Edexcel A Level Chemistry:复习笔记7.3.3 Reactions of Carboxylic Acids

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 + CO32- → 2RCOO- + CO2 + H2O
    • Sodium hydrogen carbonate: RCOOH + NaHCO3 → RCOO-Na+ + CO2 + H2O
    • Ionic equation with hydrogen carbonates: RCOOH + HCO3- → RCOO- + CO2 + H2O

 

7.3.1-Weak-acids

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

     

3.6-Carboxylic-Acids-Derivatives-Production-of-Esters-1

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

 

 

 

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