Question

How does concentration affect SN1 reactions?

Answer 1

Increasing the concentration of the nucleophile has no effect on the rate of reaction. Increasing the concentration of the substrate increases the rate.

The hydrolysis of t-butyl bromide is a typical `"S"_"N"1` reaction:

t-Bu-Br + H₂O → t-Bu-OH + HBr

The mechanism of the reaction is

The reaction involves two separate steps.

First the leaving group departs, and then the nucleophile attacks the carbocation.

The first step is the rate-determining step.

The rate law for the reaction is then

`"rate" = k[t"-BuBr"]`

The rate law does not involve `["H"_2"O"]`, so changing the concentration of the nucleophile has no effect on the rate.

The rate law does involve `[t"-BuBr"]`, so increasing the concentration of the substrate increases the rate.

Likewise, decreasing the concentration of the substrate decreases the rate.