SN1 - First-order Nucleophilic Substitution
Influence of the Substrate on the 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 reactions, particularly when the bimolecular substitution () 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, -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.