Question: Which of the following ketones racemise in aqueous solution containing acidic or basic impurities?

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My answer: A and B should racemise in aqueous solution, for the carbonyl groups in A and B contain alpha-hydrogens, leading to the possibility of enolisation, hence racemization. Compound C can't form an enol, hence shouldn't racemise.

It's very clear that compound C doesn't racemise, my confusion is about A and B.

My logic (stability of carbanion) says that A and B should both racemise, however, the answer I have is only B.

I agree that the carbanion formed by A would be less stable than that formed by B, but not so much so to ignore the possibility of enolisation and racemization.

Could someone please help? Is there anything obvious that I'm missing? or is it related to data? A good explanation would help. Thanks a lot.

  • 1
    $\begingroup$ To answer you need to look for a chiral centre that has a labile proton. $\endgroup$ – user1945827 Feb 5 '18 at 14:54

You are right that the racemization occurs through the enol form.

The chiral center is on the carbon attached to the chlorine atom. In B, that carbon participates in the enol form, but in A not. Therefore, in A the enol is formed but there is no racemization.


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