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Author: Subject: Learning to respect NBS the hard way
jam640
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shocked.gif posted on 24-11-2007 at 10:43
Learning to respect NBS the hard way


Oh boy.
I'm still kinda shaking. :D
I decided to do a reaction I'd wanted to do for a long time today, an alpha-bromination of a ketone using NBS catalyzed by ammonium acetate. I had done this reaction previously on a millimolar scale and decided to scale up as I would need a larger amount of the product. Scaled up to 0.5 moles, approx, which is not huge.

The reaction proceeded as usual. I dissolved the ketone in diethyl ether, added NBS slowly followed by 10mol% ammonium acetate which heated up the reaction mixture to very slow reflux and caused apparent formation of bromine (ie deep red color). The reaction ran as usual for 2-3 hours, occasionally heated just a slight to get it going. Then I decided that I had to stop for the day, even though the reaction wasn't completely finished as it still was deep red. So turned of heating, and left it to cool.

Go to my computer, surf the net and BLAM. Now the reaction mixture is no longer in the flask but in my entire "laboratory". I was scared shitless. The entire room was fuming, and alpha-halogenated ketones aren't exactly pleasing to eyes, head and throat.

It was terrible, to say the least. I had to run in there, shut everything off. Coughed my lungs out, got out and went outside to get some air and think about what to do. The reaction mixture was everywhere, roof, floor, walls. But I in several consecutive "run-ins" with outdoor air inbetween managed to get everything in control. Still reeks and stinks everywhere but it's pretty much managed now. I was seconds from evacuating the premisses and calling the fire department.

Days like these you are glad that you are alive. And this was _definately_ the last last last time I _ever_ did some chemistry outside a correctly equipped laboratory with fume hoods. Man, this stinks. And me who really needed the product. :D
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[*] posted on 24-11-2007 at 10:45


The funny thing is that the reaction stampeded after several hours, after I had turned of the heating.
Strange stuff. I had a long reflux condenser attached, and it was really really stampeding as the stopcocks (3 necked RBF) blew right across the room and smashed. Oh boy.
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[*] posted on 24-11-2007 at 11:35


Maybe an unstable intermediate builds up at room temp. that normally breaks down as it forms at reflux temp.?
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[*] posted on 24-11-2007 at 12:09


I'm getting a bit paranoid about the health effects.
I inhaled some, of course.
It was butyrophenone that was brominated.
Does anybody know toxicity of alpha-bromobutyrophenone?
Could not find any MSDS or toxicological data. I feel perfectly fine, almost, but I'm shivering and getting abit paranoid.
Should I worry?
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[*] posted on 24-11-2007 at 13:11


I would really really appreciate an prompt reply if anyone has knowledge of the compound in question or similar compounds. I'm getting a little freaked out.
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[*] posted on 24-11-2007 at 13:22


Probably no problem with acute toxicity, maybe some chemical pneumonitis in a few days. Any medical treatment would just be supportive. I had chemical pneumonitis from chlorine exposure long ago. It was like a bad chest cold for about two weeks.

Drink lots of fluids.
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[*] posted on 24-11-2007 at 13:31


since i just found about about the hazerdous reactions of TCCA a few weeks ago
i'll join the disaster club.

if your still alive the toxicity of your inhalation is not going to kill you. why? elemental bromine is toxic but the metabolic breakdown of its ions is really not.

http://www.epa.gov/chemrtk/pubs/summaries/ethanbrm/c14185tp....

this link show that methyl bromide breaks down into methanol and hydrobromic acid
but ethyl bromide breaks down into bromide ions and ethyl alcohol.

so metabolism of your product will ...not kill you


i guess in addtion to water vitamin E will catch possible free radicals

[Edited on 24-11-2007 by roamingnome]
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[*] posted on 24-11-2007 at 13:41


Thank you, I'm calmer now. :)
I really appreciate your posts.
You just get so paranoid and panicy when stuff like this happen, so you most likely also get some placebo effects.
My thorat is sore and my eyes still hurt a bit but other than that I feel pretty fine. Thanks again!
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[*] posted on 24-11-2007 at 17:58


Your title fascinated me. I was very eager to find out why you respected the NBS, National Bureau of Standards, now metamorphosed into NSIT. But they don't give away freebies like they used to...

