In chemistry esters are classes of chemical compounds. Esters are usually derived from
an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl
group. Commonly esters are derived from carboxylic acids and alcohol. Esters comprise most naturally
occurring fats and oils, which are fatty acid esters of glycerol. 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.
Nomenclature Etymology
The word 'ester' was coined in 1848 by German chemist Leopold Gmelin, probably as a contraction
of the German Essigäther - acetic ether. IUPAC nomenclature of Esters
Ester names are derived from the parent alcohol and the parent acid, where the latter may
be an organic or an inorganic acid. 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 CH3(CH2)6CO2(CH2)5CH3.
The chemical formulas of organic esters are typically written in the format of RCO2R',
where R and R' are the hydrocarbon parts of the carboxylic acid and 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 a lactone is gamma-valerolactone.
Orthoesters An uncommon class of organic esters are the
orthoesters, which have the formula RC(OR')3. Triethylorthoformate3) is derived, in terms
of its name from orthoformic acid3) and ethanol. Inorganic esters
Ester is a general term for the product derived from the condensation of an acid and an alcohol.
Thus, the nomenclature extends to inorganic oxo acids, e.g. phosphoric acid, sulfuric
acid, nitric acid and boric acid. For example, triphenyl phosphate is the ester derived from
phosphoric acid and phenol. Organic carbonates, such as ethylene carbonate, are derived from
carbonic acid and ethylene glycol. 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. 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 usually 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 characteristic pleasant,
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
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 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 using a dehydrating agent: sulfuric acid not
only catalyzes the reaction but sequesters water. 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 is used to activate the carboxylic acid to further reaction. DMAP 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 specialized organic synthetic operations
but is considered too 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 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
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 Favorskii rearrangement of α-haloketones
in presence of base Baeyer-Villiger oxidation of ketones with
peroxides Pinner reaction of nitriles with an alcohol
Nucleophilic abstraction of a metal-acyl complex Hydrolysis of orthoesters in aqueous acid
Cellulolysis via esterification Reactions
Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic
but is attacked by strong nucleophilies. 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 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:
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.
DIBAH reduces esters to aldehydes. 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 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. 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. See also
Cyanate ester Oligoester
Polyolester Transesterification
Transamidification References
External links An introduction to esters
Molecule of the month: Ethyl acetate and other esters