Question

Which acid of each pair shown here would you expect to be stronger?
(i) CH₃CO₂H or CH₂FCO₂H
(ii) CH₂FCO₂H or CH₂ClCO₂H
(iii) CH₂FCH₂CH₂CO₂H or CH₃CHFCH₂CO₂H
(iv) 

Answer 1

We know that atoms or groups which decrease the electron density on C-atom of -COOH group make the acid stronger while atoms or groups which increases the electron density at C-atom of -COOH group make the acid weaker.

(i) Due to -I effect of F-atom, FCH₂-COOH is a stronger acid than CH₃-COOH

(ii) Due to stronger -I effect of F-atom than Cl atom, FCH₂COOH is a stronger acid than ClCH₂COOH.

(iii) Due to -I effect of fluorine atom at β -position than at γ -position, CH₃ - CH (F) CH₂-COOH is stronger than CH₂(F) CH₂CH₂COOH.