AS/A-level Chemistry - Ester

Esters

• Named from the parent carboxylic acid from which it is derived, remove ‘-oic acid’ and replace with ‘-oate’
• The alkyl chain attached to the oxygen of the COO group is then added as the first word in the name e.g. methyl, ethyl, propyl etc. – this comes from the alcohol e.g. methanol - methyl

Esterification 

Alcohol + carboxylic acid ➔ ester + water

• Alcohol warmed with a carboxylic acid
• Concentrated sulfuric acid catalyst
• e.g. propanoic acid + ethanol ➔ ethyl propanoate + water

C₂H₅COOH + C₂H₅OH ➔ C₂H₅COOC₂H₅ + H₂O

In AS/A-level Chemistry, there is an experiment involving esterification, below is an example of ester making.🧪

Making Ethyl Propanoate

1. Pour 2cm of ethanol and 2cm of propanoic acid into a boiling tube
2. Carefully add a few drops of concentrated sulfuric acid to the mixture
3. Place the boiling tube in a beaker of hot water, approx. 80°C
4. Leave the boiling tube in the water for about 5 minutes
5. Pour the contents of the boiling tube into a beaker containing aqueous sodium carbonate – this removes the smell of any carboxylic acid that has not reacted- easier to detect the smell of the ester (oily drops of the ester floating on the water- smell aroma of ester)

Hydrolysis of Esters

• The chemical breakdown of a compound in the presence of water or in aqueous solution

Acid Hydrolysis:

• Ester heated under reflux with dilute aqueous acid
• Add H₂O and dilute hydrochloric acid. The ester is broken down by water, whilst the acid acts as a catalyst
• Produces a carboxylic acid and an alcohol (reverse of esterification)
• e.g. methyl 2-hydroxy benzoate + water ➔ 2-hydroxy benzoic acid + methanol

Alkaline Hydrolysis:

• Irreversible
• Ester heated under reflux with aqueous hydroxide ions (OH^-)
• Produces a carboxylate ion (salt) and an alcohol
• Saponification – the salt can be used to make a soap
• Ester + aqueous alkali (e.g NaOH or KOH) à salt + alcohol
• e.g. methyl ethanoate + KOH à sodium ethanoate + methanol

Uses of Esters

• Many food flavourings, often mixtures of esters are used
• Many perfumes contain mixtures of esters in a solvent that quickly evaporates e.g. ethanol
• Esters are often used as solvents e.g. ethyl ethanoate is used in nail varnish
• Esters can be used as plasticisers, these are added to polymers to make them more flexible

References: 

• esterification - alcohols and carboxylic acids (chemguide.co.uk)
• Esterification (Alcohol & Carboxylic acid) - Reactions Mechanism & Uses with Videos (byjus.com)
• 15.9: Hydrolysis of Esters - Chemistry LibreTexts

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