Ester
- For other uses, see Ester (disambiguation).
In organic chemistry and biochemistry, esters are organic compounds where an organic group (symbolised 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.
Esters are named similar 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.
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".
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-Aether, an old name for acetic acid ethyl ester (ethyl acetate).
This is the general displayed formula of an ester of a carboxylic acid:
Naming of esters
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 acetate (the part from the carboxylic acid) which make it up; for example, the reaction between methanol and butyric acid yields the ester methyl butanoate (as well as water).
Ethyl ethanoate structure The simplest ester is H-COO-CH3 (methyl formate, also called methyl methanoate). The hydrogen atom on the left can be replaced with a CH3 group or additional CH2 units, producing other methyl esters, including methyl stearate, a component of biodiesel.
Physicals
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 butanoate 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 marzipan
- ethyl methanoate smells of raspberry or rum
- ethyl butanoate smells of pineapple or apricot or strawberry
- ethyl salicylate smells of mint
- ethyl heptanoate smells of grape
- butyl ethanoate smells of raspberry
- pentyl ethanoate smells of banana
- pentyl pentanoate smells of apple
- pentyl butanoate smells of pear or apricot
- octyl ethanoate smells of orange
- benzyl ethanoate smells of jasmine
- isopentyl ethanoate smells of pears; it is used as the flavouring in the manufacturing of Pear Drops
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.
Phenyl esters react to hydroxyarylketones in the Fries rearrangement.
See also
External links
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