We saw in Chapter 6 that the concerted SN2 reaction resulted in clean inversion of configuration at the reacting carbon. We can understand this intuitively because this is a one-step reaction, leading to several consequences:
There is no room for the nucleophile to get to the carbon from the same face as the leaving group;
The dipole of the bond to the (electronegative) leaving group places a partial positive charge on carbon, creating coulombic attraction with the electrons of the nucleophile;
The HOMO of the nucleophile interacts with the LUMO of the C-X bond, which is oriented towards the back side.
We can illustrate this with a typical example: cyanide reacting with S-1-bromo-1-phenylethane:
The transition state for the SN2 reaction
In the SN1 reaction, the reactive intermediate has lost the chiral information contained in a stereocenter at the reacting site. Because the carbocation is planar, the nucleophile can approace from either side. Normally the nucleophile is the solvent, which obviously surrounds the structure.
The result is that SN1 reactions result in a mixture of stereoisomers when the reaction results in a stereocenter. If this is the only stereocenter, the mixture is racemic. If there is another (unreacting) stereocenter, the result is a mixture of diastereomers (usually 1:1 or very close to that).
The one "gotcha" is one you will see next term, where a nearby group can donate a lone pair through what's called "neighboring group participation." We will ignore this complication for the moment.
So we can use this information in two ways:
In predicting the outcome of a reaction: if the mechanism should be SN2, you predict a single steroisomer that inverst the configuration of the reacting carbon. If the mechanism should be SN1, you should create two steroisomers (enantiomers or diastereomers, depending on the number of stereocenters).
In analyzing experimental evidence: if you see a single stereoisomer, it is most likely an SN2 reaction; a mixture (particularly a 1:1 mixture) suggests an SN1 mechanism.
