CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.5.3 Relative Acidities of Chlorine-substituded Carboxylic Acids

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

pKa values of ethanoic acid and chlorine-substituted derivatives table

Carboxylic Acids & Derivatives - pKa values of ethanoic acid and chlorine-substituted derivatives table, downloadable AS & A Level Chemistry revision notes

  • The pKa 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

 

Carboxylic Acids & Derivatives - Trend in Acidity of Chlorine-Substituted Carboxylic Acids, downloadable AS & A Level Chemistry revision notes

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

 

Carboxylic Acids & Derivatives - Acidity Trichloroethanoic Acid, downloadable AS & A Level Chemistry revision notes

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

 

Carboxylic Acids _ Derivatives - Acidity Ethanoic Acid, downloadable AS & A Level Chemistry revision notes

Relative acidity of ethanoic acid

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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