Esters (AQA A Level Chemistry): Revision Note

Esters

Esters

• Esters are carboxylic acid derivative that contains the ester group, -COO-

• The equation for the formation of an ester is:

alcohol + carboxylic acid $\rightleftharpoons$ester + water

• e.g., the reaction with ethanoic acid and ethanol:

CH₃COOH + CH₃CH₂OH $\rightleftharpoons$ CH₃COOCH₂CH₃ + H₂O

• An ester is named after the parent carboxylic acid from which it is derived

• The nomenclature of esters follows the pattern:

  • Remove the -oic acid suffix from the parent carboxylic acid and replace it with -oate

  • The alkyl chain attached to the oxygen atom of the -COO- group is then added as the first word in the name

    • This part of the name comes from the alcohol, e.g., propanol becomes propyl

• Ester names are confusing because the name is written backwards from the way the structure is drawn

Esters Examples Table

Uses of Esters - fragrances and flavourings

• Like aldehydes and ketones, esters are important chemicals in the flavourings and fragrances industry

• Esters have a fruitier and sweeter smell than aldehydes and ketones and are responsible for many natural flower scents and flavours

• Synthetic esters can also be used as artificial fruit flavours in confectionery products, e.g., ethyl methanoate is used as a raspberry ester fragrance

• Aromas and tastes are often due to complex mixtures of different esters

Testing for alcohols

• The sweet and fruity smell of an ester can be used as a test for the presence of an alcohol or carboxylic acid using the esterification reaction

• An unknown substance can be warmed with a carboxylic acid such as ethanoic acid in the presence of concentrated sulfuric acid (catalyst)

• Excess acid with its pungent vinegary smell can be removed by adding warm aqueous sodium carbonate solution

• If the remaining mixture has a sweet smell of an ester, this confirms the presence of an alcohol

• The warmth of the solution then causes the ester to evaporate, and the sweet smell is easily detected

Uses of Esters - plasticisers

• Esters can also be used as plasticisers, which are additives mixed into polymers to increase the flexibility of the polymer

  • Poly(chloroethene), better known as PVC, is a strong, rigid polymer suitable for making drainpipes and guttering

  • When a suitable plasticising ester is added, it can be made into cling film, which is soft and pliable

  • An ester plasticiser works by penetrating between the polymer chains and increasing the distance between them

  • This then weakens the polar effects of the carbon-chlorine bond and the rigidity of the 3D structure, allowing the polymer chains to slide over one another

Uses of Esters - solvents

• Esters are also commonly used as solvents for organic compounds

  • Ethyl ethanoate is a common solvent that has the beneficial properties of low toxicity and low volatility (its boiling point is 77˚C), as well as being relatively cheap

  • This makes it an ideal solvent for use in glues, fragrances, and nail varnishes

Naturally occurring Esters

• Triglycerides found in animal fat and olive oil are naturally occurring esters

• They are tri-esters of glycerol (propane-1,2,3-triol) and fatty acids (long-chain carboxylic acids) such as stearic acid (CH₃(CH₂)₁₆COOH)

• The reaction of 1 mole of glycerol with 3 moles of a fatty acid in the presence of an acid catalyst leads to the formation of the triglyceride, whereby 3 moles of water are eliminated

Related topics

• Ester Hydrolysis

• Biodiesel

• Carboxylic Acids

• Oxidation of Alcohols

• Condensation Polymers