# Why is it the middle ring of anthracene which reacts in a Diels–Alder?

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

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.

• – Mithoron May 11 '15 at 23:23

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.