2- Bromobutane asymmetric center

A compound with one asymmetric center, such as 2-bromobutane, can exist as two stereoisomers. The two stereoisomers are analogous to a left and a right hand. If we imagine a mirror between the two stereoisomers, we can see they are mirror images of each other. Moreover, they are nonsuperimposable mirror images, which makes them different molecules. 

How do we name the different stereoisomers of a compound like 2-bromobutane so that we know which one we are talking about We need a system of nomenclature that indicates the arrangement of the atoms or groups around the asymmetric center. Chemists use the letters R and S for this purpose. For any pair of enantiomers with one asymmetric center, one member... 

When a reactant that does not have an asymmetric center undergoes a reaction that forms a product with one asymmetric center, the product will always be a racemic mixture. For example, the reaction of 1-butene with HBr that we just looked at forms identical amounts of (/ )-2-bromobutane and (5)-2-bromobutane. Thus, the reaction is not stereoselective because it does not select for a particular stereoisomer. Why is this so ... 

Figure 2.19. 2-Bromobutane and are diastereotopic protons a to an asymmetric center.

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