Recommended reading relating to SN1, SN2, E1, and E2 reactions.

A nice discussion of the SN2 reaction (If you click on "leaving group" and then "TsO-", note that the "S" atom is missing. It is the hypervalent atom, with six bonds to it.)

SN-2, SN-1, E-2 and E1 : Substitution and Elimination Univ. of Alberta, CANADA Nice summary with good pics.