6.10 - 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.

Naming an ester from its two parents

An ester's name is in two parts, each coming from one of the two compounds it was made from.

1. The first part comes from the alcohol - Take the alcohol's name, drop the -ol ending and put -yl in its place:
   • Methanol → methyl, ethanol → ethyl, propan-1-ol → propyl, butan-1-ol → butyl
2. The second part comes from the carboxylic acid - Take the acid's name, drop the -oic acid ending and put -oate in its place:
   • Methanoic acid → methanoate, ethanoic acid → ethanoate, propanoic acid → propanoate, butanoic acid → butanoate

Watch the order. The name begins with the alcohol part, but chemical equations usually write the acid first.

Building an ester from its parents

Take propan-1-ol and ethanoic acid:

• Name - Propan-1-ol gives propyl and ethanoic acid gives ethanoate, making propyl ethanoate.
• Structure - Ethanoic acid is CH₃COOH and propan-1-ol is CH₃CH₂CH₂OH, so propyl ethanoate is CH₃COOCH₂CH₂CH₃ - the piece from the acid on the left of the -COO- group, and the piece from the alcohol on its right.

Working backwards to the parents

• From a name - Methyl butanoate splits into methanol and butanoic acid.
• From a structure - Find the -COO- group and split the chain. The chain including the C=O carbon came from the acid; the chain attached to the single-bonded oxygen came from the alcohol.
  • In CH₃CH₂COOCH₃, the three-carbon piece is from propanoic acid and the one-carbon piece is from methanol, giving methyl propanoate.

A molecular formula isn't enough. Ethyl ethanoate and methyl propanoate both have the formula C₄H₈O₂. It's where the -COO- group sits along the chain that tells them apart.

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.