Preventing a bromination reaction from going radical

[BlackJack21]

What steps would you take to ensure a bromination occurs slowly and doesn't proceed as a radical reaction. I have tried performing the reaction at 0 degrees celcius and adding the bromine (mixed in with a solvent) slowly drop by drop. But I think its still not working right. My instructor suggested I maybe cover the reaction vessel with tin foil to keep the UV light light out. I have a few days before I can get back to the lab so I wanted to see what anyone else thinks.

[elementcollector1]

As a general rule, reactions are slower when colder. Try placing the beaker in ice water.

[BlackJack21]

I'm just learning about halogenation and radical reactions and don't really understand how selective monobromination is done practically.

[hypervalent_iodine]

Keeping it covered should definitely help and as elementcollector said, temperature will also affect it. Try just doing it in the dark and seeing what you get with GCMS and NMR. Also, 'thinking it hasn't worked' isn't a very useful analysis unless you can justify it. Have you run any NMR on it?

[mississippichem]

Keeping it covered should definitely help and as elementcollector said, temperature will also affect it. Try just doing it in the dark and seeing what you get with GCMS and NMR. Also, 'thinking it hasn't worked' isn't a very useful analysis unless you can justify it. Have you run any NMR on it?

 

Without any characterization data, it may have worked and you wouldn't know it.

 

OP: what is the substrate you are trying to brominate? What is the solvent etc...

[Anders Hoveland]

Actually, even blue-green light has enough energy to initiate bromine radical reactions. You might consider working in a darkened room with red light. Or use incandescent light (normal light bulb) rather than fluorescent lighting.

 

It is a radical cascade mechanism, so another strategy is to reduce the reaction time. If the reaction can be limited to less than 5 minutes, there should be less of a problem. I do not think temperature is going to make much difference. A single photon of light can catalyse the reaction of about 40 molecules in this type of cascade.

 

Also remember that bromine reacts at room temperature with ketones, especially if the pH is lower, through intermediate enol tautomer. Some chemists have mistakenly used acetone as a solvent for bromine, then were surprised when the brown color disappeared after a few minutes, and lachrymatory (tear gas) bromoacetone vapors were produced.This is a different type of reaction mechanism, bromine will react with acetone even in the dark.

Edited April 20, 2012 by Anders Hoveland