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ester

ester

[es-ter]
ester, any one of a group of organic compounds with general formula RCO2R' (where R and R' are alkyl groups or aryl groups) that are formed by the reaction between an alcohol and an acid. For example, when ethanol and acetic acid react, ethyl acetate (an ester) and water are formed; the reaction is called esterification. Ethyl acetate is used as a solvent. Methyl acetate, formed by the reaction between methanol and acetic acid, is a sweet-smelling liquid used in making perfumes, extracts, and lacquers. Esters react with water (hydrolysis) under basic conditions to form an alcohol and an acid. When heated with a hydroxide certain esters decompose to yield soap and glycerin; the process is called saponification. Common fats and oils are mixtures of various esters, such as stearin, palmitin, and linolein, formed from the alcohol glycerol and fatty acids. Naturally occurring esters of organic acids in fruits and flowers give them their distinctive odors. Esters perform important functions in the animal body; e.g., the ester acetylcholine is a chemical transmitter of nerve stimuli.

Any of a class of organic compounds that can react with water (see hydrolysis) to produce an alcohol and an organic or inorganic acid. They are formed by the reverse process, esterification, in which acid reacts with alcohol to form an ester and water. Esters of carboxylic acids, the most common esters, contain the acid's carbonyl group (singlehorzbondCdoublehorzbondO; see functional group); the carbon's fourth bond is with the alcohol's oxygen atom. Hydrolysis of esters in the presence of an alkali (saponification) is used to make soaps from fats and oils. Carboxylic acid esters of low molecular weight are colourless, volatile liquids with pleasant odours; they give flavour and fragrance to fruits and flowers and are used as synthetic flavours and fragrances. Others, such as ethyl acetate and butyl acetate, are used as solvents for lacquers, paints, and varnishes. Certain polymers are esters, including Lucite (polymethyl methacrylate) and Dacron (polyethylene terephthalate). Esters of alcohols and inorganic acids include nitrate esters (e.g., nitroglycerin), which are explosive; phosphate esters, including such biologically important compounds as nucleic acids; and others that are used as flame retardants, solvents, plasticizers, gasoline and oil additives, and insecticides.

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Esters are a class of chemical compounds and functional groups. Esters consist of an inorganic or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group. Some acids that are commonly esterified are carboxylic acids, phosphoric acid, sulfuric acid, nitric acid, and boric acid. Volatile esters, particularly carboxylate esters, often have a pleasant smell and are found in perfumes, essential oils, and pheromones, and give many fruits their scent. Ethyl acetate and methyl acetate are important solvents; fatty acid esters form fat and lipids; phosphoesters form the backbone of DNA molecules; and polyesters are important plastics. Cyclic esters are called lactones. The name "ester" is derived from the German Essig-Äther (literally: vinegar ether), an old name for ethyl acetate. Esters can be synthesized in a condensation reaction between an acid and an alcohol in a reaction known as esterification.

Nomenclature

An ester is named according to the two parts that make it up: the part from the alcohol and the part from the acid (in that order), for example ethyl ethanoate (see image below).

Since most esters, or carbonate, are derived from carboxylic acids, a specific nomenclature is used for them. For esters derived from the simplest carboxylic acids, the traditional name for the acid constituent is generally retained, e.g., formate, acetate, propionate, butyrate. For esters from more complex carboxylic acids, the systematic name for the acid is used, followed by the suffix -oate. For example, methyl formate is the ester of methanol and methanoic acid (formic acid): the simplest ester. It could also be called methyl methanoate.

Esters of aromatic acids are also encountered, including benzoates such as methyl benzoate, and phthalates, with substitution allowed in the name.

The chemical formulas of esters are typically in the format of R-COO-R', in which the alkyl group (R') is mentioned first, and the carboxylate group (R) is mentioned last. For example the ester: butyl ethanoate - derived from butanol (C4H9OH) and ethanoic acid (CH3COOH) would have the formula: CH3COOC4H9. Sometimes the formula may be 'broken up' to show the structure, in this case: CH3COO[CH2]3CH3.

Physical properties

Esters 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 makes them more water-soluble than their parent hydrocarbons. However, the limitations on their hydrogen bonding also make them more hydrophobic than either their parent alcohols or their parent acids. Their lack of hydrogen-bond-donating ability means that ester molecules cannot hydrogen-bond to each other, which, in general, makes esters more volatile than a carboxylic acid of similar molecular weight. This property makes them very useful in organic analytical chemistry: Unknown organic acids with low volatility can often be esterified into a volatile ester, which can then be analyzed using gas chromatography, gas liquid chromatography, or mass spectrometry. Many esters have distinctive odors, which has led to their use as artificial flavorings and fragrances. For example:

Ester Name Structure Odor or Occurrence
Allyl hexanoate pineapple
Benzyl acetate pear, strawberry, jasmine
Bornyl acetate pine tree flavor
Butyl butyrate

pineapple
Ethyl acetate nail polish remover, model paint, model airplane glue
Ethyl butyrate banana, pineapple, strawberry
Ethyl hexanoate pineapple,waxy-green banana
Ethyl cinnamate cinnamon
Ethyl formate lemon, rum, strawberry
Ethyl heptanoate apricot, cherry, grape, raspberry
Ethyl isovalerate apple
Ethyl lactate butter, cream
Ethyl nonanoate grape
Ethyl pentanoate apple
Geranyl acetate geranium
Geranyl butyrate cherry
Geranyl pentanoate apple
Isobutyl acetate cherry, raspberry, strawberry
Isobutyl formate raspberry
Isoamyl acetate pear, banana (flavoring in Pear drops)
Isopropyl acetate fruity
Linalyl acetate lavender, sage
Linalyl butyrate peach
Linalyl formate apple, peach
Methyl acetate glue
Methyl anthranilate grape, jasmine
Methyl benzoate fruity, ylang ylang, feijoa
Methyl benzyl acetate cherry
Methyl butyrate (methyl butanoate) pineapple, apple
Methyl cinnamate strawberry
Methyl pentanoate (methyl valerate) flowery
Methyl phenylacetate honey
Methyl salicylate (oil of wintergreen) root beer, wintergreen, Germolene and Ralgex ointments (UK)
Nonyl caprylate orange
Octyl acetate fruity-orange
Octyl butyrate parsnip
Amyl acetate (pentyl acetate) apple, banana
Pentyl butyrate (amyl butyrate) apricot, pear, pineapple
Pentyl hexanoate (amyl caproate) apple, pineapple
Pentyl pentanoate (amyl valerate) apple
Propyl ethanoate pear
Propyl isobutyrate rum
Terpenyl butyrate cherry

Ester synthesis

"Esterification" (condensation of an alcohol and an acid) is not the only way to synthesize an ester. Esters can be prepared in the laboratory in a number of other ways:

Ester reactions

Esters react in a number of ways:

External links

References

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