Topological Relation and Stereoselectivity

Substitution of one H atom by the new group results in one of the enantiomers. Therefore, these H atoms in achiral molecules 1 and 2 are properly denoted as enantiotopic. Enantiotopicity is destroyed under any chiral influence, in particular in the presence of a chiral center in the same molecule. Hydrogen atoms Ha and Hb 

Stereotopic faces around planar carbonyl group 

Faces around carbonyl group are diastereotopic Faces around carbonyl group are homotopic 

Finally, let us consider the atoms Ha and Hb in achiral molecule 5 and chiral molecule 6. They are homotopic, chemically and symmetrically equivalent. The central C atom in achiral molecule 5 bears two times two of the same ligands and can be described by the general formula CH2L2. Chiral molecule 6 is not a meso-form, and there is no mirror plane bisecting this molecule since two stereogenic centers possess the same absolute configuration. 

Now we can use a similar argumentation to analyze the symmetry properties of a planar unit in the selected molecule (Fig. 3.3). 

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