Question

In a Diels-Alder reaction with benzyne and anthracene to make triptycene, why is the benzyne added to the central ring of anthracene and not on the ends? Which resonance structure of anthracene is the most stable?

Answer 1

The resonance of different aromatic rings are listed below:

       Benzene: 152 kJ/mol

       Naphthalene: 255 kJ/mol

       Anthracene: 347 kJ/mol

If the Diels-Alder reaction takes place in the central ring of Anthracene it will spare two Benzene rings. The loss in resonance energy is then calculated as

           {347 - (2 * 152)} kJ/mol = 43 kJ/mol

If the Diels-Alder reaction takes place in the central ring of Anthracene it will spare a Naphthalene ring. The loss in resonance energy is then calculated as

           {347 - 155} kJ/mol = 192 kJ/mol

As the loss in resonance energy is less for reaction at the central ring the reaction will take place in the central ring.