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?

$C_6H_5NH_2$

The correct answer is Option (3) → $C_6H_5NH_2$ **

The pK_b value measures the basic strength of the amine:

• Higher pK_b → weaker base (lower tendency to accept H⁺).

Basicity order of amines in aqueous solution: 2° > 1° (alkyl) > NH₃ > 1° (aryl like aniline)

• Alkyl amines (CH₃CH₂NH₂, CH₃NH₂): Electron-donating alkyl groups increase electron density on N → stronger base than ammonia.
• NH₃ (ammonia): Reference compound.
• C₆H₅NH₂ (aniline): The lone pair on nitrogen is delocalized into the benzene ring via resonance → less available for protonation → much weaker base.

Approximate pK_b values (conjugate acid pK_a → pK_b = 14 – pK_a):

• CH₃CH₂NH₂: pK_b ≈ 3.3
• CH₃NH₂: pK_b ≈ 3.4
• NH₃: pK_b ≈ 4.8
• C₆H₅NH₂: pK_b ≈ 10.6

Thus, aniline (C₆H₅NH₂) has the highest pK_b (weakest base).