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Question #43992

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following reaction sequence for the enantiomerically pure compound \textbf{P}, which is (S)-2-carbamoyl-2-phenylpropanoic acid: \begin{enumerate} \item \ce{Br2} (1 equiv.), \ce{NaOH} (excess), $343\text{ K}$ \item Dil. \ce{HCl} (aq) added carefully until the pH of the solution is 5.5 \item The resulting aqueous solution is heated at $373\text{ K}$ for 2 hours \end{enumerate} Given that the isoelectric point (pI) of the resulting $\alpha$ -amino acid is approximately 5.5, identify the major organic product isolated at the end of the sequence.

Options

A.
(R)-2-amino-2-phenylpropanoic acid
B.
(S)-2-amino-2-phenylpropanoic acid
C.
(R)-1-phenylethanamine
D.
(S)-2-phenylpropanamide

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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

Decarboxylation Mechanisms|Hoffmann Bromamide Rearrangement|Amino Acid Zwitterion pH Sensitivity

Why This Question Is Tricky

Common Mistakes

This problem tests two critical hidden variables.

First, the stereochemical trap: students often equate 'retention of configuration' in the Hoffmann bromamide rearrangement with 'retention of the R/S descriptor'.

However, the transformation of the amide to an amine alters the Cahn-Ingold-Prelog (CIP) priorities, causing an inversion of the nomenclature from (S) to (R) despite spatial retention.

Second, the kinetic/thermodynamic trap: students blindly apply thermal decarboxylation to any malonic acid-like structure.

They fail to recognize that at the isoelectric point, the internal acid-base neutralization forms a zwitterion.

The resulting carboxylate (\ce{-COO-}) cannot participate in the requisite 6-membered cyclic transition state, rendering the $\alpha$ -amino acid completely stable to decarboxylation at $373\text{ K}$ .

Quick Facts About This Question

Question Information

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