[Zalzul]

Bruice is trying to explain why Br₂ (with FeBr₃ as a catalyst) would add preferentially to the left-hand ring with the diagram below.



I am confused as to why the carbonyl group does not aid in resonance.  I've seen in enolate chemistry where the double bond flips between the carbonyl carbon and the alpha carbon, so why is oxygen being so "selfish" here whereas on the left, it's happily donating its electrons.

Thanks for your time.

[Zalzul]

Okay, I think I understand it more.  That carbonyl carbon has a partial positive charge which hinders its ability to donate into the benzene.  But then I still don't understand how that left oxygen is able to help the ring, since oxygen is so electronegative.  How could it not be pulling electron density out of that left ring?  I'm going to assume it has to do with aromaticity.

[Archer]

You need to understand the difference between the inductive effect of an atom (i.e. electro negativity) and mesomeric effect which is where elections are being donated to stabilise the carbocation which is the intermediate in aromatic substitutions.