Recalling that benzene has a resonance energy of 152 kJ mol^-1 and naphthalene has a resonance energy of 255 kJ mol^-1,predict the positions which would be occupied by bromine when phenanthrene (below)undergoes addition of Br₂.

Question

Quizplus · Accepted Answer

Phenanthrene has 3 rings, each of which can contribute to resonance stabilization. Since the resonance energy of naphthalene (which has 2 rings) is higher than that of benzene, we can predict that the resonance energy of phenanthrene (which has 3 rings) will be even higher. This means that the addition of Br₂ will be more favorable at the positions that allow for the most resonance stabilization. The best positions for Br₂ addition in phenanthrene are 9 and 10, which are adjacent to a fused ring system (the central ring). The Br₂ addition at these positions will create a positive charge on the adjacent carbon atoms, which can be delocalized through resonance with the other rings. This delocalization of charge leads to maximum resonance stabilization and therefore an energetically favorable addition of Br₂ at positions 9 and 10.

Recalling That Benzene Has a Resonance Energy of 152 KJ