How does concentration affect SN1 reactions?

1 Answer
Dec 15, 2014

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

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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.