Structure and SN2 Reactivity The Leaving Group

In these equations, ethanol and acetone, respectively, are the solvents for the indicated transformations. These solvents are polar (Section 1-3) and particularly good at dissolving salts. We shall come back to the influence of the nature of the solvent on the Sn2 reaction in Section 6-8. In the second example, notice that the reaction taking place at C2 has no effect on the stereocenter at C3. 

As an aid in the prediction of stereochemistry, organic chemists often use the guideline that diastereomers produce diastCTeomCTs. Replace the starting conpound in each of the two preceding exanples with one of its diastoeomers, and write the product of Sn2 displacement with the nucleophile shown. Are the resulting structures in accord with this rule  

Similarly, nucleophilic substitution of a substituted halocycloalkane may change the stereochemical relation between the substituents. For example, in the disubstituted cyclohexane below, the stereochemistry changes from cis to trans. 

In Summary Inversion of conflgnration in the Sn2 reaction has distinct stereochanical consequences. Optically active substrates give optically active products, unless the nucleophile and the leaving group are the same or meso compounds are formed. In cyclic systems, cis and trans stereochemical relations may be interconverted. 

Leaving-group ability Is a measure of the ease of its displacement 

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