4.7 EstersEdexcel IGCSE Chemistry: Revision notes

Esters are a homologous series containing the functional group -COO-. They are formed when an alcohol reacts with a carboxylic acid in the presence of an acid catalyst, in a reaction called esterification. Water is also produced as a by-product.

alcohol + carboxylic acid ⇌ ester + water

Ethyl ethanoate is the ester produced when ethanol and ethanoic acid react together in the presence of an acid catalyst (such as concentrated sulfuric acid):

ethanol + ethanoic acid ⇌ ethyl ethanoate + water

Structural formula: CH3COOCH2CH3

In the displayed formula, the -COO- link is drawn showing the C=O double bond and the C-O single bond connecting the two halves of the molecule, with the rest of the carbon chains from the original alcohol and acid attached on either side.

An ester's name has two parts:

1. The alcohol part becomes the first name (e.g. 'ethyl' from ethanol)
2. The acid part becomes the second name, ending in '-oate' (e.g. 'ethanoate' from ethanoic acid)

So ethanol + ethanoic acid gives ethyl ethanoate. Working backwards from a name, the two parts can be identified and matched to the alcohol and acid that formed them, and their structural/displayed formulae combined to build the ester (and vice versa: any ester's formula can be split into an alcohol and a carboxylic acid part at the -COO- link).

Esters are volatile compounds (they evaporate easily) with distinctive, often pleasant smells. Because of this, they are widely used as:

• food flavourings
• ingredients in perfumes

A sample of an ester such as ethyl ethanoate can be prepared in the laboratory by heating an alcohol with a carboxylic acid, with a few drops of concentrated sulfuric acid as catalyst, then separating the ester layer that forms.