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

(i)

The +I effect of −CH₃ group increases the electron density on the O−H bond. Therefore, release of proton becomes difficult. On the other hand, the −I effect of F decreases the electron density on the O−H bond. Therefore, proton can be released easily. Hence, CH₂FCO₂H is a stronger acid than CH₃CO₂H.

(ii)

F has stronger −I effect than Cl. Therefore, CH₂FCO₂H can release proton more easily than CH₂ClCO₂H. Hence, CH₂FCO₂H is stronger acid than CH₂ClCO₂H.

(iii)

Inductive effect decreases with increase in distance. Hence, the +I effect of F in CH₃CHFCH₂CO₂H is more than it is in CH₂FCH₂CH₂CO₂H. Hence, CH₃CHFCH₂CO₂H is stronger acid than CH₂FCH₂CH₂CO₂H.

(iv)

Due to the −I effect of F, it is easier to release proton in the case of compound (A). However, in the case of compound (B), release of proton is difficult due to the +I effect of −CH₃ group. Hence, (A) is a stronger acid than (B).