Enantiotopic groups and faces

The three criteria of the molecular environment, of substitution, and of symmetry, show that the two hydrogen atoms in bromochloromethane (LVIIIa) are not 

Molecules such as LVIII and LIX contain a center of prochirality, or prochiral center. It is a serious error, too often encountered in biochemical papers, to confuse enantiotopic groups or atoms and prochiral centers. Enantiotopic groups belong to prochiral centers they are not themselves prochiral. 

The presence of a center of prochirality is not an obligatory condition for a molecule to be prochiral. Indeed, other elements of prochirality exist, namely axes and planes of prochirality. Thus, the prochiral allene derivative LX displays two geminal hydrogen atoms which are enantiotopic. 

The concept of prochirality can also be applied to trigonal centers, i.e., to faces of suitable molecules. In acetaldehyde (LXI), the two faces of the molecule are not 

While the presence of enantiotopic groups in a molecule necessarily implies the presence of an element of prochirality, diastereotopic groups imply prostereoisomerism as an element of prochirality, or of proachirality. For example, chloroethylene (LXII) contains two geminal hydrogen atoms which are diastereotopic. But no 

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Because biological reactions involve chiral enzymes, enantiotopic groups and faces typically show different reactivity. For example, the two methylene hydrogens in ethanol are enantiotopic. Enzymes that oxidize ethanol, called alcohol dehydrogenases, selectively remove the pro-/ hydrogen. This can be demonstrated by using a deuterated analog of ethanol in the reaction. 

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