Influence of the Substrate on the ${\text{S}}_{\text{N}}1$ Reaction

The stability of a carbenium ion is the higher the more substituents the positively charged carbon possesses. Tertiary alkyl compounds therefore usually react in ${\text{S}}_{\text{N}}1$ reactions, particularly when the bimolecular substitution (${\text{S}}_{\text{N}}2$) is additionally prevented by steric interactions.

The stability order is basically the result of hyperconjugation, an interaction between the unoccupied p orbital of the carbocation and the σ bonding orbitals of vicinal C-H bonds. The effect of hyperconjugation is the stronger the more C-H bonds are adjacent to the positively charged, ${\text{sp}}^2$-hybridized carbon. A tert-butyl cation is significantly stabilized by hyperconjugation rather than an ethyl cation, as it possesses nine C-H bonds that are adjacent to the positively charged carbon, while in the ethyl cation only three of such C-H bonds exist. The degree to which hyperconjugation is dependent on the degree of substitution has also been proven experimentally: with X-ray analyis the C-C bonds in tert-butyl cations are shorter than in primary carbenium ions, such as in the ethyl cation.

Exercises