Group from a Chiral Carbon Racemization

LOSS OF A GROUP FROM A CHIRAL CARBON RACEMIZATION 

Any mechanism involving a chiral center that proceeds via an achiral intermediate must result in a loss of stereochemistry of that center, thus producing a racemate. While any such process could be described as a racemization, this term typically refers to the conversion of an enantiopure sample to its race-mate, which involves converting the one enantiomer into the other until an equal mixture of both enantiomers exists. 

If additional chiral centers exist in the molecule and remain unchanged during this conversion, the pair of compounds with a mixture of R and S configurations at a single site would be related as diastereomers (more specifically, epimers) rather than enantiomers, so such a process would be described as an epimerization rather than a racemization. For example, sugars can undergo epimerization at the anomeric carbon via a resonance-stabilized carbocation intermediate. 

Chiral centers can be converted to achiral intermediates by loss of a leaving group to give a planar carbocation (as in the SnI mechanism). Attempted formation of an organometallic reagent from a chiral alkyl halide often results in significant racemization and is generally not synthetically useful. Similarly, radical mechanisms result in racemization since a radical intermediate is essentially planar and thus achiral. 

Racemization by Loss of an Alpha Proton (via Enolate or Enol) 

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