sn2 and sn1 are two extremes of a sliding scale of reactions. it is possible to find many reactions which exhibit both sn2 and sn1 character in their mechanisms. for instance, it is possible to get a contact ion pairs formed from an alkyl halide in which the ions are not fully separated. when these undergo substitution the stereochemistry will be inverted (as in sn2) for many of the reacting molecules but a few may show retention of configuration. sn2 reactions are more common than sn1 reactions.