NOTE: when i say primary undergo SN2 reactions, i mean this becasue it is much more likely to occur. this is because in primary halogenoalkanes/ HA, the carbon bonded to the leaving molecule is easily accesible to the nucleophile, whereas in a teriary molecule, due to steric hinderance (i.e. crowding of carbons), the nucleophile cannot access
same thing for teriary SN1 reactions, teh rate determining step in SN1 is the formation of a carbocation, it is much easier to form a stable carbo cation than an unstable one. tertiarty are more stable than primary due to presence of mehtyl groups, which have an electron releasing effect creating a more stable carbocation