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Question #45066
The Question
Consider the following sequence of chemical evaluations at 298 K:
\textbf{Step 1:} The basic strength of five substituted anilines is compared with aniline in an aqueous medium: 2-methylaniline, 4-methylaniline, 2,6-dimethylaniline, 4-nitroaniline, and 4-methoxyaniline. Let be the number of amines from this set that are strictly \textit{less} basic than aniline.
\textbf{Step 2:} 4-Methylaniline (Compound A, given of its conjugate acid ) undergoes condensation with benzaldehyde to form a Schiff base.
\textbf{Step 3:} The rate of this Schiff base formation is highly pH-dependent and reaches a maximum at an optimal pH, denoted as . At this optimal pH, the concentration of the protonated amine is exactly 10 times the concentration of the unprotonated amine .
\textbf{Step 4:} Let be the total number of hybridized carbon atoms in the final Schiff base product.
Calculate the value of .
Options
- A.
17
- B.
25
- C.
29
- D.
31
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
Acidity and Basicity (pKa/pKb)|Schiff Base pH Dependence
Why This Question Is Tricky
Common Mistakes
The primary trap is the 'Ortho Effect' in anilines.
Students typically associate alkyl groups with electron donation (+I, +HC), expecting ortho/para-methylanilines to be more basic than aniline.
However, ortho-substitution causes Steric Inhibition of Protonation (SIP), destabilizing the solvated conjugate acid and making 2-methylaniline and 2,6-dimethylaniline less basic than aniline.
A secondary trap involves the pH dependence of Schiff base formation; students might blindly apply the Henderson-Hasselbalch equation backwards or fail to recognize that at the optimal pH (~4.0), the majority of the amine is actually protonated, but the small equilibrium concentration of the free amine is sufficient for the rate-determining nucleophilic attack.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2 |
| Concept | Acidity and Basicity (pKa/pKb) |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |