Carboxylic Acids (CIE A Level Chemistry)

• 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

Oxidation of primary alcohols (1) and aldehydes (2) gives carboxylic acids

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 nitriles by either dilute acid (1) or dilute alkali and acidification (2) will form a carboxylic acid

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

Hydrolysis of esters by either dilute acid (1) or dilute alkali and heat followed acidification (2) will form a carboxylic acid

• 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

 

Example dissociation of a carboxylic acid

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

• 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

 DIfferent reactions of carboxylic acids

Carboxylic acids undergo a wide variety of reactions