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General formula of an ester.
In chemistry, esters are organic compounds in which an organic group (symbolized by R' in this article) replaces a hydrogen atom (or more than one) in an oxygen acid. An oxygen acid is an acid whose molecule has an -OH group from which the hydrogen (H) can dissociate as an H+ ion.
The most common esters are the carboxylate esters, where the acid in question is a carboxylic acid. For example, if the acid is acetic acid, the ester is called an acetate. Esters may also be formed with inorganic acids; for example, dimethyl sulfate is an ester, and sometimes called "sulfuric acid, dimethyl ester".
Esters are named similarly to salts; although they don't really have cations and anions, the terminology follows the same pattern: a more electropositive part followed by a more electronegative part.
An ester can be thought of as a product of a condensation reaction of an acid (usually an organic acid) and an alcohol (or phenol compound), although there are other ways to form esters. Condensation is a type of chemical reaction in which two molecules with -OH groups are joined with eliminating a water molecule from their -OH groups. A condensation reaction to form an ester is called esterification. Esterification can be catalysed by the presence of H+ ions. Sulfuric acid is often used as a catalyst for this reaction. The name ester is derived from the German Essig-Äther, an old name for acetic acid ethyl ester (ethyl acetate).
Naming of esters
Ethyl acetate structure
Esters can be produced by an equilibrium reaction between an alcohol and a carboxylic acid. The ester is named according to the alkyl group (the part from the alcohol) and then the alkanoate (the part from the carboxylic acid) which make it up.[1] For example, the reaction between methanol and butyric acid yields the ester methyl butyrate C3H7-COO-CH3 (as well as water). The simplest ester is H-COO-CH3 (methyl formate, also called methyl methanoate).
For esters derived from the simplest carboxylic acids, the traditional names are recommended by IUPAC,[2] viz, formate, acetate, propionate, butyrate, though out of these only acetate may carry further substituents. For esters from higher acids, the alkane name with an -oate ending is generally preferred, e.g., hexanoate. Common esters of aromatic acids include benzoates such as methyl benzoate, and phthalates, with substitution allowed in the name.
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 parent acids. Their lack of hydrogen-bond-donating ability means that ester molecules cannot hydrogen-bond to each other, which makes esters generally more volatile than an 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 analysed using gas chromatography, gas liquid chromatography, or mass spectrometry. Many esters have distinctive odors, which has led to their widespread use as artificial flavorings and fragrances. For example:
- methyl butyrate smells of pineapple or apple
- methyl salicylate (oil of wintergreen) smells of the ointments called Germolene™ and Ralgex™ in the UK
- methyl benzoate smells of fruity-ylang ylang
- ethyl formate smells of rum
- ethyl butyrate smells of pineapple or strawberry
- ethyl salicylate smells of oil of wintergreen
- ethyl heptanoate smells of grape
- propyl isobutyrate (rum)
- isobutyl formate (raspberries)
- butyl butyrate (pineapple)
- pentyl acetate smells of banana
- pentyl pentanoate smells of apple
- pentyl butyrate smells of pear or apricot
- isopentyl acetate smells of pear or banana (it is used as the flavouring in the manufacturing of old fashioned Pear Drops)
- octyl acetate smells of fruity-orange
- benzyl acetate smells slightly of jasmine
Ester synthesis
Esters can be prepared in the laboratory in several ways:
Ester reactions
Ester saponification (basic hydrolysis)
Esters may undergo hydrolysis - the breakdown of an ester by water. This process can be catalyzed both by acids and bases. The base-catalyzed process is called saponification. The hydrolysis yields an alcohol and a carboxylic acid or its carboxylate salt.
Esters also react if heated with primary or secondary amines, producing amides.
Phenyl esters react to hydroxyarylketones in the Fries rearrangement. Specific esters undergo the Chan rearrangement and the Lossen rearrangement.
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