Question
Compare the acidic strength:
(i) 2, 4-dinitrophenol and 2,4,6-trinitro phenol.
(ii) O-amino phenol and m-amino phenol.
Solution
(i) 2, 4, 6 trinitro phenol (picric acid) is stronger acid than 2, 4-dinitrophenol as it contains three electron withdrawing NO2 groups.
(ii) NH3 group is electron donating by resonance when present at ortho and para position while it has -I effect when present at m-position (lone pair can not participate into resonance with benzene from m-position. In m-amino phenol, therefore, the electron density around H of O—H group decreases and H can easily be removed as H+ ion. Hence, m-amino phenol is more acidic than o-amino phenol.
(ii) NH3 group is electron donating by resonance when present at ortho and para position while it has -I effect when present at m-position (lone pair can not participate into resonance with benzene from m-position. In m-amino phenol, therefore, the electron density around H of O—H group decreases and H can easily be removed as H+ ion. Hence, m-amino phenol is more acidic than o-amino phenol.
