Question

If the pKb value of p-nitroaniline is 13, predict the pKb value of its ortho isomer?

(a) 9.38

(b) 11.54

(c) 13

(d) 14.22

This question was posed to me in a national level competition.

The above asked question is from Amines Chemical Reactions in portion Amines of Chemistry – Class 12

Answer 1

Correct option is (d) 14.22

For explanation I would say: NO2 is an electron withdrawing group and has a weakening effect on the basicity of anilines. Its presence at para position will be more influencing than at meta, but less weakening than at ortho position. This anomaly is due to a complex ortho effect (stearic and electronic reasons). Hence o-nitroaniline is a weaker base and has a higher pKb value than p-nitroaniline.