Orgo, destroyer

[JDHK]

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In this mechanism, I was thinking the bromine would attach itself to the para position due to the ethyl group being a donating group. Why is it that it's attached to the carbon (shown in attachment). Does Br2 and light always attach to allylic carbon?

 

[ChrisM07]

When you see this: Br2 with light, think free radical halogenation. In this case, ortho/para/meta designations will be irrelevant. The only thing here that can be affected by by free radical halogenation is the ethyl group. Like always, add the Br to the most substituted carbon (unless you reacted Br2 with a peroxide, like ROOR. Then you would add the Br to the LEAST substituted carbon).

Also, don't forget to look for any kind of shifting that would give a better, more stable carbocation. For instance, if you had a propane group attached instead of methyl, the propane would shift to give a more stable carbocation (tertiary) instead of secondary.

 

[JDHK]

Thanks Chris for always answering my questions =). i knew it was free radical halogenation, but that doesn't usually happen in benzene ring i guess?

 

[ChrisM07]

Sure thing!

Not that I know of. I don't think it would be able to form the radical. Maybe someone else can verify, but I'm pretty sure it wouldn't.

 

[hope_to_match]

Nope it wouldn't. When you see Br2 with light (or Cl2 with light). Think free radical halogenation as someone already mentioned 🙂