Free Radical rxns - DAT Destroyer O-Chem #8, 2011 Edition

[Applicant12]

I know it's a dumb question, but I'm a little confused about free radical reactions. When something undergoes Br2 and light, doesn't it add anti-markovnikov? Or am I just confusing concepts?

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[spoog74]

I know it's a dumb question, but I'm a little confused about free radical reactions. When something undergoes Br2 and light, doesn't it add anti-markovnikov? Or am I just confusing concepts?

no, its a whole different mechanism for radical reactions. Remember that with non alkenes, the radical halogenation will add to the more substituted alkyl ( forming most stable radical intermediate)...

halogenation will mostly work anti markovnikov when you are using hydroboration ( BH3, H202) or whenever you see a dehydrating reagent with hydrogen peroxide ( ONLY WITH BROMINES for the ROOR reactions) will it undergo anti markovnikov.

Hope that made sense

 

[bharat008]

Just remember that there are only 2 reactions that will go ANTI-MARKONIKOV. They use the reagents shows below:
BH3/THF in H2O2, OH- = Has H2O2
HBr/H2O2 = Has H2O2

Those are the only reactions that go anti-markonikov in ochem (could be more but for DAT purposes that's enough)

Now, if i put HCl in H2O2; it will still go MARKONIKOV.. b/c the 2 rxns mentioned above are the ONLY ones that go ANTI-MARKONIKOV.. REMEMBER!!!!

Now, for radical reactions; there are only 2 of them which include::
Cl2 in hv or H2O2 or light = produces mixture of products (exothermic)
Br2 in hv or H2O2 or light = produces only 1 MOST STABLE product (endothermic)

I2 and F2 do not occur b/c they are highly unstable. F2 is too exothermic so it reacts really FAST.. I2 is slow as hell since its highly endothermic..

Just remember, Cl2 or Br2 in H2O2 = still Cl or Br adds to the most substituted C... Only exception is that Cl2 gives you mixture of products while Br2 gives you only 1 most stable product..

HOpe that made sense..

 

[Applicant12]

Okay, thanks so much! I was getting really confused! You are all lifesavers. That was a lot clearer than I was reading in Kaplan...

 

[spoog74]

Okay, thanks so much! I was getting really confused! You are all lifesavers. That was a lot clearer than I was reading in Kaplan...

dont stress it , Kaplan goes way over board with trying to explain things that are completely unnecessary