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An ester is an organic chemical with the functional group:

Esters are formed by reacting carboxylic acids with alcohols in esterification.

After the reaction, the acid and the alcohol become connected by a $$\ce{\bond{-}COO\bond{-}}$$ link, also called an ester bond.

Esterification is an example of a condensation reaction, in which two molecules join together and a small molecule (water in this case) is released.

Concentrated sulfuric acid is often used as the catalyst for esterification.

Esters are named first for the alcohol, and then for the acid. For example, methanol reacted with ethanoic acid gives methyl ethanoate.

Esters are responsible for many aromas, particularly those of ripening fruits. Perfumes and flavourings often contain esters. Esters are also used to make polyesters and various solvents.

Methyl butanoate is associated with apple flavour.
Methyl butanoate is associated with apple flavour.