Question

Arrange the following in decreasing order of their basicity: 

• A. \( B>C>A \)
• B. \( B>A>C \)
• C. \( A>B>C \)
• D. \( A>C>B \)

Hint:

In amines, electron-donating groups (like -CH₃) increase basicity, while electron-withdrawing groups (like -NO₂) decrease it.

Answer 1

The Correct Option is B

The basicity of amines depends on the availability of the nitrogen lone pair for protonation. The following factors affect the basicity:
- Methylamine (B): Methyl groups are electron-donating via inductive effects, which increase the electron density on nitrogen, making it more basic.
- Aniline (A): The phenyl group in aniline withdraws electron density from the nitrogen via resonance, making it less basic than methylamine.
- Nitroaniline (C): The nitro group (-NO_2) is a strong electron-withdrawing group. It pulls electron density from the nitrogen through resonance, significantly decreasing its basicity.
Thus, the order of basicity is \( B>A>C \), and the correct answer is option (2).

Answer 2

The basicity of a compound is determined by its ability to donate a lone pair of electrons to a proton (H⁺). In the context of organic compounds, the availability of the lone pair on the nitrogen atom largely affects this property. Here is how we evaluate the basicity of the given options:

1. Factors affecting basicity:

The basicity of an amine depends on:

• Electron donating or withdrawing effects : Groups that donate electrons by +I (inductive) effect increase basicity, while electron withdrawing groups decrease it.
• Resonance effect: When the lone pair on nitrogen is delocalized through resonance with other atoms, basicity is reduced.

2. Analysis of given options:

• Compound A: The nitrogen's lone pair may be involved in resonance with an aromatic ring, reducing its availability for protonation and thus its basicity.
• Compound B: It likely has a strong +I effect without involvement in resonance, making its nitrogen's lone pair readily available.
• Compound C: If it contains structures leading to significant delocalization of the lone pair, its basicity is reduced similar to compound A.

3. Ordering of basicity:

Considering the above factors, Compound B has the highest basicity as its lone pair is most available. Compound A is less basic than B but more basic than C due to its lesser extent of resonance involvement compared to C. Therefore, the decreasing order of basicity is B > A > C.

Thus, the correct option is:

B>A>C