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.
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.
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.
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|
|Benzyl acetate||pear, strawberry, jasmine|
|Bornyl acetate||pine tree flavor|
|Ethyl acetate||nail polish remover, model paint, model airplane glue|
|Ethyl butyrate||banana, pineapple, strawberry|
|Ethyl hexanoate||pineapple,waxy-green banana|
|Ethyl formate||lemon, rum, strawberry|
|Ethyl heptanoate||apricot, cherry, grape, raspberry|
|Ethyl lactate||butter, cream|
|Isobutyl acetate||cherry, raspberry, strawberry|
|Isoamyl acetate||pear, banana (flavoring in Pear drops)|
|Linalyl acetate||lavender, sage|
|Linalyl formate||apple, peach|
|Methyl anthranilate||grape, jasmine|
|Methyl benzoate||fruity, ylang ylang, feijoa|
|Methyl benzyl acetate||cherry|
|Methyl butyrate (methyl butanoate)||pineapple, apple|
|Methyl pentanoate (methyl valerate)||flowery|
|Methyl salicylate (oil of wintergreen)||root beer, wintergreen, Germolene and Ralgex ointments (UK)|
|Amyl acetate (pentyl acetate)||apple, banana|
|Pentyl butyrate (amyl butyrate)||apricot, pear, pineapple|
|Pentyl hexanoate (amyl caproate)||apple, pineapple|
|Pentyl pentanoate (amyl valerate)||apple|
"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:
Esters react in a number of ways:
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