Electrophilic Substitution at the Tetrahedral Carbon Atom

No gas-phase reactions of electrophilic substitution have been known so far, while the formation of the transition state structure of Eq. (5.9) in solution is greatly (often decisively) affected by medium factors, the specific solvation and the nucleophilic catalysis. Even though the true structure of a transition state of the Se2 reaction is much more complicated than XXIV, this structure correctly reflects the stereochemistry of substitution at the tetrahedral carbon atom, namely, the retention of configuration of the carbon atom bonds observed experimentally in most Se2 reactions (the Se2 rule. Ref. [1]). 

A theoretical interpretation of this important rule was for a long time based on the assessment of relative stability of various structures of the pentacoordi-nate carbon atom which carry, unlike II in Eq. (5.2), a positive charge. The simplest model is represented by the methonium ion CH5, and the simplest Se2 reaction is an attack by a proton upon the methane molecule CH4 -I- H .  

Ab initio calculations [75] with an extended basis set and inclusion of correlation corrections have shown that the proton approaches the methane molecule along the C—H bond and follows this reaction path up to the distance C—H+(1.85A)  

After this the reaction complex rearranges into a structure of Q-symmetry (compare approach 2 on scheme IX, Sect. 5.1.1.1)—analogous to XXIV in the configuration of the bonds of the central carbon atom. 

Another possible Q-structure of XXVa type (M = H) is energetically nearly equivalent to XXV, whereas the structure of the methonium ion with D3h-symmetry is unfavored (Table 5.3). 

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