Despite the utility, there is an obvious concern over the safety of using magical element #35. It’s nasty stuff, and bromine burns can take quite some time to heal (see: Jap. J. Tox. 2005, 18, 141-147). On top of that, physiologically speaking, there are a silent few who cannot tolerate organobromides (see: Risk Management for Hazardous Chemicals by Jeffrey W. Vincoli). For what it’s worth, I learned that I was a member of this club (mind you, the hard way) my second year of graduate school, but I cannot do my job without them. Then again, I’m deathly allergic to cats and dogs, but I can’t think of life without Miss Piggy or Isabelle (respectively).
The installation of a bromine atom is a task that successful organic chemists must master. Despite the inherent danger of working with pure, elemental bromine, there are alternate ways of installing a Br atom without using the characteristically, brown/red chemical. The obvious solution to an organic chemist is to use NBS or pyridinium bromide perbromide. There’s another way that’s even cooler, is pure alchemy and covered in most general chemistry classes (high school or college). I discovered this beauty during my third year of graduate school and never gave it much thought until I started writing my dissertation. For my purposes, the procedure in question (an old Chem. Ber. paper from the 1950’s), called for dissolution of the starting material in strong, aqueous acid, followed by the slow addition of KBr then KBrO3. After 2 hours of reacting, the solution was then neutralized, which precipitated a solid that was filtered off. Analysis of the resultant solid confirmed the addition of a bromine atom.
The whole equilibrium is very interesting. As confirmed by my dear, Midwestern inorganic Jedi friend, if you add acid to a solution of KBr and KBrO3, the equilibrium shifts toward bromine formation. If, however, you add base (i.e. NaOH), the equilibrium shifts back to the left and you get sodium hypobromite—a powerful oxidant (of course, on the scale of Clorox bleach). While this chemistry has been applied as far back as the early 1900’s (J. Am. Chem. Soc. 1903, 25, 2169-2171), more recent examples have been lightly peppered through the literature (J. Org. Chem. 1981, 46, 2169-2171).
Bonus Material
Everything you’ve ever wanted to know about Br2 can be found here (Courtesy of Great Lakes Chemical Corporation).
Derek Lowe has a great story about how not to brominate like a wizard.
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3 comments:
Looks like a clever technique. In grad school I used a similar solution with KI, KIO4 and HOAc to di-iodinate aromatic rings, though in this case I was essentially generating "I+". Do you see the colour of the bromine in the reaction mixture?
Thank you for your comment Jordan. As you mentioned, acidic solutions of halo-"ates" are a good source of halonium. I find it interesting that you used potassium meta-periodate; it didn't outwardly occur to me that you could tack on an extra oxygen for the same effect. I usually think of KIO4 as a reagent for oxidative cleavage of a 1,2-diol.
Anyhow, to answer your question, I seem to recall the reaction taking on a slightly yellow color. How did it look when you made your iodonium?
It could be that I did use KIO3 and not KIO4. It's been a while.
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