Question

which of the following substituents will stabilize the charge on the carboxylic acid on the benzene ring created during hydrolysis. And why?

1. methyl group at the para postion to the carboxylic group on the benzene ring

2. methyl group at the meta position to the carboxylic acid group on the benzene ring

3. methoxy group at the meta position to the carboxylic acid group on the benene ring

4. methoxy group at the para position to the carboxylic acid group on the benzene ring

5. chloro group at the para position to the carboxylic acid group on the benzene ring

6. chloro group at the meta position to the carboxylic acid group on the benzene ring

7. chloro group at the para position to the carboxylic acid group on the benzene ring

8. Nitro group at the meta position to the carboxylic acid

9. Nitro group at the para position to the carboxylic acid.

(HAMMETT CORRELATION) not plotting graph but explaining the effect

Answer 1

1) Methyl group at the para position to the carboxylic group on benzene ring destabilizes the negative charge on carboxylate ion by both +I (Inductive Electron Reducing Group) as well as by +H(Hyperconjugation)

2) Methyl group at meta-position to the carboxylic acid group on the benzene ring doesn't affect the carboxylate because hyperconjugation doesn't occur at meta position and thus positive charge is not created and here only +I effect operates.

3) When the methoxy group is at the meta position of benzoic acid, the Resonance effect can't operate because it operates only on ortho and para position. In the case of meta position, only -I effect will take place because methoxy is an electron-withdrawing group. Thus, it will decrease electron density on the carboxylic acid group. Less the electron density on carboxylate ion more is the stability. Thus, this will stabilize the carboxylic acid group.

4) When methoxy is at the para position, Both +R and -I effects will operate. +R will increase the electron density and -I will decrease the electron density on the carboxylic acid group. Thus, it will stabilize carboxylic acid to some extent but will be less stable than methoxy on meta position.

5) When Chloro group is at the para position of benzoic acid, -I effect will operate due to the electron-withdrawing nature of the chlorine group. +R effect can also operate but is ineffective due to large energy difference between orbitals of chlorine and carbon. Thus, it will stabilize the carboxylic acid as an effect of decreasing electron density.

6) When the Chloro group is at the meta position of benzoic acid, -I effect will operate due to the electron-withdrawing nature of the chlorine group. +R effect can also operate but is ineffective due to large energy difference between orbitals of chlorine and carbon. Thus, it will stabilize the carboxylic acid as an effect of decreasing electron density.

7) Same as 5th

8) Nitro(NO₂) is an electron-donating group. When it is present at the meta position of benzoic acid, the carbanion thus formed is stabilized by resonance. As the negative charge appears on only ortho and para position, this negative charge is stabilized by the nitro group, if attached on ortho and para position. If the nitro group is attached to the meta position with respect to the halogen or activating group, the carbanion is not stabilized because the negative charge moves only on Ortho and para positions.

9) Nitro group at the para position of benzoic acid will stabilize the negative charge as explained in the 8th part.