A Dibromination Dibromance Part II

Another interesting aspect to the dibromination reaction is that three different methods have been developed to perform the same reaction in the undergraduate laboratory. Of course, many more methods of dibromination exist in the chemical literature.

Some laboratory manuals, such a Nimitz, still propose the dibromination reaction using diatomic bromine in a halogenated solvent such as dichloromethane. As far as I can tell, this dates back to a 1942 Journal of the American Chemical society (JACS) article by Marie Reimer: Preparation of Phenylpropiolic Acid. Ernst Berliner also did a series of articles on bromination reactions using diatomic bromine in acetic acid. Using diatomic bromine, except in very small quantities, in the undergraduate laboratory does not seem advisable for safety  and environmental reasons.

Using pyridinium bromide (pyridine hydrobromide perbromide) as a reagent to deliver diatomic bromine has also been around from quite some time. I found a 1948 article by Djerassi and Scholz in JACS: Brominations with Pyridine Hydrobromide Perbromide. Glacial acetic acid is often used as a solvent for this reagent. Though it may be superior to diatomic bromine, there are significant safety and environmental concerns associated with this reagent as well.

More recently, the in situ formation of bromine from hydrobromic acid and hydrogen peroxide has been proposed as a bromination method: Simple and practical halogenation of arenes, alkenes and alkynes with hydrohalic acid/H2O2 (or TBHP). This safer and more environmentally friendly method was picked up by the “green chemistry” folks at University of Oregon. I’m sure that this is a cool story – how an obscure reaction in a low-impact-factor journal came to be vaulted into undergraduate chemistry laboratory stardom. The Oregon group has focused on trans-stilbene as a starting material. However, any number of alkene starting materials can employed. From personal experience, I can say that the trans-stilbene product produces a pile of pure white crystals straight from the reaction mixture. The content of this post is explained in detail in the Journal of Chemical Education article entitled: The Evolution of a Green Chemistry Laboratory Experiment: Greener Brominations of Stilbene.



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