more notesss

The S_N1 reaction



The S_N1 reaction is a two-step reaction in which

1. The leaving group leaves, forming a carbocation. This is
   the slow step, and so the rate is dependent only on the concentration
   of the substrate.
   
2. The nucleophile attacks the carbocation. It can do this
   from either side, typically in a 50/50 ratio. Therefore about half the
   product has retained the original configuration, and about half is
   inverted.
   
3. Protonated nucleophiles, such as methanol, then lose a proton to the solvent.
   

The S_N2 reaction



The S_N2 reaction is a single-step
displacement of a leaving group by a nucleophile. During the transition
state, the bond to the nucleophile forms at the same time that the bond
to the leaving group breaks. Therefore the nucleophile is required to
approach from the back, and configuration at carbon is inverted.