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)