a sn2 reaction occurs if the backside route of attack is not blocked by other atoms in the molecule (sterically hindered by substituents on the substrate). so, this mechanism usually occurs at an unhindered primary carbon center. if there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon center, the substitution will use an sn1 rather than an sn2 mechanism, (an sn1 would also be more likely with blocked molecules because a sufficiently stable carbocation intermediary could be formed.)