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(b) Treatment of the enantiomerically pure ketone C under acidic conditions leads to a 1:1 mixture of diastereomers

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My thinking is that perhaps the explanation has something to do with the carbonyl group being prochiral and having two faces, each side generating one of two diastereoisomers?

I'm not very sure of this explanation though. What is the correct reasoning for this?

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    $\begingroup$ Oh the pain of alpha-chiral carbonyl compounds ^^' $\endgroup$
    – Jan
    Dec 29 '15 at 18:44
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Carbonyls are able to undergo keto-enol tautomerisation, this is accelerated in the presence of acid/base.

This phenomenon means that alpha-chiral carbonyls are generally regarded as configurationally unstable, as they are prone to racemisation, although in the absence of acid/base the extent to which the alpha-chiral carbonyl exists in its enol form is often negligible (the C=O, C-H combination is ever so slightly more favoured than C=C, O-H in a lot of cases).

The enol form of the carbonyl (the middle structure in my diagram below) is planar. When the enol tautomerises back to the ketone form, the proton may be transferred to either face of the planar structure, leading to both diastereomers being formed, as you describe.

enter image description here

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