Question

Give reasons for the following:

o-nitrophenol is more acidic than o-methoxyphenol.

Explain why is ortho nitrophenol more acidic than ortho methoxyphenol?

Answer 1

Due to the strong –R and –I effect of the NO₂ group, the electron density on the O–H bond decreases; hence, the proton is released easily.

After losing the proton, the remaining o-nitrophenoxide ion becomes stable by resonance.

Ortho-nitrophenoxide ion resonance is stable; hence, o-nitrophenol is a strong acid. On the other hand, due to the +R effect of the OCH₃ group, the electron density on the O–H bond increases; hence, the removal of the proton becomes difficult.

Now, the o-methoxyphenoxide ion, which remains after the loss of the proton, gets destabilised due to resonance.

The two negative charges repel each other and destabilise the o-methoxyphenoxide ion, so o-nitrophenol is more acidic than o-methoxyphenol.

Answer 2

The acidity of phenol is due to the dissociation of a hydrogen ion (H⁺) from the hydroxyl (O–H) group. This loss increases if the O–H bond polarity intensifies and H carries a significant δ⁺ charge.

The presence of the strongly electron-withdrawing NO₂ group at the ortho position in (I) increases the polarity of the O–H bond.

This helps in the release of H⁺ and increases acidity.

The reaction (II) is less acidic because –OCH₃ is an electron-releasing group and increases the electron density on O of OH, and H⁺ release is suppressed. This can be understood from the resonance structures of II, which are: