19.1Carboxylic Acids

19.1    Carboxylic Acids

Introduction to carboxylic acids

• Carboxylic acids are compounds that contain the -COOH group.
• Examples of carboxylic acids:

• Salts of carboxylic acids are called carboxylate salts. The formation of carboxylate salts shows that carboxylic acids are acidic. A carboxylate salt is formed upon the removal of H⁺ from -COOH group, leaving -COO⁻.

Physical properties of carboxylic acids

• Carboxylic acids have higher melting and boiling points than the corresponding alcohols
  1. This is because in a pure carboxylic acid, hydrogen bonding can occur between two molecules of acid to produce a dimer.
  2. This doubles the size of the molecule, making the van der Waals’ forces stronger
  3. Hence more energy is required to overcome these forces of attraction
• Small carboxylic acids are soluble in water.
  i) This is because carboxylic acid is capable of forming hydrogen bond with water moleculeii) However, the solubility decreases as the number of carbon atoms increases. This is because the presence of long hydrocarbon tail disrupts the effectiveness of hydrogen bonding

Preparation of  carboxylic acids

• There are three ways to produce carboxylic acids:
  1. Oxidation of primary alcohol
  2. Oxidation of aldehyde
  3. Acidic/alkaline hydrolysis of nitrile

• For oxidation of primary alcohol and aldehyde, refer back previous chapters

• Acidic hydrolysis of nitrile:

1. When nitrile is heated under reflux with a dilute acid such as dilute hydrochloric acid, carboxylic acid will be  formed.
2. Using ethanenitrile as an example, ethanoic acid is formed. CH3CN + 2H2O + H⁺ → CH3COOH + NH4⁺

• Alkaline hydrolysis of nitrile:

1. When nitrile is heated under reflux with an alkali such as sodium hydroxide, salt of carboxylic acid(carboxylate salt) is produced. Using ethanenitrile as an example, ethanoate ion is formed
   CH3CN + 2H2O + OH⁻ → CH3COO⁻ + NH3
2. Ethanoate ion can be converted to ethanoic acid by the addition of acid.
   CH3COO⁻ + H⁺ → CH3COOH