Regioselectivity in electrophilic aromatic substitution of benzofuran and indole

This response as to the regioselectivity of the electrophilic aromatic substitution of indole left me with a few more questions than it did answers.


In figure 3b, does the pi bond in the bottom half of the benzene on the left get pushed to the nitrogen, allowing for stabilization across the benzene ring in addition to the resonance structures of the nitrogen cation? And if so, and this is the case of a viable nitrogen cation contributing to the stabilization of a valid resonance structure, is it true that all (or most) polycyclic compounds that undergo electrophilic substitution at a location on the nitrogen-containing ring that result in the temporary resonance structure of a nitrogen cation along with stabilization across a benzene ring will be more stable than substitution at a location that would only provide the resonance across the benzene (e.g. without the possible nitrogen cation, had electrophilic substitution occurred elsewhere)?

Furthermore, does this imply that in the electrophilic aromatic substitution pattern of benzothiophene, seeing as how sulfur is less electrophilic than both oxygen and nitrogen, that we would see a similar substitution pattern at the 3-position, as opposed to the substitution at the 2-position that we would see (And that is, to my puny little brain, comprehensible) on benzofuran?

  • $\begingroup$ The positively charged nitrogen here isn't electron deficient: it has a double bond to carbon next to it and hence has a complete octet. It is not like, say, $\ce{PhCH2+}$ where the positive carbon has an empty p orbital. $\endgroup$ Feb 21, 2018 at 0:40
  • $\begingroup$ So that lack of electron deficiency - that results in it being more stable than the other resonance structure afforded by substitution at the 2-position? And does electrophilic substitution occur at the 3-position on benzothiophene? $\endgroup$ Feb 21, 2018 at 2:52
  • $\begingroup$ because when electrophile react on 3 position more resonance structures are posible than the 2 position and thus it show stability.Thats why we say that position 3 is more good for indol to attach electrophile. $\endgroup$
    – sidra
    Sep 6, 2018 at 18:33


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