Sciencemadness Discussion Board

Preventing light from screwing with an electrophilic addition reaction

Edward Elric - 17-10-2009 at 18:40

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]

ammonium isocyanate - 17-10-2009 at 19:01

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.

DJF90 - 18-10-2009 at 03:17

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.

UnintentionalChaos - 18-10-2009 at 06:25

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]

Sedit - 18-10-2009 at 09:20

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???

jon - 9-11-2009 at 15:39

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