Question

If ‘a’ is the pKb value of aniline and ‘b’, ‘c’ and ‘d’ are the pKb values of o-, m- and p- isomers of methylaniline respectively, what is the correct order of the values a, b, c and d?

(a) a > b > c > d

(b) d > c > a > b

(c) d > c > b > a

(d) b > a > c > d

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

My question comes from Amines Chemical Reactions topic in section Amines of Chemistry – Class 12

Answer 1

The correct answer is (d) b > a > c > d

To explain I would say: Every ortho substituted aniline (electron withdrawing or electron releasing group) is less basic than aniline and consequently its meta and para isomers. This is because of the ortho effect, which is due to the combination of stearic and electronic factors.