And what do I find? Another undefined abbreviation. What the hell was this NBS? it wasn't on my list of "standard" chemical abbreviations. Perhaps it stood for the Nebraskan Bullshitters Society? I turned Wiki loose on the job. Now part of Wiki has been written by the Nebraskan Bullshitters Society, but the rest is around 87.98% reliable. I found

http://en.wikipedia.org/wiki/N-Bromosuccinimide

so I guess that's what you were talking about and where the bromine came from...

Regards, Der Alte
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[*] posted on 24-11-2007 at 19:47


N-bromosuccinimide usually is used in chlorinated hydrocarbons as solvent, e.g., CCl4, in which it is soluble, but succinic acid is not, so you follow the progress by the floating succinic acid.

What a-bromoketone were you making?

Quite foolish to do so outside of a hood, you know. Had this happened inside of a hood with sash down you would have had little problem. Take a lesson.

I am at a loss to explain the incident. Your mixture did not explode but violently exited flask through reflux condenser?

Did you have stirring going and turned it off when you killed the heating? Any precipitate settle out or was this all one phase?




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[*] posted on 25-11-2007 at 00:07


It was butyrophenone.
From an reference (I can look it up later) which said diethyl ether worked bested. Ammonium acetate catalyzes the reaction by producing bromine.

And I know that it was quite foolish. I will never do the same again. I am happy if I'll get out of this without any permanent damage. However I never thought that it would react so violently, and I would transfer the solution via canula, i.e never exposing it to the open.

The reaction was in a thre necked flask, with a reflux condenser. It exited through all three necks. Stirring was on, and it had a precipitate (probably succinimide). I hadn't had the heating on the whole time, only from time to time with careful look at the temperature and ready with some ice. I have no idea what caused this. One thought was that bromine built up but did not react with the ketone, until the very end when so much bromine was present that it exploded. I don't know. Obviously a reaction that doesn't scale up well.

But I'm still alive. The night was a nightmare tho. Very hard sleeping with coughing and teary eyes. My "laboratory" is in connection with my living quarters so it stank up the place (and it still smell some). I am still shaking. And yes, I have lerned my lesson. I will never condone any experiment outside a real laboratory again, as I said.

My vision is a bit blurred. Do you guys think that I should seek medical attention? Or will this pass? I guess it is a common "side effect" of tear gas, so I don't know.
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[*] posted on 25-11-2007 at 00:22


By all means see an opthalmic surgeon as soon as possible.

I do not think NBS bromination of a ketone needs catalysis at all. And bromine is happy to fall off of succinimide, that is why it is useful as a brominating agent.

Where was that reference you found that told you to use ether?




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[*] posted on 25-11-2007 at 04:35


Quote:
Originally posted by roamingnome
since i just found about about the hazerdous reactions of TCCA a few weeks ago
i'll join the disaster club.




@roamingnome
I think I should join the disaster club, too, also because of an experiment using TCCA turning nasty. I was trying to oxidise/chlorinate ethanol with TCCA (to get chloro-acetaldehydes with chloral as the ultimate target) and I now think that I made ethyl hypochlorite instead, which "decomposes in the cold and explodes on exposure to sunlight or heat" . I can vouch for the correctness of this statement!
What was your experiment?
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[*] posted on 25-11-2007 at 10:35


Dear jam640,

I don't really know where do you belong to but I don't really agree either with the chaos that you have generated around yourself. I am research scholar persuinig Ph.D. in India. I have done lots of Halogenations (I hate them) and like reactions with hezardous chemicals. There has not been a single week during past two years when one of my reactions haven't blown up. Though my ratio of accedents is higher then the total of my fellow researchers (they are 22), I don't ever penic. Chemistry is all about taking risks. I have never consulted any MSDS till date for my reactions. And still I have managed to keep my guide the happiest person in my group with my progress. Don't worry after the things that have happened. Having said that, go to Doctor if you are not at ease, but don't feel foolish. We (and no body else in the world) don't know the chemistry 100%. We are learning and such accidents (which I would say are experiments with unexpected results) do happen when we explore new fields. Don't get me wrong, as i do not endorse irresponsible behavior in lab (which is what i usually do), but if you feel guilty, then think of me, as I must be the biggest looser having done almost 100 accidents. But I am not, neither are you. Instead of worryng about the accident ant peniking, go to docor and finish the matter. Then you will be able to divert your attention more towards exploring new ideas.

