S_N1 Reaction: Mechanism

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.

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