Question

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

Answer 1

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.