Why do tertiary halides react faster in SN1 mechanism but are slow to reactivity in an SN2 mechanism?

Question

Brooklyn Barber · Accepted Answer

Well, #S_N2# processes are essentially bond-making reactions,......

.......whereas #S_N1# processes are essentially bond-breaking reactions., and of course, are only dependent on the concentration of the electrophile. A bond making process at a tertiary carbon centre, for instance, is unlikely to be kinetically favoured, given the shrubbery around the #3^@# carbon. On the other hand, this same shrubbery is likely to favour bond-breaking kinetically, and tend to stabilize a #3^@# carbocation, which is conceived to be an intermediate in an #S_N1# process : #R_1R_2R_3C-X rarr R_1R_2R_3C^(+) +X^-# Thus, hydrocarbyl substitution at carbon favors unimolecular nucleophilic substitution and stabilizes the carbocation both thermodynamically and kinetically.

Easton Jones · Answer

undefined Tertiary halides react faster in SN1 mechanisms because the formation of a stable carbocation intermediate is favored due to the presence of three alkyl groups, which stabilize the positive charge. In SN2 mechanisms, tertiary halides are slow to react because the bulky alkyl groups hinder the approach of the nucleophile, making it difficult for the nucleophile to attack the substrate.