In substitution reactions, there are two mechanisms that will be observed. An Sn2 and Sn1 reaction mechanism.

Sn2 reactions are bimolecular in rate of reaction and have a concerted mechanism. The process involves simultaneous bond formation by the nucleophile and bond cleavage by the leaving group. The transition state looks like this. Because the reaction is concerted, Sn2 mechanisms will always lead to an inversion of stereochemistry! For reactivity using an Sn2 mechanism, primary >> secondary >> tertiary carbon centers.

On the other hand, Sn1 reactions are unimolecular in rate of reaction and have a step-wise mechanism. This process first involves bond cleavage by the LG to generate a carbocation intermediate. The stability of carbocation formation will determine if Sn1 or Sn2 reactions occur. In the second step, the electronegative nucleophile attacks the carbocation to form the product. The steps look like this. Because the nucleophile can attach either side of the carbocation, which adopts an sp2-hybridized orbital with a trigonal planar geometry, an equal amount of inversion and retention is seen, referred to as a racemic mixture. For reactivity using an Sn1 mechanism, tertiary >> secondary >>> primary carbon centers.

The strength of nucleophiles used help to determine the reaction mechanism. Strong bases will almost always proceed to Sn2 mechanism. Weak nucleophiles will generally proceed to Sn1 mechanism when a stable carbocation is present.