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

1 Answer
Oct 11, 2016

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

Explanation:

.......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^-#

Hydrocarbyl substitution at carbon thus stabilizes the carbocation kinetically and thermodynamically, and favour unimolecular nucleophilic substitution.