Overall, in a substitution reaction, one group on a carbon atom is substituted or replaced with a different group.
To accomplish a substitution, two elementary steps are required:
1. loss of a leaving group (break a sigma bond)
2. nucleophilic attack (form a new sigma bond)
In some cases, these two steps happen simultaneously in a concerted reaction mechanism (SN2). Other times, the elementary steps are two separate steps in a mechanism (SN1). To decide which pathway is favored you need to consider:
1. The degree of substitution of the carbon losing the leaving group
2. The strength of the nucleophile
3. The solvent (protic vs aprotic)
Note, these factors are listed in the order they should be considered. Below is a flow chart that models this process.
Also, remember that if the mechanism goes through a carbocation (SN1), check to see if a rearrangement is probable.
Watch the videos below for more details. Then see if you can decide yourself which pathway is favored by playing the recommended practice puzzles.