AQA A Level Chemistry复习笔记7.3.1 Carboxylic Acids

Carboxylic Acids Examples Table

Weak Acids

• Carboxylic acids with fewer than six carbon atoms per molecule are water-soluble
  • This is because water molecules can hydrogen-bond with the functional group

   

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

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

• 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

   

Preparation of esters

• Esters are formed from the condensation reaction between an alcohol and carboxylic acid in the presence of a concentrated strong acid catalyst e.g. sulfuric or hydrochloric acid
• This reaction is often referred to as esterification
• A condensation reaction involves the elimination of a small molecule not always water
  • Esterification is one example of a condensation reaction as water is eliminated from the acid and alcohol reacting together