I am getting a bit confused about why the configuration at the optical centre changes from d to l after enolization?
Any stereogenic centre next to a carbonyl group will be destroyed by enolization. Such compounds racemize quickly.
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2 Answers
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Forming an enol causes the carbon attached to the methyl group to essentially form a trigonal planar structure. This means that in the eventual conversion back to a ketone, there are two sides the hydrogen can attach to, often referred to as the re and si faces (see prochirality).
Therefore, the 'optically pure' d-isomer would actually exist in equilibrium with the l-isomer as the acetoacetic ester, which forms especially stable enols, will basically force prochirality on the carbon attached to the Me group.
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$\begingroup$ In the figure, it is mentioned that optically pure l isomer is formed, but instead it will be more like a racemic mixture? $\endgroup$– CaraNov 4, 2022 at 8:09
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3$\begingroup$ @HiyaJandial The picture doesn't say a pure optical isomer if formed: it shows a reaction path where the two pure isomers are in equilibrium. $\endgroup$ Nov 4, 2022 at 16:07
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[OP in comments] In the figure, it is mentioned that optically pure l isomer is formed, but instead it will be more like a racemic mixture?
For equilibrium reactions, the chemical equation does not tell you what the net direction of reactions will be. When you reach equilibrium, you will have some of each of the two enantiomers, and some of the enol. If there is not special circumstance (chiral cosolvent, etc.), the two enantiomers will be present in equal amounts.
I am getting a bit confused about why the configuration at the optical centre changes from d to l after enolization?
What is confusing is that the molecules are labeled "optically pure isomer". This does not make much sense in this context. It is true that no matter whether you start with optically pure d or l enantiomer, you will end up with a racemic mixture.
Molecular formulas do not indicate pure or mixture, they just denote a given molecule, so labeling them "pure" is a bit strange.
Taking a snapshot at equilibrium, some centers will have changed from d to l, and some will not have changed. Also, some centers are in the process changing from planar to d or l, and some d or l centers are in the process of changing to non-chiral. This is what we call dynamical equilibrium.