Stereochemistry of the S-2 Reaction

Back-side attack inverts the configuration of the carbon atom. 

Back- de attack in the 8 2 reaction. The 8 2 reaction takes place through nucleophilic attack on the back lobe of carbon s sp hybrid orbital. This back-side attack inverts the carbon atom s tetrahedron, like the wind inverts an umbrella. 

In some cases, inversion of configuration is readily apparent. For example, when cis-1 -bromo-3-methylcyclopentane undergoes Sn2 displacement by hydroxide ion, inversion of configuration gives /raii5-3-methylcyclopentanoL 

The S m2 displacement is a good example of a stereospedfic reaction one in which different stereoisomers react to give different stereoisomers of the product. To study the mechanism of a nucleophilic substitution, we oft look at the product to see if the reaction is stereospecific, with inversion of configuration. If it is, the 8 2 mechanism is a good possibility, especially if the reaction kinetics are second order. In many cases (no asymmetric carbon or ring, to example), it is impossible to determine whether inversion has occurred. In these cases, we use kinetics and other evidence to help determine the reaction mechanism. 

Draw a perspective structure ora Fischer projection for the products of the following 8 2 

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