SN1 mechanism (S: substitution, N: nucleophilic, 1: first order) is one of the two limiting mechanisms of nucleophilic aliphatic substitution at saturated carbon. It is a two-step mechanism:
A nucleophilic aliphatic substitution at saturated carbon occurring via SN1 mechanism is called an SN1 reaction. The rate law of an SN1 reaction is
rate = k [substrate].
According to the rate law, an SN1 reaction is first order overall, and the concentration of the nucleophile does not affect the rate. The implication is that the nucleophile does not participate in the rate limiting step or any prior steps, which suggests that the first step is the rate limiting step. Since the nucleophile is not involved in the rate-limiting first step, the nature of the nucleophile does not affect the rate of an SN1 reaction.
see also SN2 mechanism