Question

How do you determine if a reaction will be ${{S}_{N}}1$ or ${{S}_{N}}2$ ?

Answer 1

Hint There are many factors that can decide whether the reaction is ${{S}_{N}}1$ or ${{S}_{N}}2$ like the effect of the nucleophile, kinetics of the reaction, stability order of the carbocation of the alkyl halide, etc. The order of the stability of the carbocation in the ${{S}_{N}}1$ reaction will be ${{3}^{\circ }}>{{2}^{\circ }}>{{1}^{\circ }}$ and reverse order is for the ${{S}_{N}}2$ reaction.

Complete step by step answer:
There are two types Nucleophilic substitution reaction, i.e., unimolecular Nucleophilic substitution reaction ( ${{S}_{N}}1$ ) and bimolecular Nucleophilic substitution reaction ( ${{S}_{N}}2$ ).There are many factors that can decide whether the reaction is ${{S}_{N}}1$ or ${{S}_{N}}2$ like the effect of the nucleophile, kinetics of the reaction, stability order of the carbocation of the alkyl halide.
(a)- Effect of the nucleophile: If the nucleophile will attack the reactant molecule in the second step of the reaction, then it is a ${{S}_{N}}1$ reaction, and if the incoming nucleophile attaches with the reactant molecule in the first step then it is ${{S}_{N}}2$ reaction. Usually, a weak nucleophile is required in the ${{S}_{N}}1$ reaction and strong nucleophiles are required in ${{S}_{N}}2$ reaction.
(b)- Kinetic order of the reaction: If the rate of the reaction only depends on the concentration of the reactant molecule then the reaction is ${{S}_{N}}1$ reaction and if the rate of the reaction depends on the concentration of the reactant as well as the concentration of the nucleophile then the reaction is ${{S}_{N}}2$ .
(c)- Stability order of the carbocation of the reactant molecule: If the reactant molecule will form a primary carbocation then it will undergo ${{S}_{N}}2$ reaction because the stability order of the ${{S}_{N}}2$ reaction is ${{1}^{\circ }}>{{2}^{\circ }}>{{3}^{\circ }}$ . If the reactant molecule will form a tertiary carbocation then it will undergo ${{S}_{N}}1$ reaction because the stability order of the ${{S}_{N}}1$ reaction is ${{3}^{\circ }}>{{2}^{\circ }}>{{1}^{\circ }}$ .

Note: Solvents can also decide the mechanism of the reaction, strong polar solvents are needed in ${{S}_{N}}1$ reaction because the nucleophiles must remain in the solution and weak polar solvents are used for ${{S}_{N}}2$ reaction.