Recalling that benzene has a resonance energy of 152 kJ m o l − 1 and naphthalene has a resonance energy of 255 kJ m o l − 1 , predict the positions that would be occupied by bromine when phenanthrene

Question

Recalling that benzene has a resonance energy of 152 kJ m o l − 1 and naphthalene has a resonance energy of 255 kJ m o l − 1 , predict the positions that would be occupied by bromine when phenanthrene undergoes addition of B r 2 ​ . Phenanthrene positions are numbered 1 to 4 and 5 to 8 around the outer rings and 9 and 10 on the middle ring.

TestHub · Accepted Answer

Addition at C 9 ​ - C 10 ​ leaves two intact benzene rings (2 × 152 = 304 kJ/mol), the most resonance energy that can be kept. Addition in a terminal ring would leave a naphthalene unit (255 kJ/mol), which keeps less. So bromine adds at positions 9, 10.

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