Diastereotopic Hydrogens are Not Chemically Equivalent

We can conclude that when two different sets of protons split a signal, the multiplicity of the signal can be determined by using 

Enantiotopic hydrogens show one NMR signal because they are chemically equivalent. 

Enantiotopic hydrogens are chemically equivalent, so they show only one NMR signal. 

The carbon to which the enantiotopic hydrogens are attached is called a prochlral carbon. If Hq is replaced by a deuterium, the asymmetric center will have the R configuration. Thus, H is called the pro-R-hydrogen. Similarly, H is called the pro-S-hydr en because replacing it with a deuterium would generate an asymmetric center with the S configuration. 

If a carbon is bonded to two hydrogens in a compound that has an asymmetric center, then the two hydrogens are called diastereotopic hydrogens, because replacing each of them in turn with deuterium (or another group) creates a pair of diastereomers. 

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We know that diastereotopic hydrogens are not chemically equivalent because they do not react at the same rate with achiral reagents. For example, removal of H and Br to form ( )-2-butene occurs more rapidly than removal of Ha and Br to form (Z)-2-butene because ( )-2-butene is more stable than (Z)-2-butene (Section 10.6). Because diastereotopic hydrogens are not equivalent, the W + 1 rule has to be applied to them separately. 

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