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Question #43950

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The Question

L-Lysine is an essential amino acid with macroscopic $pK_a$ values of $2.18$ ( $\alpha$ -\ce{COOH}), $8.95$ ( $\alpha$ -\ce{NH3+}), and $10.53$ ( $\epsilon$ -\ce{NH3+}). It undergoes nucleophilic addition with one equivalent of benzaldehyde to form a Schiff base. The regioselectivity of this reaction is highly pH-dependent.

Let $R$ be the ratio of the concentration of the microstate with a free $\alpha$ -amine and protonated $\epsilon$ -amine to the microstate with a protonated $\alpha$ -amine and free $\epsilon$ -amine in an aqueous solution.

Calculate $R$ . Furthermore, identify the major Schiff base product formed when the reaction is conducted at $pH = 11.50$ under kinetic control.

(Assume the intrinsic nucleophilicity of the unhindered $\epsilon$ -amine is greater than the $\alpha$ -amine. Given: $10^{1.58} \approx 38$ )

Options

A.
$R \approx 38$ ; $N^\epsilon$ -benzylidene-L-lysine
B.
$R \approx 38$ ; $N^\alpha$ -benzylidene-L-lysine
C.
$R \approx 0.026$ ; $N^\epsilon$ -benzylidene-L-lysine
D.
$R \approx 0.026$ ; $N^\alpha$ -benzylidene-L-lysine

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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

Schiff Base pH Dependence|Amino Acid Zwitterion pH Sensitivity

Why This Question Is Tricky

Common Mistakes

This question tests the critical distinction between the thermodynamic availability of a nucleophile and its intrinsic kinetic reactivity.

Students often fall into the trap of assuming the group with the lower $pK_a$ ( $\alpha$ -amine) is always the major reactive site because it deprotonates first.

While true at lower pH (e.g., pH 8) where the $\alpha$ -amine's free concentration is 38 times higher, this logic fails at high pH.

At $pH = 11.50$ , both amines are fully deprotonated.

The 'hidden variable' is the shift from thermodynamic control (concentration of active species) to kinetic control (intrinsic nucleophilicity).

Since the $\epsilon$ -amine lacks the steric hindrance of the $\alpha$ -position, it is kinetically superior, making it the major site of Schiff base formation at highly basic pH.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Schiff Base pH Dependence
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free