10.14 - Esters

Esters, and how one is made

An ester is a compound made when a carboxylic acid and an alcohol react together.

What makes a compound an ester

• Every ester contains the -COO- group: a carbon joined to two oxygen atoms, one by a double bond and the other by a single bond, with that singly bonded oxygen joined on to a carbon chain.
• The group sits in the middle of the molecule, with a carbon chain running off each side (in an ester of methanoic acid, such as methyl methanoate, HCOOCH₃, a hydrogen atom takes the place of the chain on the acid side).
• You can identify an ester:
  • From its structure - Find the -COO- group with a carbon chain joined to its singly bonded oxygen
  • From its name - Every ester's name ends in -oate

Making an ester

A carboxylic acid and an alcohol are warmed together with an acid catalyst.

carboxylic acid + alcohol → ester + water

When they react:

• The carboxylic acid loses the -OH from its -COOH group
• The alcohol loses the hydrogen from its -OH group
• The remaining parts join together to form the ester, and the lost atoms form water.

Ethyl ethanoate

The ester made from ethanoic acid and ethanol is ethyl ethanoate:

CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O

Its structural formula is CH₃COOCH₂CH₃.

Properties and uses of esters

• Esters are volatile (they evaporate easily) and have distinctive, sweet or fruity smells.
• For example, ethyl ethanoate has a characteristic pear-drop smell.

Uses

• Perfumes - Because they're volatile and smell pleasant, esters evaporate easily from the skin to release their scent.
• Food flavourings - Esters are added to foods and drinks to give them fruity flavours and aromas.