Stereospecific reactions bimolecular nucleophilic

The general procedures previously outlined in this section for the formation of the carbon to azide bond have been widely employed in the steroid field, particularly as a stage in the stereospecific synthesis of aminosteroids. Bimolecular nucleophilic displacement reactions of sterols substituted with j -toluenesulphonyl, methanesulphonyl or halogeno groups etc. with azide ion proceeds with Walden Inversion and enables the stereospecific introduction of the azido group, which may then be reduced to an amino group. 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

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