In a blatant plug for the Reagent Guide, each Friday I profile a different reagent that is commonly encountered in Org 1/ Org 2.
If you’ve ever had the “pleasure” of working with bromine (Br2), you’ll know that this dense orange liquid is a pain in the butt for two reasons. First of all, it fumes like a bastard. Once you open the bottle, orange fumes start spewing everywhere, and if you haven’t put the bottle deep into the fume hood, you will soon be savoring the unforgettable aroma of Br2 (mixed with HBr) in your nostrils. Secondly, it’s extremely dense (d=3.19) and therefore drops of it tend to fall from whatever you’re using to dispense it with, Jackson Pollock style, leaving little violently fuming orange puddles behind.
In contrast, NBS (N-bromo succinimide) is a gleaming white crystalline solid and easy as pie to work with. But don’t be deceived. It packs a punch. It will do many of the same reactions as bromine – attached to the electron-withdrawing nitrogen of succinimide, the bromine has a partial positive charge and is therefore electrophilic.
There are two major reactions NBS is used for in Org 1/ Org 2: allylic bromination (the most common) and also as a replacement for Br2 in the formation of bromohydrins.
Allylic bromination is the replacement of a hydrogen on a carbon adjacent to a double bond (or aromatic ring, in which case it’s called benzylic bromination). NBS is used as a substitute for Br2 in these cases since Br2 tends to react with double bonds to form dibromides. The advantage of NBS is that it provides a low-level concentration of Br2, and bromination of the double bond doesn’t compete as much.
Once Br2 is formed, the reaction proceeds much like other free-radical halogenation reactions: homolytic cleavage of the Br2 with light or head (initiation), followed by abstraction of the allylic H (propagation step #1) and subsequent reaction of this radical with another equivalent of Br2 to give the desired product. The remaining Br radical then reacts with another equivalent of the hydrocarbon in this chain reaction until the limiting reagent is consumed.
NBS can also serve as a replacement for Br2 in formation of halohydrins.
Recall that alkenes react with Br2 to form “bromonium ions”, which are 3-atom rings with a positive charge on the bromine. Well, NBS will also form bromonium ions with alkenes. When water (or an alcohol) is used as a solvent, it will attack the bromonium ion, resulting in formation of the halohydrin. Note that the stereochemistry is always “trans”.
P.S. You can read about the chemistry of NBS and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF.