A guide to the two pathways for nucleophilic substitution.
SN1 Mechanism
The mechanism involves two-steps.
With a tertiary halide:
In the first step, which is the slow step, the C-Cl bond will break and both electrons will go to Cl. A carbocation intermediate and Cl- form.
The second (fast) step sees the hydroxide, OH-, attacking the carbocation and forming the alcohol.
In the slow step, there is only one halogenoalkane, hence the rate is first order with respect to halogenoalkane and zero order with respect to hydroxide.
The rate equation:
rate = k[RX]
The overall order is one, hence SN1. Tertiary halides favour the SN1 mechanism - there are more electron-donating alkyl groups to stabilise the carbocation intermediate (carbocation is more likely to form).
SN2 Mechanism
The SN2 mechanism is a one-step reaction.
With a primary halide:
The hydroxide attacks the carbon from behind the C-Cl bond. The C-O bond is formed while the C-Cl bond breaks with, in the transition state, the OH, C and Cl groups along the same axis.
This is the only step, so it is the rate determining step. One halogenoalkane and one OH- are involved - rate is first-order with respect to both the halogenoalkane and the hydroxide.
The rate equation:
rate = k[RX][OH-]
Overall, a second-order mechanism (SN2) favoured by primary halides (less bulky alkyl groups = less steric hindrance).
