Part 8: THERMODYNAMICS

In order to do calculate the resonance energy of anthracene, we need to compare the experimental heat of hydrogenation (given = -116.2 kcal/mol) with that of isolated (non-interacting) double bonds. (We did this in class for benzene)

Since anthracene has 7 double bonds we get:

7 x -28.6 kcal/mol = -200.2 kcal/mol

which is 84 kcal/mol more EXOTHERMIC than that of anthracene (compare -200.2 and -116.2)

so we can conclude that anthracene is 84 kcal/mol more stable than 7 isolated double bonds.

Resonance energy (anthracene) = 84 kcal/mol

The Diels-Alder reaction to A gives a naphthalene system (resonance energy = 61 kcal/mol)
and,
the Diels-Alder reaction to B gives two benzenes (resonance energy = 2 x 36 kcal/mol = 72 kcal/mol)

Therefore the greater stability of B compared to A drives the reaction (since two benzenes have greater resonance stablitity than a naphthalene) and despite the statistical effect (2 ways to get A and only 1 way to get B)