Pictures of the Day CH320M328M

SN2 reactions are initiated through reaction of a nucleophile on the backside of the electrophile carbon-halogen bond. The charge distribution of the alkyl halide electrophile (where the electrons are!) promotes this process as seen above. The nucleophile with a large negative charge (red color) is attracted to the backside of the C-X bond due to partial positive charge (blue color) present there. Thus, by understanding electron distribution (where the electrons are!), you can correctly understand and predict that the nucleophile will initiate the SN2 reaction via backside attack of the C-X bond. Click here to see the SN2 mechanism movie.

An E2 reaction involves the deprotonation of a C-H bond with simultaneous (concerted) formation of a new pi bond and loss of a leaving group such as halide anion to give an alkene. The most important thing to remember about E2 reactions is that an anti-periplanar geometry is required for the reaction to take place, since this geometry is the only one that properly aligns the orbitals involved with the reaction. Click here to see a movie of an E2 reaction.