Selective Allylic Bromination

[shoehornlettuce]

Hey, I'm just wondering if somebody could help me bridge a few concepts. I'm a bit confused as to why NBS is needed for bromination of allylic carbons. I understand that Bromine gas alone will form a trans dibromine via a bridged halonium ion but when the reaction is performed under sufficient energy conditions, wouldn't the bromine radicals abstract a hydrogen from the allylic carbon forming an allylic radical? The allylic radical would be more stable than even a tertiary radical due to resonance so what am I missing? Any help would be appreciated, thanks.

---

Members don't see this ad.

 

[MedPR]

I thought NBS was just an alternative to using Bromine gas. Where did you read that it was necessary?

 

[shoehornlettuce]

I thought NBS was just an alternative to using Bromine gas. Where did you read that it was necessary?

I was going over my textbook and they specifically introduce NBS for allylic bromination. If that's the case I guess I just assumed that NBS was needed for it cause all the examples in my book use NBS for alkenes and Br2 for alkanes.

 

[MedPR]

I was going over my textbook and they specifically introduce NBS for allylic bromination. If that's the case I guess I just assumed that NBS was needed for it cause all the examples in my book use NBS for alkenes and Br2 for alkanes.

Actually, that's how my class was too. We covered radical reactions first (Br2, Cl2) and then we went into Alkenes and that's where NBS was introduced. My teacher told us that NBS (in alcohol, right?) followed the same mechanism as Br2, but I took it at a CC (retake) so maybe there is something special about NBS that I wasn't taught.

Someone else will have a more definitive answer for you eventually. I'm interested now as well.

 

[shoehornlettuce]

Yeah, the mechanisms for both are almost identical aside from bromine gas being formed from NBS for propagation vs. bromine gas being already present. Hopefully somebody else can chime in. Thanks for the response though.

 

[MT Headed]

I'm shooting from the hip here, but I was pretty sure NBS was handy when you only wanted a little bromine released at a time. Using genuine Br2 could release a lot of radicals all at once and lead to unwanted side reactions. But i'm recalling this from a year ago. Does wiki say anything about the benefits of NBS vs bromine?

 

[MedPR]

I'm shooting from the hip here, but I was pretty sure NBS was handy when you only wanted a little bromine released at a time. Using genuine Br2 could release a lot of radicals all at once and lead to unwanted side reactions. But i'm recalling this from a year ago. Does wiki say anything about the benefits of NBS vs bromine?

Not sure, but here's a discussion on this very question from SDN circa 2007.

http://forums.studentdoctor.net/showthread.php?t=405313

 

[shoehornlettuce]

In case anybody cares, from what I've been able to gather it seems they both do the same thing except that NBS produces less Br2 so you get more of the desired product (assuming allylic bromination is what you are going for). Apparently using Br2 on an alkene under proper heat conditions to generate the Br radical will result in two products. You get the desired bromination of the allylic carbon but some of the Br2 will also react with the double bond to form a dibromo product. Obviously this isn't a problem in alkanes because there is no double bond for the Br2 to react with. In summation, it's more efficient. This makes sense but if I'm wrong please correct me and thanks again for all the help.