Question

Why do only compounds that yield tertiary carbocations (or resonance‐stabilized carbocations) undergo SN1?

Answer 1

Compounds that yield tertiary or resonance‐stabilized carbocations undergo `"S"_N1` reactions because their activation energies are lower than for `"S"_N2` reactions.

You always have a competition between the `"S"_N1` and `"S"_N2` mechanisms. The question is, "Which one predominates?"

t-Butyl bromide is extremely hindered to back-side attack. So the `E_"a"` for an `"S"_"N"2` attack is quite high.

But both hyperconjugation and the electron-donating effects of the alkyl groups stabilize the 3° carbocation.

So the `E_"a"` for an `S_N1` attack is low, and the `"S"_"N"1` reaction predominates by a large amount.

Allyl bromide, CH₂=CH-CH₂Br, is a 1° halide. The `E_"a"` for an `"S"_"N"2` attack is normal.

But resonance stabilizes the allyl cation, CH₂=CH-CH₂⁺.

The `E_"a"` for formation of the cation is so low that the `"S"_N"1` reaction predominates by a large amount.