What are the stereochemical implications of bimolecular and unimolecular nucleophilic substitution?
If a chiral molecules undergoes nucleophilic substitution what happens to its stereochemistry?
SN1: These reactions occur with the production of a cationic intermediate. These contain an sp2 hybrdised carbon atom with vacant p orbitals (hence the positive charge). The nucleophile can attack from above or below the plane of the molecule therefore producing a racemic mixture of product. If a single enantiomer is reacted with a nucleophile and the reaction proceeds via unimolecular nucleophilic substitution then equal amounts of both possible enantiomer will be formed (providing the carbon atom remains a chiral centre).
SN2: These reactions proceed via a transition state (anionic if the nucleophile is anionic) where the nucleophile attacks from one side and the leaving group leaves from the other. This results in something that resembles an umbrella turning inside out as the three unaffected groups switch sides. This means that if one enantiomer is reacted with a nucleophile then only a single enantiomer will be present in the products, although it's absolute configuration may have changed.