7.3.3 Reactions of Carboxylic Acids

• 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 + CO₃^2- → 2RCOO^- + CO₂ + H₂O
  • Sodium hydrogen carbonate: RCOOH + NaHCO₃ → RCOO^-Na⁺ + CO₂ + H₂O
  • Ionic equation with hydrogen carbonates: RCOOH + HCO₃^- → RCOO^- + CO₂ + H₂O

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 hydrogen gas 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

   

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