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

Dec 13, 2014

Compounds that yield tertiary or resonance‐stabilized carbocations undergo ${\text{S}}_{N} 1$ reactions because their activation energies are lower than for ${\text{S}}_{N} 2$ reactions.

You always have a competition between the ${\text{S}}_{N} 1$ and ${\text{S}}_{N} 2$ mechanisms. The question is, "Which one predominates?"

t-Butyl bromide is extremely hindered to back-side attack. So the ${E}_{\text{a}}$ for an $\text{S"_"N} 2$ attack is quite high.

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

So the ${E}_{\text{a}}$ for an ${S}_{N} 1$ attack is low, and the $\text{S"_"N} 1$ reaction predominates by a large amount.

Allyl bromide, CH₂=CH-CH₂Br, is a 1° halide. The ${E}_{\text{a}}$ for an $\text{S"_"N} 2$ attack is normal.

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

The ${E}_{\text{a}}$ for formation of the cation is so low that the $\text{S"_N} 1$ reaction predominates by a large amount.