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[*] posted on 25-11-2007 at 14:38


I have never used NBS but its use was covered in my organic course lectures and theory texts. So I looked this use up. My notes said it is particularly useful for brominating allylic alpha positions (without touching the double bond) and the benzylic position (without adding to the ring).

Edit: Also said was that its formation of Br2 is slow and steady.

Edit: The reaction must be light catalyzed, or a radical initiator used.

Of course, this is just general information.

[Edited on by Magpie]

[Edited on by Magpie]




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[*] posted on 25-11-2007 at 19:57


That's a very narrow explication of NBS use.

It is also very useful for making alpha-bromo carboxylic acids, by the bromination of the acyl chloride. Such a-bromoacids are useful intermediates in the preparation of a-aminoacids, by treating the bromo acid with conc NH4OH.

Neither hv nor a radical initiator are required. In fact those conditions would favor ring bromination if your substrate was an aromatic one.

[Edited on 26-11-2007 by Sauron]




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[*] posted on 25-11-2007 at 20:19


Dear mitesh

Yes, chemistry involves risks, but we manage our risks by taking appropriate and adequate precautions in advance. For example, I bet you do your preps in a fume hood, wear gloves and goggles or a face plate or maybe even a SCBA depending on what you are using. You would not prepare a half molar scale of a known military grade lachrymator's homolog on an open bench, would you?

Skipping MSDS is fine as they are so often full of nonsense, and everything useful in them you ought to know already.

If you have survived 100 "explosions" (which surely were merely deflegrations at worst, as opposed to high order detonations) then I deduce that you WERE prepared, and forearmed with adequate countermeasures.

What happened to the thread author would have been a minor annoyance in a fume hood, don't you agree?

Remember, most of us on this forum are doing our procedures at home in residential neighborhoods. We can't afford to have an "accident" and have the neighbors' kids crying because their eyes are burning. Can't afford to have the fire dept, hazmat teams and cops out. So damned right we MANAGE our risk taking. We had better, or else we'd be in deep trouble.




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[*] posted on 26-11-2007 at 01:45


Yes, I agree with both.
Accidents and chemistry go hand in hand, although you should always take the necessary precautions and I tend to read MSDS whenever available (and I feel the need to, as with new potential dangerous compounds).

The mistake I did was not to think of a worst case scenario. I knew the product was most likely very nasty, however I thought that it would stay in the reaction flask and that I could transfer the solvent after the reaction was finished using a syringe i.e never exposing it to the open. However as we all know that it not what happened, and I should have anticipated what to do IF what happened happened which would have meant that I would never have done the reaction. I was foolish, as we all are some times. I am just very glad that noone seems to have gotten hurt. If someone I know was in my house and got ill from this I do not know what I would have done. It was very careless of me to do this, and I very much regret it. As said chemistry and accidents tend to go hand in hand, but you should always take necessary precautions and sometimes (like this) it means that you have to have the necessary equipment to do a certain reaction. As Sauron says, if this had been done properly in a fume hood there would not have been much of a problem involved.

My healt is better now. I had very sore eyes yesterday, and somewhat blurred vision. Could not see a doctor as it was a sunday and today my vision is back to normal. My throat is soar tho, and breathing is somewhat heavy. Kinda feels like a tough cold. Hopefully it will pass soon.

My living quarters still stink, so I've moved out to a friends house. I will get some "odor cleaners" later today to see if it works. I am very happy to recieve some tips on removing the smell. It seems that the lachramatory effect is gone, and I recognize the smell mostly as unreacted starting material, i.e butyrophenone.

