Question

primary alkyl halide undergo nucleophilic substitution through SN2 mechanism while tertiary through SN1. why?

Answer 1

SN1 is a slow reaction which occurs with formation of an intermediate and in turn depends on stability of intermediate. Tertiary carbocation is more stable than 1 degree carbon.
SN2 depends on amount of +ve charge on position of nucleophile attack. So primary undergo SN2