Recall that when a tertiary halide reacts in an aqueous solution to give a nucleophilic substituted product, only the substrate participates in the rate-determining step. This unimolecular S_N1 process occurs through a multistep mechanism.

In the first step, the haloalkane ionizes through a high-energy transition state, generating a tertiary carbocation intermediate and a halide ion. The ionizing ability of the polar solvent, water, facilitates the departure of the halide. The heterolytic cleavage is a slow and highly endothermic process with large activation energy, making this a rate-determining step.

In the absence of the solvent, that is, in the gas phase, the activation energy is almost seven-folds higher.

The ions produced from the first step are stabilized by water molecules through solvation. The carbocation formed is a strong electrophile that can readily react with a weak neutral nucleophile like water — the solvent of this reaction.

In the second step, water acts as a Lewis base and donates its electrons to the carbocation, generating a protonated species — the oxonium ion. Since a new bond forms, the process is strongly exothermic with a low-energy transition state.

In the third step, the oxonium ion loses a proton to water, which now acts as a Brønsted base, resulting in two products: tert-butyl alcohol and a hydronium ion.

To summarize, the S_N1 mechanism consists of two core steps for substitution, and when an uncharged nucleophile is used, one additional proton-transfer step is involved in the end.

The multistep S_N1 reaction differs from the single-step S_N2 reaction in that the bond between the carbon atom and the leaving group breaks before the nucleophilic attack.

Moreover, while an S_N1 reaction includes two transition states and one intermediate, an S_N2 reaction has only one transition state and no intermediate.