Esters

Esters are made when an alcohol reacts with a carboxylic acid, with a little concentrated sulfuric acid as a catalyst; water is made too. They have the –COO– group, and they are volatile with fruity smells, so they are used in perfumes and flavourings. An ester is named from its alcohol first and its acid second: ethanol and ethanoic acid make ethyl ethanoate.

Key facts

  • Alcohol + carboxylic acid ⇌ ester + water, with concentrated sulfuric acid as a catalyst. Ethanol + ethanoic acid → ethyl ethanoate.
  • Esters are volatile with fruity smells: they are used in perfumes and flavourings.
  • An ester is named from its alcohol first (ethyl) and its acid second (ethanoate).

Worked examples

Example 1

What is the functional group of the esters?

  1. An ester has a –COO– group joining two carbon chains.
  2. Esters are made from an alcohol and a carboxylic acid.

Answer: –COO–

Example 2

Which ester is made when ethanol reacts with ethanoic acid?

  1. From the alcohol ethanol: "ethyl".
  2. From the acid ethanoic acid: "ethanoate".
  3. So the ester is ethyl ethanoate, made with a little concentrated sulfuric acid as a catalyst.

Answer: ethyl ethanoate

Example 3

Name the ester made from methanol and propanoic acid.

  1. The alcohol gives the first word, ending in "-yl": methanol → methyl.
  2. The acid gives the second word, ending in "-anoate": propanoic acid → propanoate.
  3. So the ester is methyl propanoate. Water is the other product.

Answer: methyl propanoate

Example 4

Which catalyst is used when an alcohol reacts with a carboxylic acid to make an ester?

  1. Esterification is catalysed by a few drops of concentrated sulfuric acid.
  2. Water is also made: alcohol + carboxylic acid ⇌ ester + water.

Answer: concentrated sulfuric acid

Example 5

What are esters used for?

  1. Esters are volatile and have fruity smells.
  2. So they are used in perfumes and flavourings.

Answer: perfumes and food flavourings