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Question #44033
The Question
Consider the basicity of Cytosine (4-amino-1H-pyrimidin-2-one) and Aniline in aqueous solution at 298 K. Upon treatment with one equivalent of dilute \ce{HCl_{(aq)}}, both molecules form thermodynamically stable monoprotonated cations. Identify the major site of protonation for Cytosine and Aniline, respectively, and the correct comparative basicity of their exocyclic \ce{-NH2} groups.
Options
- A.
Cytosine: N3 ; Aniline: \ce{-NH2} ; Cytosine exocyclic \ce{-NH2} is less basic than Aniline \ce{-NH2}
- B.
Cytosine: Exocyclic \ce{-NH2} ; Aniline: \ce{-NH2} ; Cytosine exocyclic \ce{-NH2} is more basic than Aniline \ce{-NH2}
- C.
Cytosine: N1 ; Aniline: \ce{-NH2} ; Cytosine exocyclic \ce{-NH2} is equally basic to Aniline \ce{-NH2}
- D.
Cytosine: N3 ; Aniline: para-carbon ; Cytosine exocyclic \ce{-NH2} is more basic than Aniline \ce{-NH2}
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Mechanism_Trap
Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.
Concepts Behind This Question
Aniline Resonance Pitfalls|Nitrogenous Base Keto-Enol Tautomerism|Basicity-Solvation Competition
Why This Question Is Tricky
Common Mistakes
The question targets the 'Resonance Misapplication' and 'Nitrogenous Base Tautomerism' traps.
Students often rely on the heuristic that an \ce{-NH2} group is always the primary basic site, incorrectly equating the exocyclic amine of nitrogenous bases to Aniline.
The hidden mechanism is the amidine-like resonance in Cytosine, which 'locks' the exocyclic lone pair and transfers the basicity entirely to the ring nitrogen (N3).
This demonstrates that resonance stabilization of the ground state can completely shift the thermodynamic site of protonation away from the apparent functional group.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Amines - Class 12 |
| Concept | Aniline Resonance Pitfalls |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |