July 02, 2020, 01:36:49 AM
Forum Rules: Read This Before Posting


Topic: Reactions of substituted benzenes  (Read 882 times)

0 Members and 1 Guest are viewing this topic.

Offline Zalzul

  • Regular Member
  • ***
  • Posts: 17
  • Mole Snacks: +1/-0
  • Pharmacy student
Reactions of substituted benzenes
« on: April 29, 2014, 01:03:13 AM »
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.

Offline Zalzul

  • Regular Member
  • ***
  • Posts: 17
  • Mole Snacks: +1/-0
  • Pharmacy student
Re: Reactions of substituted benzenes
« Reply #1 on: April 29, 2014, 01:25:44 AM »
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.

Offline Archer

  • Chemist
  • Sr. Member
  • *
  • Posts: 1001
  • Mole Snacks: +85/-20
  • Gender: Male
Re: Reactions of substituted benzenes
« Reply #2 on: April 29, 2014, 02:16:16 AM »
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.

 
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Sponsored Links