7.3.2 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

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

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

The synthesis of a triglyceride from glycerol and fatty acid chains