Arrange the following in increasing order of their \(pK_b\) values.
(A) 
(B) 
(C) 
(D) \(NH_3\)
(E) \(CH_3-NH_2\)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → E < D < B < C < A
pKb = −log Kb, so a stronger base has a smaller pKb. Increasing pKb is therefore decreasing basic strength.
Three factors decide the basic strength:
Alkyl group (+I effect): pushes electron density onto nitrogen; aliphatic amines are stronger bases than \( \ce{NH3} \).
Aryl group: the lone pair of nitrogen is delocalised into the benzene ring by resonance, so aromatic amines are much weaker bases than \( \ce{NH3} \).
Substituent on the ring: an electron-releasing group (–\( \ce{OCH3} \)) increases the basic strength of aniline; an electron-withdrawing group (–\( \ce{NO2} \)) decreases it, as its −R and −I effects pull the lone pair further away.
The compounds are:
(E) \( \ce{CH3NH2} \): methanamine (aliphatic), pKb 3.38
(D) \( \ce{NH3} \): ammonia, pKb 4.75
(B) \( \ce{H3CO-C6H4-NH2} \): 4-methoxyaniline (aryl amine with an electron-releasing group), pKb about 8.7
(C) \( \ce{C6H5NH2} \): aniline, pKb 9.38
(A) \( \ce{O2N-C6H4-NH2} \): 4-nitroaniline (aryl amine with an electron-withdrawing group), pKb about 13
Shortcut: Options (3) and (4) both start with E and end with A. The –\( \ce{OCH3} \) group makes B more basic than aniline, so B comes before C: Option (3).
NCERT (2026-27 edition):
Basic Character of Amines – “… Larger the value of Kb or smaller the value of pKb, stronger is the base.”
Basic Character of Amines – “… pKb value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom. Their pKb values lie in the range of 3 to 4.22. On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.”
Basic Character of Amines – “… In case of substituted aniline, it is observed that electron releasing groups like –OCH3, –CH3 increase basic strength whereas electron withdrawing groups like –NO2, –SO3H, –COOH, –X decrease it.”