Carboxylic acids & Esters (Oxford AQA International A Level Chemistry)

Carboxylic Acids

• Carboxylic acid is the name given to the family of compounds that contain the carboxyl functional group, -COOH

• The general formula of a carboxylic acid is C_nH_2n+1COOH which can be shortened to RCOOH

• The nomenclature of carboxylic acid follows the pattern alkan + oic acid, e.g. propanoic acid

• There is no need to use numbers in the name as the carboxyl group, COOH, is always on the number 1 carbon atom

Carboxylic acids

• Carboxylic acids partially ionise in solution

  • This means that carboxylic acids are weak acids

• The concentration of hydrogen ions is sufficient to react with an aqueous solution of sodium carbonate or sodium hydrogen carbonate to produce carbon dioxide

• These reactions are a useful test for the possible presence of a carboxylic acid:

  • Sodium carbonate:

2RCOOH + Na₂CO₃ → 2RCOO^-Na⁺ + CO₂ + H₂O

Esters

Preparation of esters

• Esters are formed from the condensation reaction between an alcohol and carboxylic acid in the presence of a concentrated strong acid catalyst e.g. sulfuric or hydrochloric acid

• This reaction is often referred to as esterification

• A condensation reaction involves the elimination of a small molecule not always water

  • Esterification is one example of a condensation reaction as water is eliminated from the acid and alcohol reacting together

Esterification

Structure and naming of Esters

• 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

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