Before You Begin

Question #45001

Difficulty: 8/10 | 🟢 Free

The Question

Compound $\mathbf{\Theta}$ is a synthetic fat-soluble vitamin derivative prepared by replacing the terminal hydroxyl group of all-\textit{trans}-retinol ( $\ce{C19H27CH2OH}$ ) with an $N$ -phenylamino group ( $\ce{-NH-C6H5}$ ). A solution of $\mathbf{\Theta}$ in $n$ -octanol is equilibrated with an equal volume of an aqueous phosphate buffer at $pH = 6.5$ at $298\text{ K}$ . Which of the following correctly identifies the predominant protonation state of $\mathbf{\Theta}$ , its primary phase location at equilibrium, and the dominant electronic effect governing the nitrogen's basicity?

Options

A.
Protonated, Aqueous phase, $+I$ effect
B.
Unprotonated, $n$ -octanol phase, Resonance effect
C.
Protonated, $n$ -octanol phase, $+I$ effect
D.
Unprotonated, $n$ -octanol phase, $-M$ effect

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Vitamin Solubility Logic|Basicity-Solvation Competition|Aniline Resonance Pitfalls

Why This Question Is Tricky

Common Mistakes

The question tests the student's ability to correctly identify the dominant electronic effects governing the basicity of a complex, multi-functional molecule.

The primary trap is the misapplication of electronic effects: students may either assume the massive retinyl chain exerts a strong $+I$ effect (treating it like a highly basic aliphatic amine) or erroneously assume extended conjugation ( $-M$ effect) into the polyene chain, ignoring the $sp^3$ hybridized $\ce{-CH2-}$ spacer.

By correctly identifying that resonance with the phenyl ring is the sole dominant factor lowering the $pK_a$ to $\approx 4.8$ , the student can accurately predict that the molecule remains unprotonated at $pH = 6.5$ and thus retains its fat solubility.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Biomolecules - Class 12
Concept	Vitamin Solubility Logic
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free