Carboxylic Acids, Esters and Acyl Chlorides

Carboxylic Acids, Esters and Acyl Chlorides

Carboxylic Acids

  • Carboxylic acids are an organic compounds containing a carboxyl group -COOH.
  • The name of a carboxylic acid is derived from the corresponding alkane by replacing the ‘e’ of the alkane with ‘oic acid’ (e.g., ethane gives ethanoic acid).
  • In a molecule of a carboxylic acid, the -OH group is polar and can form hydrogen bonds, hence carboxylic acids have high melting and boiling points.
  • Carboxylic acids react with metals/metal oxides/hydroxides/carbonates to form a salt and water (acid-base reactions).
  • They also react with alcohols in the presence of a catalyst (usually concentrated sulfuric acid) to form esters, a process known as esterification.

Esters

  • Esters are organic compounds derived from carboxylic acids and alcohols.
  • They are generally characterised by the presence of a carbonyl group adjacent to an ether group -COO-.
  • The naming of esters follows the format: alkyl + carboxylic acid name with ‘oic acid’ replaced by ‘oate’. For example, the ester formed from methanol and ethanoic acid is called methyl ethanoate.
  • Esters are widely used in a variety of applications due to their distinctive odours and are commonly found in flavours and fragrances.
  • Hydrolysis of esters, with the presence of a dilute acid or alkali, forms a carboxylic acid and an alcohol.

Acyl Chlorides

  • Acyl chlorides are also known as acid chlorides and are typically more reactive than carboxylic acids.
  • They are characterised by the presence of a carbonyl group adjacent to a chloride group -COCl.
  • Acyl chlorides react quickly and completely with water to form a carboxylic acid and hydrogen chloride. This reaction is used to test for the presence of acyl chlorides.
  • They can also react with alcohols or phenols to form an ester and hydrogen chloride (HCl).
  • The presence of sulfuric acid or pyridine can catalyse the reaction of acyl chlorides with alcohols or phenols.
  • The name of an acyl chloride is derived from the corresponding carboxylic acid by replacing ‘oic acid’ with ‘oyl chloride’. For example, ethanoyl chloride is derived from ethanoic acid.