Sauron: The reference can be found here http://dx.doi.org/10.1002/chin.200423054
There is also another article that uses diethyl ether and UV-radiation here http://dx.doi.org/10.1016/j.tetlet.2006.12.101

I am aware that NBS often is told to work perfectly without catalysis, however when I researched this particular reaction (i.e alpha-bromination of ketones) all references I could ifnd involved some kind of catalysis. And as I had some ammonium acetate lying around I figured to give it a shot. I Still have a hard time figuring out what really happened, but my best guess would be that bromine concentration was building up over the course of hours and then when it finally reached a high enough concentration it all reacted at once.

[Edited on 26-11-2007 by jam640]
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[*] posted on 26-11-2007 at 02:18


As a hypothesis it's as good as any, if it stands scrutiny. For example, it only makes sense if the supposedly lagged reaction of Br 2 and the ketone + catalyst was exothermic enough to VERY rapidly heat the solvent to the point where it went from very mild reflux (and heat already turned off, so maybe no boiling at all) to violent egress in almost no time at all. Frankly, this does not strike me as likely. Halogenations do not happen that way.

BTW you told us a half molar basis so you had half a mol of the ketone and half a mol NBS and 0.05 mol NH4OAc, but just how much Et2O did you have in there as solvent (for immediate purposes read: "heat sink").

Was the top of the reflux condenser perhaps closed with a dryug tube? Or was it open?

I don't recall what butyrophenone smells like, but if it smells like acetophenone at all, it is not unpleasant. Maybe just opening all the windows and letting some breeze get through there for a day or two will bring the smell down to a dull roar.

Ever have pneumonia? If so then you will know what pulmonary edema feels like. Try to take a deep breath, if it hurts to do so you have fluid in your lungs (pilmonary edema). Your sore (not soar) throat and upper lungs got seared with that bromine and bromoketone and now that the irritant has been removed and vice versa, your body will eventually reabsorb the fluid and the worst you will have is some scar tissue in your lungs and airway. See a nose & throat specialist and have your airway and lungs checked out. And do see an opthamologist in case you have any corneal damage.

I've had pneumonia, and much later chemically induced pulmonary edema (from nitrosyl chloride) so I am speaking from my own experience. Not fun.




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[*] posted on 26-11-2007 at 02:59


Yes, I don't know. It just strikes me as very odd that the reaction had been going with slight heating for several hours, and then suddenly react so violently. It feels like some intermediate/reactant must have collected up, to react all at once. Also the solution darkened over time, to become very dark red at the end. After the explosion there "reaction mixture" was light yellow. When I turned of the heating and left the lab there was no reflux or boiling at all, but the reaction flask was still warm. The amount of diethyl ether was slightly over 300 mL and it was a 1L RBF. Maybe too little? The reflux condenser was "semi-closed" by a septum with a needle stuck through to relieve pressure.

The smell is not very unpleasant, but it smells "chemically" and it doesn't feel very good staying in it. :D A friend was over to check out the situation yesterday and he thought that the smell was reminiscent of some chemical solvent/cleaning agent. All windows are open as we speak to let it out. Hopefully it will be gone in some days. I am also going ot buy some "odour removers" to see if that helps (but I doubt it).

Never had pneumonia, but taking deep breaths hurts (but just slightly) so I think you're right about the pulmonary edema. I feel pretty ok, but it feels very uncomfortable when going up the stairs fast etc. I will probably go see a doctor later this week and see what they have to say. But at the moment I'm more worried about sanitizing my house. :o :D

And again, thank you everyone for your replies. They have been much appreciated and helpful!
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[*] posted on 26-11-2007 at 03:32


Aha.

I think that closing the reflux condenser was a Very Bad Idea. I bet you did pressurize the system enough so that the ether was somewhat superheated, I bet at the end the needle/septum let go (or maybe one of the glass stoppers that you presumably had in the other two necks) and the sudden depressurization and reduced bp resulted in the violent egress of the flask contents.

Also I think 300 ml ether was indeed too little solvent, was that a direct scaleup from the lit ref you cited? I get they operated on a 10 mmol scale or something (typical these days).

For the Br2 to build up like that would imply that the halogenation was not proceeding (hardly) at all. Which does not speak well for the alleged catalyst. Your ref is from Cheminform, I wonder, is that journal peer reviewed? Or is it like Tet.Lett. these days, full of guano?

