Trigonal carbon, electrophilic substitution

A possible justification for frontside attack in electrophilic substitution is that ab initio molecular orbital calculations for the CH5+ cation, the species that would be formed if H+ attacked methane, indicate that the most stable structure would not be a trigonal bipyramid, in which carbon uses a p orbital to bond to two protons, but would be a relatively unsymmetrical structure that has a smallest H—C—H bond angle of about 37° (Figure 4.10).85 For further discussion of SB2 substitution on carbon, see Section 10.3.86... 

The actual mode of interaction between Co(III) and the alkane was not elucidated. It could involve electron transfer as described above or it may be an example of a general class of electrophilic substitutions at saturated carbon centers in which attack at a a bond occurs via a trigonal (three-center) transition state,3008 e.g.,... 

The stereochemistry of the direct attack can be expected to resemble the corresponding reactions at a saturated carbon (Sections 5.1.1.1 and 5.1.1.2)—inversion for nucleophilic substitution, and retention, or perhaps occasionally inversion, for electrophilic substitution. In practice, SN2 reactions at trigonal carbon are rare,503 and their stereochemistry, where inversion is known,504 barely established. Electrophilic attack, like the reactions of vinyllithium reagents with protons, aldehydes and carbon dioxide, takes place invariably... 

Figure 4.11 illustrates the three possible stereochemical outcomes of a substitution reaction. The Sn2 substitution gives inversion because the nucleophile must attack the carbon from the back side because the full nucleophile orbital must overlap well with the empty electrophile orbital (Section 4.2.7). The SnI substitution often gives racemization because the intermediate carbocation is trigonal planar, and the nucleophile can attack equally from top or bottom faces (Section 4.2.8). 

There are two important consequences of this analysis (1) the more substituted carbon is a stronger electrophile and is therefore more susceptible to nucleophilic attack and (2) the more substituted carbon has significant carbocationic character, which means that its geometry is described as somewhere between tetrahedral and trigonal planar, allowing nucleophilic attack to occur at that position even though it is terdaiy. 

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