Question

How do you increase the rate of an $SN1$ reaction?

Answer 1

We know that this reaction is a nucleophilic substitution reaction which means that the functional group already present in the compound is replaced by another functional group that is also a nucleophile. A nucleophile is an electron-rich species. This reaction takes place in a polar solvent.

Complete step by step solution:
The $SN1$ reaction is a two-step reaction that involves the substitution of a nucleophile in the presence of a polar solution. This reaction depends on the concentration of substrate that is being used. On increasing the concentration of substrate, the rate of the reaction also increases. The reaction does not depend on the concentration of the nucleophile.
The second step is the attack of nucleophiles on the carbocation. This results in the attachment of the electron rich nucleophile onto the electron deficient electrophile. This attachment can happen in two ways. The first possibility is if the reaction occurs by the attachment of the nucleophile at the same site as that of the previous nucleophile. This leads to the formation of a product that has identical geometry as that of the reactant but contains a different functional group.
The rate determining step of $SN1$ is a unimolecular equation (mechanism of two steps) which is the step of forming the carbocation: $R-X\xrightarrow{{}}{{R}^{+}}+{{X}^{-}}$ and therefore, the rate is giving by: $Rate=k\left[ R-X \right].$ To increase the rate of the reaction we can increase the concentration of the alkyl halide.

Note:
Remember that the second possibility is if the nucleophile is attached at a position that is opposite to the site of the previous functional group. This means that the attack will be at the backside of the carbocation. Resulting in the formation of a compound that is inverted with respect to position of the functional group. This leads to the formation of two substances called enantiomers.