Edward Elric
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Preventing light from screwing with an electrophilic addition reaction
Light is all around us.
So, when someone says that a reaction can take place because light is there, then I can't help but think that the reaction is inevitable even if a
person wants a particular reaction.
Ok, so let's take a basic rxn. This thread is mostly about Markovnikov reactions.
So, let's say I take this: 1-Butene
And I mix it with this: HBr
Let's say I want the the reactants to go through a Markovnikov reaction path. Ideally, I would get 2-Bromobutane.
Free-radical addition (Anti-Mark.) is going to occur regardless, right, because there is light? Thus, I'm going to get 1-Bromobutane?
Therefore, despite my best efforts to remove light from the environment, I'm going to have 1-Bromobutane and 2-Bromobutane as products?
How do scientists go about forcing Bromide into one path? Is there a way to block out light?
This anti-mark. thing only happens with bromide, right?
[Edited on 18-10-2009 by Edward Elric]
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ammonium isocyanate
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Free radical addition, IIRC, would require alot of light, specifically of higher wavelengths, or better yet a radical initiator. Normal light in the
laboratory would probably not cause anti-markovnikov addition to an appreciable extent.
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DJF90
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When a chemist wants antimarkovnikov addition, they use a source of UV and a radical initiator. Without a radical initiator, you almost certainly need
UV, and this can be eliminated by working under red or yellow coloured lamps (these colours are very low in energy with respect to UV), much like a
dark room in a photographic lab.
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UnintentionalChaos
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The addition of HBr should have few to no issues with light. That's just bromine, chlorine, etc. You may simply wrap the flask in aluminum foil to
keep out light.
Furthermore, Markovnikov/antimarkovnikov addition isn't a hard rule. Since the selection is only based on stability of the product and steric issues,
you will get a significant amount of the anti-markovnikov product along with the expected markovnikov product.
This is why fractionating columns exist, why preparative chromatography is used, and sometimes very expensive reagents are used where it might appear
that simpler ones would work. No reaction is without side products.
[Edited on 10-18-09 by UnintentionalChaos]
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'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Sedit
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| Quote: | | How do scientists go about forcing Bromide into one path? |
Radical scavengers and antioxidants have been used to force Markovnikov addition in this reaction IIRC. You seem to suggest that removal of light from
a reaction is a hard thing to do???
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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jon
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it's not that touchy keep it on the freeze sealed up it's dark in the freezer.
the cooler the better.- c i mean
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