Arrange the following in cncreasing order of their basic strength

(I) \(C_6H_5NH_2\) (II) \(C_2H_5NH_2\) (III) \(NH_3\)

I < III < II

The correct answer is option 2. I < III < II.

Basicity refers to the ability of a compound to donate a pair of electrons to accept a proton (H⁺). In the case of amines, basicity is determined by the availability of the lone pair of electrons on the nitrogen atom.

Several factors influence the basicity of amines:

i. Inductive Effect

ii. Resonance Effect

iii. Solvation Effect

Let us evaluate the basicity of each compound in detail:

I. Aniline (\(C_6H_5NH_2\))

Aniline is an aromatic amine with the structure \(C_6H_5NH_2\). The nitrogen atom is attached directly to a benzene ring. The lone pair of electrons on the nitrogen can participate in resonance with the \(\pi \)-electrons of the benzene ring. This delocalization reduces the availability of the lone pair for protonation.

Effect on Basicity: Because the lone pair is partially delocalized into the benzene ring, it is less available to accept a proton. This decreases the basicity of aniline.

Conclusion: Aniline is less basic compared to aliphatic amines and ammonia.

II. Ethylamine (\(C_2H_5NH_2\))

Ethylamine is a primary aliphatic amine with the structure \(C_2H_5NH_2\). The nitrogen atom is bonded to an ethyl group (\(C_2H_5\)). The ethyl group is an electron-donating group via the inductive effect (+I effect). This increases the electron density on the nitrogen atom, making the lone pair more available for protonation.

Effect on Basicity: The increased electron density on nitrogen makes ethylamine more basic than both aniline and ammonia.

Conclusion: Ethylamine is the most basic among the given compounds.

III. . Ammonia (\(NH_3\))

Ammonia is a simple molecule with the formula \(NH_3\), where nitrogen has a lone pair of electrons. There are no resonance effects or strong electron-donating groups attached to the nitrogen in ammonia. As a result, the lone pair is available for protonation, but not as available as in ethylamine, where an electron-donating alkyl group is present.

Effect on Basicity: Ammonia is more basic than aniline (which has the lone pair delocalized) but less basic than ethylamine (which has the lone pair stabilized by an electron-donating group).

Conclusion: Ammonia is intermediate in basicity between aniline and ethylamine.

Final Order of Basicity

Based on the above explanations, the correct order of increasing basic strength is:

\(\text{Aniline (I)} < \text{Ammonia (III)} < \text{Ethylamine (II)}\)

The correct answer is option 2. I < III < II.