• 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