Ester

In chemistry, an ester is a compound derived from an acid in which the hydrogen atom of at least one acidic hydroxyl group of that acid is replaced by an organyl group. These compounds contain a distinctive functional group. Analogues derived from oxygen replaced by other chalcogens belong to the ester category as well. According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well, but not according to the IUPAC.
Glycerides are fatty acid esters of glycerol; they are important in biology, being one of the main classes of lipids and comprising the bulk of animal fats and vegetable oils. Lactones are cyclic carboxylic esters; naturally occurring lactones are mainly 5- and 6-membered ring lactones. Lactones contribute to the aroma of fruits, butter, cheese, vegetables like celery and other foods.
Esters can be formed from oxoacids, but also from acids that do not contain oxygen. An example of an ester formation is the substitution reaction between a carboxylic acid and an alcohol, forming an ester, where R stands for any group 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 essential oils and pheromones. They perform as high-grade solvents for a broad array of plastics, plasticizers, resins, and lacquers, and are one of the largest classes of synthetic lubricants on the commercial market. Polyesters are important plastics, with monomers linked by ester moieties. Esters of phosphoric acid form the backbone of DNA molecules. Esters of nitric acid, such as nitroglycerin, 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 is a trimethylstannyl ester of acetic acid, and dibutyltin dilaurate is a dibutylstannylene ester of lauric acid, and the Phillips catalyst is a trimethoxysilyl ester of chromic acid.

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

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 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.
Image:Butyl acetate ester example.png|thumb|Butyl acetate, an ester derived from a residue of butanol and acetic acid . The acidic hydrogen atom from acetic acid molecule is replaced by the butyl group.
The chemical formulas of organic esters formed from carboxylic acids and alcohols usually take the form or RCOOR', where R and R' are the organyl parts of the carboxylic acid and the alcohol, respectively, and R can be a hydrogen in the case of esters of formic acid. For example, butyl acetate, derived from butanol and acetic acid would be written. Alternative presentations are common including BuOAc and.
Cyclic esters are called lactones, 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 orthoesters. One of them are the esters of orthocarboxylic acids. Those esters have the formula, where R stands for any group and R stands for organyl group. For example, triethyl orthoformate is derived, in terms of its name from esterification of orthoformic acid with ethanol.

Esters of inorganic acids

Esters can also be derived from inorganic acids.

Perchloric acid forms perchlorate esters, e.g., methyl perchlorate
Sulfuric acid forms sulfate esters, e.g., dimethyl sulfate and methyl bisulfate
Nitric acid forms nitrate esters, e.g. methyl nitrate and nitroglycerin
Phosphoric acid forms phosphate esters, e.g. triphenyl phosphate and methyl dihydrogen phosphate
*Pyrophosphoric acid forms pyrophosphate esters, e.g. tetraethyl pyrophosphate, ADP, dADP, ADPR, cADPR, CDP, dCDP, GDP, dGDP, UDP, dTDP, MEcPP, HMBPP, DMAPP, IPP, GPP, FPP, GGPP, ThDP, FAD, NAD, NADP.
*Triphosphoric acid forms triphosphate esters, e.g. ATP, dATP, CTP, dCTP, GTP, dGTP, UTP, dTTP, ITP, XTP, ThTP, AThTP.
*Tetraphosphoric acid forms tetraphosphate esters, e.g. hexaethyl tetraphosphate, adenosine tetraphosphate, Ap4A.
Carbonic acid forms carbonate esters, e.g. dimethyl carbonate and 5-membered cyclic ethylene carbonate
Trithiocarbonic acid forms trithiocarbonate esters, e.g. dimethyl trithiocarbonate
Chloroformic acid forms chloroformate esters, e.g. methyl chloroformate
Boric acid forms borate esters, e.g. trimethyl borate
Chromic acid forms di-tert-butyl chromate

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

Thiosulfuric acid forms two types of thiosulfate esters, e.g. O,''O-dimethyl thiosulfate and O'',S-dimethyl thiosulfate
Thiocyanic acid forms thiocyanate esters, e.g. methyl thiocyanate , but forms isothiocyanate "esters" as well, e.g. methyl isothiocyanate, although organyl isothiocyanates are not classified as esters by the IUPAC
Phosphorous acid forms two types of esters: phosphite esters, e.g. triethyl phosphite, and phosphonate esters, e.g. diethyl phosphonate

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

Sulfurous acid, which is unstable, forms stable dimethyl sulfite
Dicarbonic acid, which is unstable, forms stable dimethyl dicarbonate

In principle, a part of metal and metalloid alkoxides, of which many hundreds are known, could be classified as esters of the corresponding acids 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 orthotitanic acid).

Structure and bonding

Esters derived from carboxylic acids and alcohols contain a carbonyl group C=O, which is a divalent group at C atom, which gives rise to C–C–O and O–C–O angles. Unlike amides, 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 and more volatile than the corresponding amides. The pK_a of the alpha-hydrogens on esters of carboxylic acids is around 25.
Many carboxylic acid 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.

Physical properties and characterization

Esters derived from carboxylic acids and alcohols 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 ν_C=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, 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:

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 azeotrope 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. 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.