How does polarity affect SN1 reactions?

In #S_N1# reactions, there is a so-called carbocation intermediate. Solvents of higher polarity tend to stabilize these carbocations and therefore, #S_N1# reactions occur more readily in polar solvents.

For many problems in organic chemistry, it is often a good idea to take a look at the mechanism in question. Here is the mechanism for a general #S_N1# reaction.

The first step involves bromine as leaving group breaking away to generate the carbocation (positively charged carbon compound). Once the carbocation is formed, the #CH_3CH_2OH# nucleophile quickly attacks at the positively charged carbon. Notice that since the first step is slow - namely, the formation of carbocation - it will determine the overall rate of the reaction. The more favorable the carbocation formation, the faster the rate of the overall #S_N1# reaction.

Looking at solvent polarity, as a general rule, a highly polar solvent will stabilize a charged ionic species such as a carbocation much better than a non-polar solvent. This is because the solvent itself has partial positive and partial negative regions (i.e. dipole moments ) which could interact favorably with the charged carbocation.