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I'm wondering why maleic anhydride adds to the middle cycle of anthracene, and not the outer two.

Reaction of anthracene with maleic anhydride

I would have expected that a Diels–Alder with the outer ring would be better, because I expected a naphtalene part to be lower in energy than two benzene parts (more resonance stabilisation).

I guess it has to do with reactant based arguments that the atomic coefficients for the two center carbon atoms (C-9 and C-10) are higher than from the outer cycle (C-1 and C-4). I ran a calculation using http://www.chem.ucalgary.ca/SHMO and the coefficients on C-9 and C-10 were 0.44, whereas those on C-1 and C-4 were only 0.31.

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To provide a reason for the observed regioselectivity, it is helpful to draw anthracene's aromatic π-electron system in alternance of single and double bonds. In this instance, it is more beneficial than "the ring" symbolizing the delocalised electron system, as this helps you to account for the precise number of π-electrons before the reaction (starting materials), during the reaction (the mechanism), and after the reaction (the product).

Anthracene numbering

For the Diels–Alder reaction, you may imagine two different pathways. I invite you to draw the mechanisms by yourself:

  • A reaction that involves carbon atoms #1 and #4 (or #5 and #8). Possible, by mechanism. There are five double bonds remaining in conjugation, and you count one six-membered ring in the state of "a benzene ring" (the very left one). In the very right six-membered ring, there is only a single double bond, too.
  • Alternatively, a Diels–Alder reaction with carbon atoms #9 and #10. This is more favourable then the former example, because both the very left as well as the very right ring of the product are in the state of "a benzene ring", each of these rings contain 6 π-electrons, the prerequisite of a Hückel aromatic compound. It is so much favourable to the former, that this is the reaction observed.

It may be helpful to add that benzene, naphthalene and anthracene are of course Hückel-aromatic compounds; with 6, 10 or 14 π-electrons they fit into the rule of $(4n + 2)$. Yet gradually, as experimentally found, in this group of three, benzene is the most, anthracene the least aromatic compound.

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