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