Question

The CORRECT order of acidity of the following compounds is: 
 

• A. I>II>III
• B. II>III>I
• C. I>III>II
• D. III>II>I

Hint:

When comparing acidity, always consider the stability of the conjugate base formed after deprotonation. Greater resonance and electronegativity effects increase conjugate base stability and acidity.

Answer 1

The Correct Option is B

Step 1: Understanding acidity in the given compounds.

The acidity of a compound is influenced by the stability of the conjugate base after deprotonation. A more stable conjugate base implies higher acidity. In this case, the compounds I, II, and III differ based on the position and number of hydrogen atoms attached to the nitrogen atom in the pyrrole ring.

• Compound I: The nitrogen atom is protonated (\( \text{NH}_2^+ \)), leading to a strong acid since the conjugate base formed (\( \text{NH} \)) is neutral and stable.
• Compound II: The nitrogen atom is protonated (\( \text{NH}_3^+ \)), but it has an additional proton, which results in relatively less stability of the conjugate base compared to Compound I.
• Compound III: The nitrogen is unprotonated, and the lone pair on nitrogen participates in resonance with the pyrrole ring, reducing its acidic nature.

Step 2: Analyzing the options.

• Compound II has the highest acidity due to the greater stability of the conjugate base after deprotonation (\( \text{NH}_2^+ \) becoming \( \text{NH}_3^+ \)).
• Compound III follows due to partial resonance stabilization.
• Compound I is the least acidic because the conjugate base is less stabilized.

Step 3: Conclusion.

The correct order of acidity is:

\[ II > III > I \]