Final answer:
Tertiary alkyl halides are unreactive to SN2 mechanisms due to the sterically hindered environment around the central carbon and are instead prone to undergoing E2 or SN1 mechanisms.
Step-by-step explanation:
Tertiary alkyl halides are unreactive to the process of SN2 mechanisms. This is because these compounds prefer to undergo substitution reactions through a different path known as the SN1 mechanism, where a three-step process occurs: first the dissociation of the leaving group to form a carbocation, then the planar carbocation can be attacked from either side by the nucleophile, and finally, the departure of the proton if necessary.
Due to the stable nature of tertiary carbocations, formed thanks to the inductive effect provided by adjacent carbon groups, the SN2 mechanism is not favored for tertiary alkyl halides. This is because the SN2 mechanism involves a one-step bimolecular reaction that requires a good backside attack on the central carbon, which is hindered in tert-carbocations by steric bulk. Therefore, SN2 reaction rates follow the trend of methyl < primary < secondary < tertiary alkyl halides, with tertiary being the least reactive for this kind of mechanism. The tertiary alkyl halides are more likely to react via the E2 mechanism to form alkenes, or through the SN1 route for substitutions.