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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 (hydroxy) group is replaced by an -O-alkyl (alkoxy) group. The most common type of esters are carboxylic acid esters (R1-C(=O)-O-R2); other esters include phosphoric acid, sulfuric acid, nitric acid, and boric acid esters. Volatile esters often have a 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; 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.
Additional recommended knowledge
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 sulfuric acid ester.
Since most esters 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.
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:
"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:
Categories: Functional groups | Esters
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ester". A list of authors is available in Wikipedia.|