SN1 versus SN2

TODO

How to determine which mechanism is occuring?

Its easy for methyl, primary (1o) and tertiary (3o) halides and tosylates.  Methyl and 1o halides and tosylates will undoubtedly follow the SN2 pathway since they can not form stable carbocations required for SN1 and they have least steric hindrance (favors SN2).  Tertiary (3o) will follow SN1 pathway since they can form stable cations.   Tertiary (3o) have the most steric congestion which limits nucleophilic attack in SN2 like reactions.  Secondary halides and tosylates are difficult to predict (in practice they are even more difficult to predict).  Bare in mind that elimination reactions (E1/E2) can compete with substitution and often times elimination products form as well, especially in the presence of nucleophiles that are very basic.