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 destabilized due to resonance.

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