SN1 reaction

Occurs with tertiary halogenoalkanes. In these, the carbon atom bonded to the halogen atom is also bonded to three other carbon atoms (alkyl groups).
Mechanism:
1. The carbon–halogen bond breaks, forming a tertiary carbocation. This is an example of heterolytic fission as both electrons in the bond go to the halogen.

2. The tertiary carbocation is immediately attacked by the hydroxide ion.
The rate of the reaction only depends on the concentration of the halogenoalkane only, as shown in the first (slow) step of the mechanism.
rate = k[halogenoalkane]

SN2 reaction

Occurs with primary halogenoalkanes. In these halogenoalkanes, the halogen atom is bonded to a carbon atom, which is bonded to one other carbon atom and two hydrogen atoms.
Mechanism: The nucleophile (OH-) donates a pair of electrons to the δ+ carbon atom, forming a new covalent bond. At the same time, the C─Br bond is breaking, with the Br atom taking both electrons in the bond. The Br atom leaves as a Br− ion.
This is an example of heterolytic fission of a covalent bond as Br takes both electrons in the C-Br bond.
The rate of the reaction depends on both the concentration of the halogenoalkane and the concentration of the hydroxide ions present.
rate = k[halogenoalkane][OH-]