My remarks assume that you were not using an excess of NBS, which would account for the dark red color even if the reaction was proceeding apace.

NBS is not magic. It is just a convenient easily handled source of Br2, period. This reaction would work without the ammonium acetate and with Br2 itself.

I'd stick to same scale but use 2X-3X solvent, preferably THF instead of Et2O and put this in a 3L or 5 L flask in case of sudden boilup. Do not restrict the reflux condenser, instead connect it to another condenser set for downward distillation and connect the receiving flask to that with a vacuum adapter closed to the air with a mineral oil bubbler. See? Not a closed system but air tight, no way to pressurize, and at worset if you get a vigorous boilup you will catch it all in the receiver (which ought to be large enough.) You may lose some mineral oil but who cares?




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[*] posted on 26-11-2007 at 04:07


NBS is used in both ionic and free radical brominations. This is most likely of the ionic type, where the point of using NBS is to provide a low concentration of free bromine in the reaction mixture. Because I'm lazy I shall write our substrate having a hydrogen atom to be substituted as (S)-H, thus our product is (S)-Br and to show the nitrogen and bromine atoms we write N-bromosuccinimide as (R)N-Br, then succinimide that is formed in the reaction is (R)N-H. The reaction proceeds as follows:

1. (R)N-Br + HBr --> (R)N-H + Br2
2. Br2 + (S)-H --> (S)-Br + HBr

First NBS reacts with some HBr in the reaction mixture to give free bromine. Where the HBr comes from is not that important, but it may come from impurities in NBS or be a product of a free-radical side reaction. Bromine then reacts with your substrate in a reaction that gives HBr, which liberates more bromine. This way the concentration of Br2/HBr in the reaction mixture stays pretty much the same, which improves selectivity. They say crude NBS still having a bit bromine in it works very well in practice.

The free radical way is similar, but bromine and alkyl readicals are involved in the mechanism. Now I'll use * to mark the lone electron, so bromine radical would be Br*, our substrate alkyl radical (S)*. First the reaction is initiated to give bromine radicals, then the reaction proceeds as follows:

1. Br* + (S)-H --> (S)* + HBr
2. (R)N-Br + HBr --> (R)N-H + Br2
3. (S)* + Br2 --> (S)-Br + Br*

Now let's focus on ketone bromination. In the ionic reaction between a ketone and bromine it is the enol form of the ketone that reacts with bromine, giving the brominate ketone and HBr. Now, the interconversion of the keto and enol tautomers is catalyzed by acids and bases, so the reaction is very slow in neutral conditions. But as the reaction slowly proceeds some HBr is released, which further increases the reaction rate, which is why the reaction is autocatalytic. If you simply mix enough bromine with acetone and wait, you should get a lachrymatory shower after an induction period.

Traditionally you add the halogen slowly, the induction period is lowered by adding some acid to the reaction mixture or irradiating with UV light (free-radical halogenation releases the acid). Further the reaction might be performed in the precense of solid calcium carbonate that neutralizes the acid, so that you don't get a runaway. I'm not completely certain if the calcium carbonate is also used to catalyze the enol formation or if the method relies in a free-radical mechanism.

Anyway, I don't have access to the article so I'm not sure how the reaction is performed there. My guess would be that somehow your NBS decomposed so that bromine accumulated, unreacted NBS keeping the acidity of the reaction mixture low thus hindering the reaction. Then at some point all the NBS had somehow decomposed, acidity began to rise and it was time for runaway bromination. Why do they use ammonium acetate, I do not know. You could argue that it would catalyze enol formation but then again I don't think ammonium acetate dissolves in ether at all. It might also have some other sort of effect, formation of acetyl hypobromite (AcOBr) perhaps? Or should it be used as an aqueous solution, so that you had two phases? Good stirring would be necessary anyway as your reaction mixture would not be homogeous. Anyway, doing this reaction with NBS without free-radical initiation sounds a bit fishy, but I have no experience. Personally I would likely add hydrogen peroxide to a vigorously stirred two-phase mixture of the ketone in DCM and an aqueous solution of HBr or H2SO4/NaBr or whatever. The rate of addition can be judged by the color of the reaction mixture.

