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19.2Reactions of Carboxylic Acids

19.2    Reactions of Carboxylic Acids

Acidity of carboxylic acids

  • Carboxylic acids are acidic because they can donate a proton to form carboxylate ion and hydroxonium The presence of hydroxonium ions makes the solution acidic.
    RCOOH + H2O ⇌ RCOO⁻ + H3O⁺
  • However, carboxylic acids are only weak acids. For instance ethanoic acid has a pH of about 2-3.

 

Reaction with bases

  • Since carboxylic acids are acids, they will react with a base such as sodium hydroxide to produce a carboxylate salt and water. This is a simple neutralisation reaction

 

  • Take ethanoic acid and sodium hydroxide as an example: CH3COOH + NaOH → CH3COONa + H2O
    …and the ionic equation is:
    H⁺ + OH⁻ → H2O

 

Reaction with metals

  • Carboxylic acids will react with reactive metals such as sodium to produce a carboxylate salt and hydrogen gas.
  • Take ethanoic acid and sodium metal as an example:
    2CH3COOH + 2Na → 2CH3COONa + H2

 

Reaction with carbonates and hydrogencarbonates

  • Carboxylic acids will react with carbonates and hydrogencarbonates to produce a carboxylate salt, carbon dioxide gas and water.

 

  • Take ethanoic acid and sodium carbonate as an example: 2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O
    …and the ionic equation is:
    2H⁺ + CO3²⁻ → CO2 + H2O

 

  • Take ethanoic acid and sodium hydrogencarbonate as an example:
    CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O
    …and the ionic equation is:
    H⁺ + HCO3⁻ → CO2 + H2O

 

  • This is a useful test to distinguish carboxylic acids from alcohols and phenols. This is because alcohols and phenols are not acidic enough to react with these

 

Converting carboxylic acids to acyl chlorides

  • Acyl chlorides have the -OH in the -COOH group of the carboxylic acid replaced by chlorine

 

 

  • Take ethanoic acid as an example, ethanoyl chloride is produced

 

  • Acyl chlorides are very reactive and can be used to produce a range of other organic compounds
  • There are three methods to produce acyl chlorides from carboxylic acids:
    • Reaction with phosphorus(III) chloride, PCl3.
      • Take ethanoic acid as an example, the equation is: 3CH3COOH + PCl3 → 3CH3COCl + H3PO4
    • Reaction with phosphorus(V) chloride, PCl5.
      • Take ethanoic acid as an example, the equation is:
        CH3COOH + PCl5 → CH3COCl + POCl3 +  HCl

 

  • Reaction with thionyl chloride, SOCl2.
    • Take ethanoic acid as an example, the equation is: CH3COOH + SOCl2 → CH3COCl + SO2 + HCl
    • This method is generally preferred because SO2 and HCl are both gases, and can be separated from the mixture easily