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

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.