Read the passage carefully and answer the questions.
Amines are considered as derivatives of ammonia obtained by replacement of hydrogen atoms with alkyl or aryl groups. In nature they occur among proteins, vitamins, alkaloids and hormones. These can be primary, secondary or tertiary depending on the number of hydrogen groups replaced. They can be prepared from nitro compounds, alkyl halides, nitriles, amides etc. Due to unshared pair of electrons they behave as Lewis bases. Influence of the number of hydrogen atoms at nitrogen atom is displayed in physical properties such as solubility, boiling point and chemical reactions including basicity. Basic character of amines is expressed in terms of their \( K_b \) and \( pK_b \) values. Three classes of amines can be distinguished with p-toluene sulphonyl chloride which reacts with primary and secondary amines. Amino group when attached to benzene ring behaves as an activating and ortho para directing group.
Which amine will have highest \( pK_b \) value?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(C_6H_5NH_2\)
The basic strength of an amine is expressed by its base dissociation constant \( K_\mathrm{b} \), or by \( \mathrm{p}K_\mathrm{b} = -\log K_\mathrm{b} \). A stronger base has a larger \( K_\mathrm{b} \) and therefore a smaller pKb; the weakest base has the highest pKb.
Basicity depends on how freely the lone pair on nitrogen can accept a proton. The +I effect of alkyl groups makes aliphatic amines stronger bases than ammonia, while in aniline the lone pair is delocalised over the ring by resonance, so aniline is a weaker base than ammonia.
The pKb values in water are:
\( \displaystyle \ce{C6H5NH2}\,(9.38) \gt \ce{NH3}\,(4.75) \gt \ce{CH3NH2}\,(3.38) \gt \ce{CH3CH2NH2}\,(3.29) \)
So aniline, the weakest base of the four, has the highest pKb.
Why the other options are incorrect
Option (1) \(CH_3CH_2NH_2\) — Incorrect. Ethanamine is the strongest base here because of the +I effect of the ethyl group, so it has the lowest pKb (3.29).
Option (2) \(NH_3\) — Incorrect. Ammonia (pKb 4.75) is weaker than the aliphatic amines but much stronger than aniline, because its lone pair is not delocalised.
Option (4) \(CH_3NH_2\) — Incorrect. Methanamine is an aliphatic amine with a low pKb (3.38), close to that of ethanamine.
NCERT (2026-27 edition):
Basic Character of Amines – …
