No Interchangeability by a Symmetry Operations

4 No Interchangeability by a Symmetry Operation If geminal protons (CH2) in a molecule cannot be interchanged through a symmetry element, 

The following similar achiral molecules contain diastereotopic methylene protons 3-hydroxyglutaric acid, glycerol, citric acid, diethyl acetal, and a cyclic acetal (respectively, Fig. 4.37, structures f, g, h, i, and j) structure j involves the additional concept of magnetic equivalence (Section 4.13). 

From the above discussion, diastereotopic protons cannot be placed in the same set since they are not chemical-shift equivalent. However it is not uncommon for diastereotopic protons to appear to be chemical-shift equivalent in a given magnetic field in a particular solvent. Such accidental chemical shift equivalence can usually be detected by using an instrument with a higher magnetic field or by changing solvents. 

Students are familiar with the terms applied to relationships between stereoisomeric molecules homomeric molecules (superposable molecules), enantiomeric molecules (nonsuperposable mirror images), and diastereomeric molecules (stereoisomers that are not mirror images of one another). These familiar terms are parallel to the terms that we have introduced above homotopic, enantiotopic, and diastereotopic, which are applied to nuclei or groups within the molecule. 

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