Q.18 The following reaction is an example of ___________.
(A) enolization (B) racemization (C) isomerization (D) epimerization
The given reaction is an example of enolization, i.e. keto–enol tautomerization of a carbonyl compound bearing an α‑hydrogen next to a phosphate group.
Question restatement and concept
The structure on the left is a keto form of a phosphoglycerate derivative, and the structure on the right is its corresponding enol (double bond with adjacent OH) formed by migration of a proton from the α‑carbon to the carbonyl oxygen. This interconversion of a carbonyl compound and its enol tautomer is termed enolization (keto–enol tautomerism) in organic and biochemistry.
In the reaction, the carbon skeleton remains the same and no new stereocenter is created or inverted; only the position of a hydrogen and a double bond changes, converting C=O to C=C–OH. Therefore, option (A) is correct.
Option A: Enolization
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Definition: Enolization is the process by which a carbonyl compound (aldehyde or ketone, or related derivatives like 3‑phosphoglycerate) with an α‑hydrogen converts to its enol form through proton transfer and double‑bond shift.
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Mechanism features:
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Deprotonation at the α‑carbon forms an enolate intermediate.
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Reprotonation at oxygen gives the enol (–C=C–OH), while the reverse gives back the keto form, making the process a reversible tautomerization.
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In the figure, the left structure (keto) ⇌ right structure (enol) illustrates precisely this keto–enol tautomerism adjacent to a phosphate group, so enolization matches the transformation.
Option B: Racemization
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Definition: Racemization is the process by which one pure enantiomer converts into an equilibrium mixture of both enantiomers (a racemic mixture), usually via loss of stereochemical information at a chiral center.
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Key requirement: A chiral center whose configuration (R/S) is scrambling so that both mirror‑image forms are produced in equal amounts.
In the given reaction, no chiral center is present in the starting keto compound, and the interconversion is between two constitutional tautomers, not between mirror images. Because no racemic mixture of enantiomers is formed, racemization is not correct.
Option C: Isomerization
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Definition: In biochemistry, isomerization refers broadly to the conversion of one isomer into another, which can involve constitutional isomers or stereoisomers without changing overall molecular formula.
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Relevance here: Keto–enol tautomerization is indeed a type of isomerization because the keto and enol forms are constitutional isomers (different connectivity of atoms).
However, the question specifically offers enolization as a more precise term for the keto ⇌ enol interconversion, so the best and expected answer is enolization rather than the general term isomerization.
Option D: Epimerization
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Definition: Epimerization is the interconversion of two epimers, stereoisomers that differ in configuration at only one of several chiral centers (commonly at a single carbon in sugars or amino acids).
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Key requirement: The molecule must have at least two stereocenters, and the reaction must invert configuration at exactly one of them while leaving the rest unchanged.
In the shown reaction, there is no defined chiral center being inverted between product and reactant; the change is in bond type (keto to enol) at the same carbon skeleton. Therefore, epimerization does not describe this process, making option (D) incorrect.
