NBS (N-Bromo Succinimide) | Chemistry Optional Notes for UPSC PDF Download

N-Bromosuccinimide Is A More Convenient Alternative To Bromine (Br₂)

• 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 a little trail of 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.

NBS As A Reagent For Allylic Bromination

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.

Allylic Bromination With NBS: How It Works

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 As A Reagent For Bromohydrin Formation From Alkenes

• 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“.
  
• There are tons of other uses for NBS beyond what you see in Org 1/Org 2, of course, but those are the basics.