[qw098]

Hi guys!

I was just looking through my organic textbook, and read something that was fairly interesting but the textbook didn't answer the question it asked! My professor however is notorious at asking us the same questions that the textbook doesn't answer on his exams!

The question is this: Why does halogenation (bromination) of phenanthrene and anthracene occur under very mild conditions (i.e. does not require Lewis Acid) to give addition products? Also, why does addition only occur in the middle of the ring?

I thought a long time on this question but have absolutely no idea! I was thinking that because with a catalyst, there is a positive charge induced on one of the bromines that helps undergo nucleophilic substitution on benzene, but with phenanthrene or anthracene the fused benzene rings don't need this added positive charge on bromine to undergo nucleophilic substitution?

I honestly have no clue, and any help greatly be appreciated! Thanks! 

Please find the attached scan from my textbook (Just the diagram)

[OC pro]

Has something to do with resonance energies. Two (separated) benzene rings in a molecule add up more resonance energy than compared to a nahpthalene unit. (Benzene 151 kJ/mol, napthalene 255kJ/mol --> 2x 151 > 255 kJ/mol). Formation of sigma-complex is easier.

Therefore naphthalene is ~12000 fold more reactive in electrophilic aromatic substitutions than benzene under comparable conditions.  

[Honclbrif]

March's Advanced Organic Chemistry, 6th edition, pp 58-61. Good discussion of the aromaticity of fused aromatic systems. Addresses these exact questions.

[Honclbrif]

In case you have access to a different edition, it is in chapter 2 (aromaticity) under the heading "six-membered rings"

[qw098]

Thank you very much guys!

Answered my questions exactly!