bromination of alkenes

[Oh_Gee]

From GS-6 BS, how come the answer says there is a carbocation intermediate when there are no such things during bromination of alkenes according to here

http://www.masterorganicchemistry.com/2013/03/06/bromination-of-alkenes-how-does-it-work/

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[justadream]

I think this is because you are looking at a conjugated diene addition. There are carbocations involved:

http://chemwiki.ucdavis.edu/Organic_Chemistry/Conjugation/Electrophilic_Attack_on_Conjugated_Dienes

 

[Oh_Gee]

I think this is because you are looking at a conjugated diene addition. There are carbocations involved:

http://chemwiki.ucdavis.edu/Organic_Chemistry/Conjugation/Electrophilic_Attack_on_Conjugated_Dienes

so if there was just one alkene, there would be no carbocation intermediate according to my original link, but in TBR OChem I pg 43, it shows there is but that is with alkene+ HBr whereas in the link, the rxn is alkene + Br2? can you please elaborate

 

[justadream]

so if there was just one alkene, there would be no carbocation intermediate?

My alkene chemistry is not that great since I didn't study it (it's not on the MCAT).

The reaction you pulled up (addition of two Br molecules) appears NOT to involve carbocations. However, note that the conjugated diene reactions (the one I showed you) is an addition of ONE hydrogen and ONE halogen (not TWO halogens).

If you want to do a similar reaction with just a single alkene (that is, adding H and a Br), you can have mechanisms involving carbocations. For example, see: http://www.masterorganicchemistry.com/reaction-guide/addition-of-hbr-to-alkenes/

I would just focus on the conjugated diene reactions as these appear to be MCAT-testable whereas the other stuff is not.

 

[Oh_Gee]

My alkene chemistry is not that great since I didn't study it (it's not on the MCAT).

The reaction you pulled up (addition of two Br molecules) appears NOT to involve carbocations. However, note that the conjugated diene reactions (the one I showed you) is an addition of ONE hydrogen and ONE halogen (not TWO halogens).

If you want to do a similar reaction with just a single alkene (that is, adding H and a Br), you can have mechanisms involving carbocations. For example, see: http://www.masterorganicchemistry.com/reaction-guide/addition-of-hbr-to-alkenes/

I would just focus on the conjugated diene reactions as these appear to be MCAT-testable whereas the other stuff is not.

oh wow totes forgot to check the BS topics list i downloaded. i can't find alkenes on it so I guess you're right but i don't see conjugated dienes either...

DAMN YOU GS FOR WASTING MY TIME

 

[justadream]

oh wow totes forgot to check the BS topics list i downloaded. i can't find alkenes on it so I guess you're right but i don't see conjugated dienes either...

DAMN YOU GS FOR WASTING MY TIME

lol maybe you missed it but I said above that you should still focus on conjugated dienes. I think these are testable (even though they involve alkenes). I'd just know it to be sure. I know it is in TBR OChem and the TBR people didn't keep that section in there for nothing.