6.14 : Predicting Products: S_N1 vs. S_N2

Nucleophilic substitution reactions of alkyl halides can proceed via an S_N1 or an S_N2 mechanism with distinct stereochemical outcomes.

Factors like the structure of the substrate, the strength of the nucleophile, and the nature of the solvent promote one mechanism over the other.

In S_N2 reactions, the nucleophile attacks the substrate simultaneously as the leaving group departs. Therefore, bulky substituents on the substrate prevent the incoming nucleophile from forming a bond. Consequently, less hindered substrates favor S_N2 reactions.

In S_N1 reactions, the substrate first dissociates to give the carbocation intermediate, stabilized through inductive effect and hyperconjugation.

The electron releasing inductive effect of alkyl groups stabilizes the charge on the cation. In hyperconjugation, filled sp³ orbitals of alkyl groups and the vacant p orbital of the carbocation overlap to stabilize the carbocation.

Therefore, increased alkyl substitution stabilizes the carbocation, making tertiary halides most suitable for S_N1 reactions.

The rate laws of mechanisms suggest that the nature and concentration of nucleophiles affect only S_N2 reaction rates.

While the reactions are relatively fast in the presence of strong nucleophiles like the hydroxide ion, weak nucleophiles like water slow down S_N2 reactions.

In S_N1 reactions, nucleophiles do not participate in the rate-determining step, and hence, both strong and weak nucleophiles are effective.

In S_N2 reactions, polar protic solvents negatively influence the reaction rate, as they cage the nucleophiles through hydrogen bonds, delaying their approach towards the substrate.

Polar aprotic solvents, on the contrary, destabilize the nucleophiles, thereby decreasing the activation energy and increasing the reaction rate.

In S_N1 reactions, polar protic solvents facilitate the departure of the leaving group by stabilizing the ions through solvation.