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

1 Answer
Ernest Z.
Dec 13, 2014

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?"

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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.

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