Just like acid–base reactions, nucleophilic attack will only occur if the reactants are more
reactive than the products. Thus, the nucleophile must be more reactive than the leaving
group.NUCLEOPHILIC SUBSTITUTION REACTIONS
Nucleophilic substitution reactions are perfect examples for demonstrating nucleophile–
electrophile reactions. In both SN1 and SN2 reactions, a nucleophile forms a bond with a substrate
carbon and a leaving group leaves.
SN1 Reactions
Unimolecular nucleophilic substitution (SN 1 ) reactions contain two steps. The first step is the
rate-limiting step in which the leaving group leaves, generating a positively charged carbocation.
The nucleophile then attacks the carbocation, resulting in the substitution product. This
mechanism is shown in Figure 4.5 below.
Figure 4.5. Mechanism of SN1 Reaction
Step 1: Formation of the carbocation (rate-limiting); Step 2: Nucleophilic attack.The more substituted the carbocation, the more stable it is because the alkyl groups act as electron
donors, stabilizing the positive charge. Because the formation of the carbocation is the rate-limiting
step, the rate of the reaction depends only on the concentration of the substrate: rate=k[R–L],
where R–L is an alkyl group containing a leaving group. This is a first-order reaction; anything that
accelerates the formation of the carbocation will increase the rate of an SN1 reaction.