Reactions of Carboxylic Acids (Edexcel International A Level Chemistry)

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Carboxylic Acids - Reactions

  • In aqueous solution they are only slightly ionised, to give low concentrations of hydronium ions and alkanoate ions (often called carboxylate ions)

  • This partial ionisation in solution means that carboxylic acids are weak acids
  • This means that the position of the equilibrium lies to the left and that the concentration of H+ is much smaller than the concentration of the carboxylic acid
  • However, 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
    • Ionic equation with carbonates: 2RCOOH + CO32- → 2RCOO- + CO2 + H2O
    • Sodium hydrogen carbonate: RCOOH + NaHCO3 → RCOO-Na+ + CO2 + H2O
    • Ionic equation with hydrogen carbonates: RCOOH + HCO3- → RCOO- + CO2 + H2O

Weak acids, downloadable AS & A Level Chemistry revision notes

Carboxylic acids are weak acids that do not fully dissociate in water, the position of the equilibrium lies to the left

Reaction with LiAlH4

  • Carboxylic acids can undergo reduction when they react with a reducing agent such as lithium tetrahydridoaluminate, LiAlH4, suspended in dry ether at room temperature 
  • A carboxylic acid will be reduced to a primary alcohol, for example

CH3CH2COOH (l) + 4[H] → CH3CH2CH2OH (l) + H2O (l)

  • Addition of water at the end will destroy any excess lithium tetrahydridoaluminate

Reaction with bases

  • Carboxylic acids can form salts with metals, alkalis and carbonates.
  • In the reaction with metal oxides a metal salt and water are produced
    • For example in reaction with magnesium the salt magnesium ethanoate is formed:

2CH3COOH (aq) + MgO (s) → (CH3COO)2Mg (aq) + H2O (l) 

  • In the reaction with alkalis a salt and water are formed in a neutralisation reaction
    • For example in reaction with potassium hydroxide the salt potassium ethanoate is formed:

CH3COOH (aq) + KOH (aq) → CH3COOK (aq) + H2O (l)

  • In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced
    • For example in reaction with potassium carbonate the salt potassium ethanoate is formed:

2CH3COOH (aq) + K2CO3 (s) → 2CH3COOK (aq) + H2O (l) + CO2 (g)

Reaction with phosphorus(V) chloride 

  • Carboxylic acids react with solid phosphorus(V) chloride to form an acyl chloride
  • For example, propanoic acid will react with phosphorus(V) chloride to form propanoyl chloride, phosphorus trichloride oxide and hydrogen chloride 

CH3CH2COOH (l) + PCl5 (s) → CH3CH2COCl (l) + POCl3 (l) + HCl (g)

  • In this reaction, steamy fumes of HCl are produced 
  • The liquid products can be separated by fractional distillation 

Reaction with alcohols

  • When carboxylic acids react with alcohols an ester is formed
  • Esters are compounds with an -COOR functional group and are characterised by their sweet and fruity smells
  • They are prepared from the condensation reaction between a carboxylic acid and alcohol with concentrated H2SO4 as catalyst
    • This is also called esterification

  • The first part of the ester’s name comes from the alcohol and the second part of the name comes from the carboxylic acid
    • E.g. Propanol and ethanoic acid will give the ester propyl ethanoate

 

Carboxylic Acids & Derivatives Production of Esters, downloadable AS & A Level Chemistry revision notes

Esters are formed from the condensation reaction between carboxylic acids and alcohols

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.