In organic chemistry and biochemistry esters are the functional group (Rī-COOR") consisting of an organic radical 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"). The most common esters found in nature are fats and vegetable oils, which are esters of glycerol and fatty acids.

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:

Naming of esters:

             O
             ||
             C - CH3
            /
 CH3 - CH2-O

   ethyl      acetate/ethanoate 
 (comes from    (comes from
 the alcohol)      the acid)

CH₃-COO-CH₃ methyl acetate is the simplest ester. One of the oxygens has a double bond. The CH₃ group on the right can be replaced with 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 acid of similar molecular weight.

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

ethyl methanoate 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

Esters 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.

See also: transesterification