In contrast to an SN2 reaction, in which the bond-making addition of the nucleophile and the bond-breaking departure of the leaving group occur in a single step, the SN1 reaction involves two separate steps: first the departure of the leaving group and then the addition of the nucleophile.
In the SN1 reaction, the bond between the substrate and the leaving group is broken when the leaving group departs with the pair of electrons that formerly composed the bond. As a result, the carbon atom to which the bond was formerly made is left with a positive charge. This positive charge on a carbon atom is called a carbocation, from "carbon" and "cation", the word for a positively charged atom. The formation of a carbocation is not energetically favored, so this step in the reaction is the slowest step and determines the overall rate of the reaction. The step which controls the overall rate of a reaction is called the rate-determining step.
Only after the leaving group has departed and a carbocation has formed, a nucleophile forms a bond to the carbocation, completing the substitution. This step is more energetically favorable and proceeds more quickly.
There are several important consequences to the unimolecular nature of the rate-determining step in the the SN1 reaction. First, since the rate is controlled by the loss of the leaving group and does not involve any participation of the nucleophile, the rate of the reaction is dependent only on the concentration of the substrate, not on the concentration of the nucleophile. Second, since the nucleophile attacks the carbocation only after the leaving group has departed, there is no need for back-side attack. The carbocation and its substituents are all in the same plane (Figure 1), meaning that the nucleophile can attack from either side. As a result, both enantiomers are formed in an the SN1 reaction, leading to a racemic mixture of both enantiomers. Finally, since the nucleophile does not participate in the rate-determining step, the strength of the nucleophile does not affect the rate of the SN1 reaction.
Figure 1: SN1 reaction showing unimolecular step and formation of racemic mixture
What factors govern the rate the probability of an SN1 reaction? The single most important factor is the stability of the carbocation. Alkyl substituents increase the stability of a carbocation, so increasing alkyl substitution of the carbon atom increases the probability of an SN1 reaction occurring.
Recall that, as in the case of an SN1 reaction, the above trend regarding degree of substitution is just a trend and the real factor that determines whether an SN1 reaction can occur is the stability of the carbocation. From above, we would expect an SN1 reaction not to occur at the site of a primary carbon atom. Indeed, such reactions do not occur in ordinary alkanes. However, in molecules in which the carbon next to the site of substitution contains a double bond, the SN1 reaction is possible. The reason is that the positive charge on the carbocation can be delocalized among multiple possible resonance structures (see Resonance and delocalization), making the carbocation dramatically more stable. This effect can occur when the carbon atom of interest is next to one double bond (allylic) or a benzene ring (benzylic). Note that in the allylic case, because of the delocalization of the positive charge, the nucleophile can attack at multiple sites (Figure 2); this effect is absent in the benzylic case due to the need to preserve aromaticity. In summary, the key to the SN1 reaction is the stability of the carbocation.
Figure 2: SN1 reaction at a primary carbon atom due to delocalization and resonance
Jonathan Mooney (McGill University)