Question

Arrange the following in increasing order of basic strength:

• A. Aniline $<$ Ammonia $<$ Ethylamine
• B. Ammonia $<$ Aniline $<$ Ethylamine
• C. Ethylamine $<$ Ammonia $<$ Aniline
• D. Aniline $<$ Ethylamine $<$ Ammonia

Hint:

Aromatic amines are always less basic than aliphatic amines and ammonia.
This is due to the resonance delocalization of the nitrogen lone pair into the aromatic ring.
Conversely, aliphatic alkyl groups push electron density onto nitrogen, boosting its basicity.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks us to arrange aniline, ammonia, and ethylamine in the increasing order of their basic strengths.

Step 2: Key Formula or Approach:
The basic strength of a nitrogenous base is determined by the ease with which its nitrogen atom can donate its lone pair of electrons to a proton (\(H^+\)).
\[ \text{Basic Strength} \propto \text{Electron Density on Nitrogen} \propto \frac{1}{\text{Resonance Delocalization}} \]

Step 3: Detailed Explanation:

• Structure of Aniline: In aniline (\(C_6H_5NH_2\)), the lone pair of electrons on the nitrogen atom is conjugate with the \(\pi\)-electrons of the benzene ring.
  
• Resonance Effect in Aniline: The lone pair is delocalized into the ring via resonance, making it less available for protonation.
  
• Basicity of Ammonia: Ammonia (\(NH_3\)) has a lone pair on the nitrogen atom that is fully localized, as there are no conjugate systems or alkyl groups.
  
• Structure of Ethylamine: Ethylamine (\(CH_3CH_2NH_2\)) contains an ethyl group, which is an electron-donating group.
  
• Inductive Effect in Ethylamine: The ethyl group exerts a \(+I\) inductive effect, increasing the electron density on the nitrogen atom and making its lone pair highly available for donation.
  
• Comparative Summary: Aniline is the weakest base because of resonance, ammonia is stronger than aniline, and ethylamine is the strongest due to the \(+I\) effect.
  

Step 4: Final Answer:
The correct increasing order of basic strength is Aniline $<$ Ammonia $<$ Ethylamine, which corresponds to option (A).