Tertiary halogenoalkanes will undergo a SN1 reaction. This means that there is 1 molecule in the rate determining step, which is the slowest step. Because of the bulky alkyl groups surrounding the central carbon attached to the halogen atom, there is very little space for the nucleophile to attack. This is called steric hindrance. Because of this, the weak carbon-halide bond must first be broken without any other molecular interefence and once a carbocation is formed, the nucleophile will be able to attack the now positive carbon atom to form a covalent bond.