6.12:
SN1 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 SN1 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 SN1 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 SN1 reaction differs from the single-step SN2 reaction in that the bond between the carbon atom and the leaving group breaks before the nucleophilic attack.
Moreover, while an SN1 reaction includes two transition states and one intermediate, an SN2 reaction has only one transition state and no intermediate.
6.12:
SN1 Reaction: Mechanism
Kinetic studies of ionization of a tertiary halide in a protic solvent suggest that only the substrate participates in the rate-determining step (slow step). The nucleophile is involved only after the slowest step. The SN1 reaction takes place in a multiple-step mechanism.
Firstly, the haloalkane ionizes to generate a carbocation intermediate and a halide ion. This heterolytic cleavage is highly endothermic with large activation energy. The ionization of the substrate, facilitated by a polar protic solvent, is the slowest of all steps, making it the rate-determining step of an SN1 reaction. The ions formed are stabilized through solvation. In the second step, the reactive carbocation intermediate behaves as a strong electrophile and is attacked by the nucleophilic solvent molecule that quickly donates an electron pair to generate an oxonium ion. This process is exothermic. In the third step, the solvent abstracts a proton from the oxonium ion to yield the final nucleophilic substituted product.
Thus, the SN1 reaction consists of two core steps for substitution and an additional step of proton loss. The mechanism further suggests that several factors such as the stability of the carbocation, the nature of the leaving group, and the nature of the solvent used, favor the SN1 mechanism.