For the Biblical Ester, see Esther.

For the town, see Ester, Alaska.

Esters are usually encountered as sweet smelling organic compounds commonly produced by many plants and fruits. However, the most common esters found in nature are fats and vegetable oils, which are esters of glycerol and fatty acids.

In organic chemistry and biochemistry esters are substances that have the functional group (R´-COOR") (the carbon is double-bonded to one oxygen atom and single-bonded to another) and consist of an alkane united with the residue of any oxygen acid, organic or inorganic. An ester is a product of the reaction of an acid (usually organic) and an alcohol (the hydrogen of the acid R-COOH is replaced by an alkyl group R"). Esters mainly result from the condensation (this is, a reaction that produces water) of a carboxylic acid and an alcohol. The process is called esterification. This reaction can be catalysed by the presence of H+ ions. Sulphuric 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).

Naming of esters

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.

Physical properties

Esters can 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. But 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 they cannot form hydrogen bonds between ester molecules, which makes them 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

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

ethyl butanoate smells of pineapple

pentyl ethanoate smells of banana

pentyl pentanoate smells of apple

pentyl butanoate smells of pear or apricot

octyl ethanoate smells of orange

Esters may undergo hydrolysis - the breakdown of an ester by water. Esters may also be decomposed by strong acids or bases into an alcohol and a carboxylic acid or a salt of carboxylic acid.

Ester hydrolysis