By the way, have you read the article? Do you know the theory behind what you are doing, how to perform the reaction and why? Lack of understanding is the foremost reason behind lab accidents, and even though science involves unknown factors, the advice our PhD student is giving sounds alarming. Do not listen to him. Get to know your stuff and then do it well and don't be stupid. A day in the library saves a week in the lab, if not more.

Then about cleaning up the mess. To neutralize halogenated ketones I would try washing with a base, depending on the place you want to clean. Sodium carbonate, ammonia, sodium hydroxide should all react with the haloketone so that you'd get rid of the worst. Maybe use some fabric softener as a PTC in your cleaning mixture.

Edit: I agree with Sauron about the use of septum and a needle here, but there are a few misconceptions that need to be sorted out.
- in the reaction NBS ends up as succinimide, not succinic acid.
- initiation and catalysts depend on mechanism of the reaction that is being performed, NBS is merely a reagent
- free-radical halogenation is not used for ring bromination, which proceeds through electrophilic substitution, an ionic process

Good luck, but do not rely on it alone!

[Edited on 11/26/2007 by trilobite]
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[*] posted on 26-11-2007 at 04:45


Sauron: No, it was not a direct scale-up of solvents but an apparently somewhat optimistic estimation. The system was constantly pressure relieved by opening one of the necks, and had been so a minute before the explosive reaction took place. But in retrospect it would have been much wiser to use a system such as you explained. I have no idea on the validity of ChemInform but the article was reasonable well written and similar reactions have been reported before.

trilobite: Thank you for the most informative reply. Your theory sounds very plausible. I have read the article numerous times, and studied other similar systems. I have a degree in chemistry (although my mind isn't what it should be these days) and I have performed some halogenations before and am aware of the mechanistic aspects. Sometimes you just don't spend enough time in the "library" as you should tho. The article writers argues that ammonium acetate works by reacting with NBS to form Br2 and N2. Mixing NBS and ammonium acetate in 1:1 ratio apparently gave a complex that liberated N2/Br2 upon dissolving in diethyl ether (and some other solvents, I don't have the ref in front of me currently and I'm a bit stressed at the moment).
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[*] posted on 26-11-2007 at 05:21


That doi to Cheminform was just a citation of the article. The article is Chemical Communications, 4, 470-471 (2004)

Fortunately I have RSC access.

So abide a few minutes and I will return with the full text.

Here you go. First off what it says was that Et2O was solvent for cyclic ketones while CCl4 was preferred for acyclic ketones such as your butyophenone. Just as I told you. CCl4 at relux. I do not believe you would have had this problem in this solvent. Especially if you have used sufficient solvent.

If you could not get CCl4 or chloroform then DCM would have been prudent. They list about 10 solvents with reaction times. CCl4 and Et2O were 30 minutes so why were you running yours so long?

I quote from p 470: bottom of left column

"Reactions in Et2O required cooling with water to avoid sudden boiling of the solvent."

You were heating a reaction that required cooling in that solvent and you were using the wrong solvent for the type of ketone you were brominating.

What happened was SPECIFICALLY cautioned about.

The authors were quite right.

The case is solved. Chemist error.

Did you rely only on the 1 page graphical citation in Cheminform, and not bother to obtain the two page Chem.Comm. paper?

Cheminform did not include the warning.

[Edited on 26-11-2007 by Sauron]

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trilobite
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[*] posted on 26-11-2007 at 05:26


You are welcome. I think NBr3 is formed, a dark red explosive substance, it decomposes to Br2 and N2.

6(R)N-Br + 2NH4OAc --> 6(R)N-H + 2NBr3 + 2HOAc --> 6(R)N-H + 3Br2 + N2 + 2HOAc

Maybe it was NBr3 behind all this? I'm not certain. I suppose that the acetic acid would have some effect on the keto-enol kinetics, but it is only a weak acid, so maybe not. Sounds like a method you might not want to scale up.

[Edited on 11/26/2007 by trilobite]
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