FireLion3
Hazard to Others
Posts: 218
Registered: 11-1-2014
Member Is Offline
Mood: No Mood
|
|
Halogenation of alkyl side chains, selectivity?
What causes some side-chain halogenations to take place adjacent to the aromatic ring, while others take place at the at the second-to-last carbon on
the side chain? I see that the aromatic ring is electron withdrawing which has something to do with this, but what sort of conditions favor
halogenation at the second to last carbon on the side chain? Electrophilic versus Nucleophilic halogenating agents?
|
|
|
Metacelsus
International Hazard
Posts: 2539
Registered: 26-12-2012
Location: Boston, MA
Member Is Offline
Mood: Double, double, toil and trouble
|
|
Benzylic halogenation takes place by a radical mechanism; the benzylic radical is stable due to conjugation. Under what conditions / on what
substrates does halogenation occur at the second to last carbon? It seems to me that benzylic halogenation (if possible) would be preferred.
|
|
|
FireLion3
Hazard to Others
Posts: 218
Registered: 11-1-2014
Member Is Offline
Mood: No Mood
|
|
I read that for instance N-Bromosuccinimide would brominate at a different position than would elemental bromine. Something to do with the fact that
NBS has electrophilic bromine while elemental bromine is nucleophilic. I'm trying to get a good idea of how might an electrophilic agent target a
different carbon than a nucleophilic agent. I have an idea, but I am not really sure.
The carbon adjacent to the aromatic ring will have a lower BDE for its hydrogen since the aromatic ring activates it, making them more nucleophilic,
while hydrogen near the end of the chain will not be activated, making them less nucleophilic? Does this have anything to do with where the
halogenation will take place depending on the nature of the agent?
Edit:
Is there any specific technique to making these activation energies behave in inverse? As in, instead of the carbon-hydrogen adjacent to the aromatic
being the most likely to undergo bond disassociation, make the carbons toward the end of the chain more flexible? I was imagining some sort of lewis
acid might negate some of the nucleophilicity of the aromatic ring?
[Edited on 20-3-2014 by FireLion3]
|
|
|
Nicodem
Super Moderator
Posts: 4230
Registered: 28-12-2004
Member Is Offline
Mood: No Mood
|
|
FireLion3, when you claim extraordinary things like these, you are expected to give the references. What is the point of saying that you read
something, if you don't say where? How is anybody going to answer your questions without you giving the reference to what you talk about? So, please
read the forum guidelines and improve the quality of your posts.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
