Stereochemistry of Molecules with Two or More Asymmetric Carbons

Stereochemistry of Molecules with Two or More Asymmetric Carbons 

In the preceding section, we saw there are four stereoisomers (two pairs of enantiomers) of 2-bromo-3-chlorobutane. These four isomers are simply all the permutations of (R) and (S) configurations at the two asymmetric carbon atoms, and C  

A compound with n asymmetric carbon atoms often has 2 stereoisomers. This formula is called the 2 rule, where n is the number of chirality centers (usually asymmetric carbon atoms). The 2 rule suggests we should look for a maximum of 2 stereoisomers. We may not always find 2 isomers, especially when two of the asymmetric carbon atoms have identical substituents. 

3-Dibromobutane has fewer than 2 stereoisomers. It has two asymmetric carbons (C and C ), so the 2 rule predicts a maximum of four stereoisomers. The four permutations of (/ ) and (S) configurations at and are shown next. Make molecular models of these structures to compare them. 

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