AQA A Level Chemistry

Revision Notes

7.3.2 Esters

Esters

Esters

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

7.3.2 Structure of methyl ethanoate, downloadable AS & A Level Chemistry revision notes

Structure of methyl ethanoate

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 confectionary 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
  • 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 if reduced allowing the polymer chains to slide over one another

 

 

 

 

 

 

Intermolecular forces in PVC

7.3.2 Addition of plasticiser to PVC, downloadable AS & A Level Chemistry revision notes

Addition of a plasticiser reduced rigidity in PVC

Uses of Esters – solvents

  • Esters are also commonly used as solvents for organic compounds
    • Ethyl ethanoate is a common solvent which 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

Triglycerides Basics - Formation of a triglyceride (2), downloadable AS Level & A Level Biology revision notes

The synthesis of a triglyceride from glycerol and fatty acid chains

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