10. CARBOXYLIC ACIDS and ESTERS - other families of organic compounds

We find members of the homologous series called carboxylic acids in fruits and in vinegar and many in fragrances and food additives as carboxylic acid derivatives called esters. ( ester preparation and uses of esters).

• Carboxylic acids form another homologous series and have the functional group -COOH.
• The structures of the first three members are given below: Names end in ...oic acid.
  • methanoic acid (old name 'formic acid')
    • or or
  • ethanoic acid (old name 'acetic acid', in vinegar)
    • or or
  • propanoic acid (old name 'propionic acid')
    • or or
• Vinegar contains ethanoic acid (old name 'acetic acid')
  • Ethanoic acid can be manufactured on a large scale by oxidising the alcohol ethanol.
    • See in section 9 oxidation of the alcohol ethanol
  • It is used as a preservative and in food flavourings.
• Ethanoic acid is used in the manufacture of the fibre, acetate rayon.
• Citrus fruits like oranges and lemons and many soft drinks contain the tri-carboxylic acid citric acid. and contribute to the 'tarter' or 'sour' taste of fruit. The molecule contains three acidic carboxylic acid groups -COOH.

  • Citric acid is a natural preservative (E330 on food labels) and is found in the largest quantities in lemons, limes and grapefruit. It is an anti-oxidant. Metal salts from citric acid, i.e. citrates, are used in dietary supplements to deliver trace metal minerals in a biologically available/absorbable chemical form.
  • Citric acid can be used in baking powder to react with sodium bicarbonate giving the raising action from carbon dioxide gas formation. The same combination can be used to give the fizzy drink effect in medicines like ant-acid stomach powders.

• Aspirin is a carboxylic acid. Aspirin is a drug used for pain relief and is taken regularly by those at risk from heart attacks (see also Drugs).
• Ascorbic acid (vitamin C) is another carboxylic acid and is present in fresh fruit and vegetables and is vital for good health AND the body cannot synthesise it, so you must eat fruit and vegetables regularly!
  • A lack of vitamin C can cause the disease scurvy. The symptoms of scurvy are skin sores, spongy gums and bleeding from mucous membranes. This is one example of malnutrition diseases caused by a vitamin deficiency in a diet.
• Carboxylic acids are weak acids, typically solutions are around pH 3 (yellow-orange-pink with universal indicator).
  • They are called weak acids because only a few % of the molecules in aqueous ionise to release protons (hydrogen ions, H⁺).
    • e.g. for ethanoic acid (vinegar)
    • 98% CH₃COOH(aq) <==> CH₃COO^-(aq) + H⁺(aq) 2%
  • [ see theory of acids and bases]
• They react and are neutralised by bases (insoluble or soluble - alkalis and carbonates) ... with examples ...
  • metals react to form salts and hydrogen e.g.
    • ethanoic acid + magnesium ==> magnesium ethanoate + hydrogen
    • 2CH₃COOH + Mg ==> (CH₃COO)₂Mg + H₂
  • alkalis (soluble bases) react to form a carboxylic acid salt and water  e.g.
    • ethanoic acid + sodium hydroxide ==> sodium ethanoate + water
    • CH₃COOH + NaOH ==> CH₃COONa + H₂O
    • OR ethanoic acid + potassium hydroxide ==> potassium ethanoate + water
    • CH₃COOH + KOH ==> CH₃COOK + H₂O
  • insoluble bases dissolve and react to form salt and water e.g.
    • zinc oxide + ethanoic acid ==> zinc ethanoate + water
    • ZnO + 2CH₃COOH ==> (CH₃COO)₂Zn + H₂O
  • carbonate and hydrogencarbonate bases to produce a carboxylic acid salt, water and carbon dioxide  e.g.
    • ethanoic acid + sodium hydrogen carbonate ==> sodium ethanoate + water + carbon dioxide
    • CH₃COOH + NaHCO₃ ==> CH₃COONa + H₂O + CO₂
    • OR ethanoic acid + sodium carbonate ==> sodium ethanoate + water + carbon dioxide
    • 2CH₃COOH + Na₂CO₃ ==> 2CH₃COONa + H₂O + CO₂
    • OR propanoic acid + sodium carbonate ==> sodium propanoate + water + carbon dioxide
    • 2CH₃CH₂COOH + Na₂CO₃ ==> 2CH₃CH₂COONa + H₂O + CO₂
  • aqueous ammonia solution forms ammonium salts e.g.
    • methanoic acid + ammonia ==> ammonium methanoate
    • HCOOH + NH₃ ==> HCOONH₄
    • ethanoic acid + ammonia ==> ammonium ethanoate
    • CH₃COOH + NH₃ ==> CH₃COONH₄
    • Strictly speaking, ammonium hydroxide doesn't really exist, but in older texts you will find these reactions written in this way e.g.
      • propanoic acid + ammonium hydroxide ==> ammonium propanoate + water
      • CH₃CH₂COOH + NH₄OH ==> CH₃CH₂COONH₄ + H₂O
• ESTERS: Carboxylic acids react with alcohols to form members of another homologous series called esters. Concentrated sulphuric acid acts as a catalyst in this reaction.
  • + + H₂O
    • sometimes more simply written as
    • CH₃COOH + CH₃CH₂OH CH₃COOCH₂CH₃ + H2O
  • For more details see the preparation & formation of ethyl ethanoate and other esters in section 9 for lots more details.
  • Structures of other esters made from ethanoic acid:
    • methyl ethanoate using methanol, and
    • propyl ethanoate from using propan-1-ol (n-propyl alcohol).
  • and what would the structure of their original alcohols be and what would the structure of butyl ethanoate be?
• Esters occur widely in nature and are usually sweet/pleasant smelling liquids and widely used as fragrances (e.g. perfumes) and food flavourings. Natural substances are used in many cosmetics but many mixtures contain synthetic organic compounds.
  • Examples of plant ester sources:
    • Lavender oil essence is distilled from the lavender plant
  • Examples of flavouring esters:
    • Pear drop sweet essence is an ester.
  • Factors affecting perfume design e.g. using esters:
    • Designing a perfume - several issues to address by way of design factors.
    • The perfume needs to be a mixture of compounds to give a prolonged perfumery effect.
    • The perfumer chemist has to design the mixture to give a particular fragrance which includes ...
      •  the top note - the first fragrant molecule to be released,
      • and the low note, the last molecule to be vapourised.
  • -

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Advanced Level Organic Chemistry revision notes

Revise KS4 Science GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10)