7.5.4 Relative Acidities of Chlorine-substituted Carboxylic Acids

• 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

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

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

Relative acidity of ethanoic acid