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Which of the following does not undergo electrophilic aromatic substitution? enter image description here

Answer is option (D). But I am not able to understand why. All of them have lone pairs. I can notice only two differences. Functional group in Option (d) has the largest size and lowest electronegativity.

I think that may be because of a large difference in size of Selenium and Carbon, pie bond formation in between them will be a bit difficult, but that bond is not playing any role here except stabilizing the compound.

Low electronegativity seems like a good thing to me as it does not deactivates the compound.
So what am I missing here?

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  • $\begingroup$ @AvatarShiny Couldn't you say the same for sulphur? $\endgroup$ – Gaurang Tandon Feb 16 '18 at 5:36
  • $\begingroup$ Se has bigger d orbitals. It's softer. @Gaurang $\endgroup$ – Avnish Kabaj Feb 16 '18 at 5:37
  • $\begingroup$ @AvatarShiny I still can't understand why will d- orbitals effect the reaction. $\endgroup$ – V J Feb 16 '18 at 5:41
  • $\begingroup$ @AvatarShiny Oh so you're saying that it will be easier to take electrons from Se because of d orbitals? That must be it. Thank you. $\endgroup$ – V J Feb 16 '18 at 5:57
  • $\begingroup$ @VJ Your thinking is correct. Selenium is the largest atom of the series, it is in Period 4 while the others are in Periods 2 and 3. It's larger size precludes effective overlap with the carbon pi orbitals. Whereas molecules A-C are "aromatic" because the heteroatom lone pair is involved in overlap with the dienyl system to make a 6 pi electron ring, the selenium atom can't participate like this so molecule D is not "aromatic" like the others $\endgroup$ – ron Feb 16 '18 at 14:56
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Wikipedia disagrees. Selenophene does undergo EAS, and apparently faster than thiophene.

None of the proposed answers to the original question is correct.

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