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SN1 - First-order Nucleophilic Substitution

Influence of the Substrate on the SN1 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 SN1 reactions, particularly when the bimolecular substitution (SN2) is additionally prevented by steric interactions.

Relative stabilities of carbenium ions.

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, sp2-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.

Comparison of the methyl cation with the 1,1-dimethylethyl cation.


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