Question

Arrange the following in decreasing order of their acidic strengths.

• A. A > C > B > D
• B. A > D > C > B
• C. A > D > B > C
• D. D > A > C > B

Answer 1

Answer: (B)

A > D > C > B

Answer 2

The acidic strength of a compound is determined by the stability of its conjugate base. A more stable conjugate base corresponds to a stronger acid.

1. Compound (A): This is a protonated cyclic ether (oxonium ion). Such species are very acidic. Upon losing a proton, it forms a neutral cyclic ether, which is a stable conjugate base.
2. Compound (B): This is pyrrolidine, a cyclic secondary amine. Amines are very weak acids.
3. Compound (C): This is 2-methylcyclopentanol, a cyclic secondary alcohol. Alcohols are weakly acidic.
4. Compound (D): This is N-methylpyrrolidinium ion, a protonated cyclic secondary amine (ammonium ion). Such species are acidic.

Comparison:

• Protonated species (A and D) are significantly more acidic than neutral species (B and C).
• Comparing (A) and (D): Protonated ethers (A) are much more acidic than protonated amines (D). So, A > D.
• Comparing (B) and (C): Alcohols (C) are more acidic than amines (B) because oxygen is more electronegative than nitrogen, stabilizing the negative charge better in the conjugate base. So, C > B.
• Comparing (D) and (C): Protonated amines (D) (pKa of conjugate acid ~ 10.3) are more acidic than alcohols (C) (pKa ~ 16-18). So, D > C.

Combining these inequalities (A > D, D > C, C > B), the decreasing order of acidic strengths is A > D > C > B.