed by sodium methoxide:
CH3OH + CO → CH3O2CH
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 + 1⁄2 O2 → C2H3O2CCH3 + H2O
Direct routes to this same ester are not possible because vinyl alcohol is unstable. Other methods
Reactions Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic but is attacked by strong nucleophiles (amines, alkoxides, hydride sources, organolithium compounds, etc.). The C–H bonds adjacent to the carbonyl are weakly acidic but undergo deprotonation with strong bases. This process is the one that usually initiates condensation reactions. The carbonyl oxygen in esters is weakly basic, less so than the carbonyl oxygen in amides due to resonance donation of an electron pair from nitrogen in amides, but forms adducts. 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 may also be displaced by stronger nucleophiles such as ammonia or primary or secondary amines to give amides: (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 is
used to reduce esters to two primary alcohols. The related reagent
sodium borohydride is slow in this reaction.
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 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 commonplace use in artificial flavorings and fragrances when those odors aim to be mimicked.
pear, strawberry, jasmine
nail polish remover, model paint, model airplane glue
sweet, wintergreen, fruity, medicinal, cherry, grape
banana, pineapple, strawberry
pineapple, waxy-green banana
lemon, rum, strawberry
apricot, cherry, grape, raspberry
cherry, raspberry, strawberry
pear, banana (flavoring in
fruity, ylang ylang, feijoa
pineapple, apple, strawberry
apricot, pear, pineapple
blackberry, pineapple, cheese, wine
Amide, an ester analog with oxygen replaced by nitrogen
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "esters". ^ Leopold Gmelin, Handbuch der Chemie, vol. 4: Handbuch der organischen Chemie (vol. 1) (Heidelberg, Baden (Germany): Karl Winter, 1848), page 182. Original text:
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.
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.
^ a b March, J. Advanced Organic
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An introduction to esters
Molecule of the month:
v t e
Only carbon, hydrogen and oxygen
Only one element apart from C, H, O
Disulfide Sulfone Sulfonic acid Sulfoxide Thial Thioester Thioether Thioketone Thiol
Selenol Selenonic acid Seleninic acid Selenenic acid
Isothiocyanate Phosphoramide Sulfenyl chloride Sulfonamide Thiocyanate
See also chemical classification, chemical nomenclature (inorganic, organic)
v t e
Methyl formate Methyl acetate Methyl propionate Methyl butyrate Methyl pentanoate
Ethyl formate Ethyl acetate Ethyl propionate Ethyl butyrate Ethyl pentanoate Ethyl hexanoate Ethyl heptanoate Ethyl octanoate Ethyl decanoate
Propyl acetate Propyl propanoate Isopropyl acetate Isopropyl palmitate
Amyl acetate Isoamyl acetate Sec-Amyl acetate Pentyl propanoate Pentyl butyrate Pentyl pentanoate Pentyl hexanoate