Trigonal molecules enantiotopic atoms

Just as there are enantiotopic and diastereotopic atoms and groups, so we may distinguish enantiotopic and diastereotopic faces in trigonal molecules. Again we have three cases (1) In formaldehyde or acetone (G), attack by an achiral reagent A from either face of the molecule gives rise to the same transition state and product the two faces are thus equivalent. (2) In butanone or acetaldehyde (H), attack by an achiral A at one face gives a transition... 

The concept of heterotopic atoms, groups, and faces can be extended from enantiotopic to diastereotopic types. If each of two nominally equivalent ligands in a molecule is replaced by a test group and the molecules that are generated are diaster-eomeric, then the ligands are diastereotopic. Similarly, if reaction at one face of a trigonal atom generates a molecule diastereomeric with that produced at the alternate face, the faces are diastereotopic. 

The two prochiral faces of a trigonal atom C(x,y,z) are enantiotopic and designated Re and Si according to whether x, y, and z trace a clockwise Re) or counterclockwise Si) path in order of decreasing Cahn-Ingold-Prelog precedence. An acetaldehyde molecule that lies in the plane of the paper, for example, presents either the Re or Si face according to how it is oriented. 

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