Relative Acidities of Chlorine-substituted Carboxylic Acids (CIE A Level Chemistry)

• Electron-withdrawing groups bonded to the carbon attached to the -COOH group make the carboxylic acids stronger acids
• This is because the O-H bond in the undissociated acid molecule is even further weakened as the electron-withdrawing group draws even more electron density away from this bond
• Furthermore, the electron-withdrawing groups extend the delocalisation of the negative charge on the -COO^- group of the carboxylate ion
• The -COO^- group is now even more stabilised and is less likely to bond with an H⁺ ion
• Chlorine-substituted carboxylic acids are examples of carboxylic acids with electron-withdrawing groups

pK_a values of ethanoic acid and chlorine-substituted derivatives table

• The pK_a values of ethanoic acid and chloro-substituted derivatives show that the more electron-withdrawing groups there are on the carbon attached to the -COOH group, the stronger the acid

Comparing the relative acidities of chlorine substituted derivatives of ethanoic acid

The more chlorine atoms there are in the carboxylic acids, the stronger the acid is

• Trichloroethanoic acid is the strongest acid as:
  • The O-H bond in CCl₃COOH is the weakest since there are three very strong electronegative Cl atoms withdrawing electron density from the -COOH group
  • When the O-H is broken to form the carboxylate (-COO^-) ion, the charge density is further spread out by the three electron-withdrawing Cl atoms
  • The carboxylate ion is so stabilised that it is less attracted to H⁺ ions

The equilibrium of trichloroethanoic acid and the trichloroethanoate ion

Relative acidity of trichloroethanoic acid

• Ethanoic acid is the weakest acid as:
  • It contains an electron-donating methyl group which strengthens the O-H bond
  • The methyl group donates negative charge towards the -COO^- group which becomes more likely to accept an H⁺ ion

The equilibrium of ethanoic acid and the ethanoate ion 

Relative acidity of ethanoic acid