Carboxylic Acids (Cambridge (CIE) A Level Chemistry): Revision Note

Production of Carboxylic Acids

• Carboxylic acids are compounds with a -COOH functional group

• They can be prepared by a series of different reactions

Oxidation of primary alcohols & aldehydes

• Carboxylic acids can be formed from the oxidation of primary alcohols and aldehydes by either acidified K₂Cr₂O₇ or acidified KMnO₄ and reflux

• The oxidising agents themselves get reduced causing the solutions to change colour

  • In K₂Cr₂O₇ the orange dichromate ions (Cr₂O₇^2-) are reduced to green Cr³⁺ ions

  • In KMnO4  the purple manganate ions (MnO₄^-) are reduced to colourless Mn²⁺ ions

Oxidation of primary alcohols and aldehydes

Hydrolysis of nitriles

• Carboxylic acids can also be prepared from the hydrolysis of nitriles using either dilute acid or dilute alkali followed by acidification

  • Hydrolysis by dilute acid results in the formation of a carboxylic acid and ammonium salt

  • Hydrolysis by dilute alkali results in the formation of a sodium carboxylate salt and ammonia; Acidification is required to change the carboxylate ion into a carboxylic acid

• The -CN group at the end of the hydrocarbon chain is converted to a -COOH group

Hydrolysis of nitriles

Hydrolysis of esters

• Esters are formed from the condensation reaction between an alcohol and a carboxylic acid

• Hydrolysis of esters by dilute acid or dilute alkali and heat followed by acidification will reform the alcohol and the carboxylic acid

  • Hydrolysis by dilute acid is a reversible reaction where an equilibrium is established

  • Hydrolysis by dilute alkali is an irreversible reaction as all the ester is broken down to form a sodium carboxylate salt and an alcohol; acidification is required to change the carboxylate ion into a carboxylic acid

Hydrolysis of esters

Reactions of Carboxylic Acids

• Carboxylic acids are weak acids as they do not completely dissociate in water

• 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

• The solution has a pH value of less than 7

Carboxylic acid reaction types

• Carboxylic acids are reactive compounds which can undergo many types of reactions including:

  • Redox reactions with reactive metals

  • Neutralisation reactions with alkali

  • Acid-base reactions with carbonates

  • Esterification with alcohols

  • Reduction by LiAlH₄

Redox reactions with reactive metals

• This reaction produces a salt and hydrogen gas

• General word equation:

Metal + carboxylic acid → salt + hydrogen

• Example reaction equation:

Sodium + ethanoic acid → sodium ethanoate + hydrogen

2Na + 2CH₃COOH → 2CH₃COO^-Na⁺ + H₂

Neutralisation reactions with alkali

• This reaction produces a salt and water

• General word equation:

Alkali + carboxylic acid → salt + water

• Example reaction equation:

Sodium hydroxide + ethanoic acid → sodium ethanoate + water

NaOH + CH₃COOH → CH₃COO^-Na⁺ + H₂O

Acid-base reactions with carbonates

• This reaction produces a salt, water and carbon dioxide gas

• General word equation:

Carbonate + carboxylic acid → salt + water + carbon dioxide

• Example reaction equation:

Sodium carbonate + ethanoic acid → sodium ethanoate + water + carbon dioxide

Na₂CO₃ + 2CH₃COOH → 2CH₃COO^-Na⁺ + H₂O + CO₂

Esterification with alcohols

• This reaction produces an ester and water

• General word equation:

Alcohol + carboxylic acid → ester + water

• Example reaction equation:

Ethanol + ethanoic acid ⇌ ethyl ethanoate + water

CH₃CH₂OH + CH₃COOH ⇌ CH₃COOCH₂CH₃ + H₂O

Reduction by LiAlH₄

• This reaction produces a primary alcohol and water

• General word equation:

Reducing agent + carboxylic acid → primary alcohol + water

• Example reaction equation, using LiAlH₄:

4[H] + ethanoic acid → ethanol + water

4[H] + CH₃COOH → CH₃CH₂OH + H₂O