Sciencemadness Discussion Board

better way to benzyl bromide

menchaca - 5-7-2006 at 07:28

Hi, im interested in the better way to obtain benzyl bromide.I , ve read that can be as simple as reflux toluene and bromine in prsence of Mn(IV)oxide is that correct?can somebody help me in temperatures, times of refluxing, etc..?

evil_lurker - 5-7-2006 at 13:06

N-bromosuccinimide is the best brominating agent for toluene, and the best catalyst is sunlight. Why I say NBS is the best is because all you get is the alpha brominated product. For sheer simplicity boiling toluene in the bright midday sunlight and slowly dripping in your bromine is the way to go.

Coupla pointers... be sure to pre-distill your toluene first to eliminate as many metal ions as possible... those metal ions promote nuclear substituted products. When you get ready to do the reaction, simply boil some of the toluene off in the reaction flask which will dry everything out. Be sure to dry your bromine by adding an equal volume of H2SO4 to it as well. Adding in an excess of toluene is not a bad idea also as it will prevent the formation of benzal bromide (you'll end up with some regardless). Keep your condensor wrapped in aluminum foil as well, as this will keep the toluene from reacting when cool (promotes nuclear substitution). Be sure to vent off the HBr formed into water to recycle back into bromine for later (calcium cloride drying tube wouldn't hurt either to keep things nice and dry).

Flip - 5-7-2006 at 20:36

I'll back that up, and NBS is certainly the safest way to go, but reagents cost $$$. If you want to go with aq. Br, sunlight is all that you need. I've seen it done with expensive lamps and such at certain wavelengths, and by chemical catalysis, but the truth is that sunlight is all you really need. If you go the easy way, just make sure to take good care how you handle the bromine and especially how you store it when not in use.

I had never heard that about the foil around the condenser... instead of insulating it though, why not just control the temperature of the liquid in the cooling jacket...?

evil_lurker - 5-7-2006 at 21:33

The reasoning behind insulating the condensor with aluminum foil is not to protect it from thermal irradiation, but to attempt to insulate it from photo irradiation.

menchaca - 7-7-2006 at 09:49

well i thought that bromine could be stored diluted in methylene chloride, then add toluene and reflux. Will methylene chloride interfere in the reaction? or its low boiling point will make reaction not possible??
I would like to make it as simple as possible!!:) thanks!! :)

menchaca - 7-7-2006 at 09:55

ouch!! i forgot it

How expensive is order N-Br-succinimide for Spain I´ve asked my chemsupplier about it and has no idea..and i´ve never ordered a chemical..:(

evil_lurker - 7-7-2006 at 09:59

I think the free radicals formed would brominate the methylene chloride as well as the toluene.

And there is no need to store bromine in methylene chloride anyways. Just get a good glass bottle with PTFE cap and put in the freezer.

Ephoton - 20-12-2007 at 14:30

yes methylene bromide comes to mind when using methylene chloride as a solvent

MagicJigPipe - 20-12-2007 at 15:49

I've never put my bromine in the freezer (only I2). I keep my freezer very cold, around -20*F (-28*C). Wouldn't that freeze the Br2 and cause possible breakage? I mean, even a normal freezer might if it's below -7*C, right? Or does Br2 not expand when it freezes?

syntelman - 20-12-2007 at 16:14

No, solid bromine has a higher density than in liquid form (as do most compounds except water).
If bottled properly bromine doesn't need to be stored in the freezer really, just keep it in a well ventilated area. In order to minimize vaporization just cool it in some ice before use.

Sauron - 20-12-2007 at 16:17

I hope that anyone trying to make benzyl bromide has the good sense to do it in a fume hood. If not, they will be sorry.

Nasty stuff.

garage chemist - 20-12-2007 at 16:40

The toluene + halogen method for production of benzyl bromide and chloride is far from optimal for the laboratory- careful fractionation is needed to get a reasonably pure product, and you can never completely ensure absence of ringchlorinated impurities.

I make my benzyl chloride from benzyl alcohol and concentrated HCl. There is a method on Rhodium- you simply heat BzOH with an excess of fuming HCl under stirring, the BzCl separates as an organic phase. Separate, wash with water, dry, and distill.
This gives very pure BzCl since benzyl alcohol today is made from benzaldehyde in a completely chlorine-free process.
The only impurity in BzCl prepared this way is BzOH, which can be removed by fractionation, or converted to BzCl by heating and bubling in HCl gas.

Siddy - 20-12-2007 at 21:11

But how can Benzyl Alcohol be obtained OTC?

[Edited on 23-12-2007 by Siddy]