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jam640
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Sorry Sauron, I didn't have SciFinder access when I looked it up earlier today so I just did hasty Google and that was what I found first.
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jam640
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I have attached the full article now.
And it actually gives a caution of the reaction between NIS and ammonium acetate which gave an explosion, possibly accounted for by the formation of
NI3. 
And I think trilobite is right, this reaction should not be done on anything but microscale. 
[Edited on 26-11-2007 by jam640]
[Edited on 26-11-2007 by jam640]
Attachment: nbs.pdf (116kB)
This file has been downloaded 1945 times
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Sauron
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Yes I posted this a few minutes ago upthread.
See bottom of paragraph 5, in left column, p 470
REACTIONS IN ET2O REQUIRE COOLING WITH WATER TOAVOID SUDDEN BOILING OF SOLVENT.
Yet you were heating a reaction that required cooling to hold it to 25 C and you were using Et2O when the authors advised CCl4 at 80 C (reflux) for
acyclic ketones, like butyrophenone.
End of story. Not a scale problem. Chemist error.
The warning was plain. What happened was exactly what they described, sudden boiling of solvent.
Numerous other solvents were described as suitable, merely requiring longer rxn times. In case you could not get CCl4 you had other options but you
went with the ONLY solvent that was prone to sudden boilup.
Bad choice.
The consequences are punishment enough I am sure, and you know better than anyone that things could have been MUCH worse. You could have gotten a
facefull of that lachrymatory fountain jetting from the flask. All that ether COULD have ignited and you'd have had a nasty explosion and fire and the
cops just LOVE ether explosions and are conditioned to treat them all as drug lab accidents.
You could have been blinded, or killed.
So I hardly need to berate you.
But face it squarely. Don't blame the scale. Don't blame the mechanism or NBr3 bogeyman. You messed up, in two ways. Three IMO if you start from
working with ether without a fume hood.
Case closed. Chemist error. Sorry, fella, you upgefucked.
[Edited on 26-11-2007 by Sauron]
Sic gorgeamus a los subjectatus nunc.
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trilobite
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I think the word you are looking for is an exotherm. But as you know, ether itself isn't responsible for exotherms, the reactions are. NBr3 is no
bogeyman, it most likely is the intermediate species in this reaction since nitrogen is evolved. Even the authors seem to recognize this but it might
be something they wish not to shout out loud. They still do note that the same reaction with the analogous N-iodosuccinimide is an explosive hazard
and even make note of the possible formation of NI3 which is notoriously unstable.
Whatever the reason for the runaway, it is a scale problem if there are exotherms that need cooling. The larger your reaction vessel is, the less you
have surface area per mass unit and the harder it is to cool. It might very well be that ether isn't the best solvent to conduct this reaction, no
denying that. I don't know the reason for it since I haven't tried the reaction, but I already provided two explanations for what happened. You could
easily determine which one is it by following the colours and nitrogen evolution during the reaction.
Your frustfration is understood since you don't seem to grasp even the basics that well, but still try very hard. It is easy to dismiss scientific
arguments and go ad hominem when something is too difficult to explain. Surely you must realize that the runaway wouldn't have happened if it wasn't a
result of the actual reaction mechanism and thermodynamics? Sudden boiling of solvent is the real bogeyman explanation here and there is no need to
read the article for that one, it was a runaway after all.
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Sauron
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Trilobite, did you bother to read the paper that the thread author himself said he used as basis for his procedure?
It STATED that this runaway was going to happen just as it DID happen. It INSTRUCTED anyone following this procedure to COOL any reaction mix using
ether as solvent in order to maintain a temperature of 25 C. It TAUGHT that ether was best choice for CYCLIC ketones, while CCl4 was better for
ketones such as the thread author's substrate. At 80 C not 25 C.
So this hapless fellow did what?
He selected the wrong solvent for his type of ketone.
He selected the ONLY solvent out of a long list that required a warning about the necessity of cooling to 25 C in order to avoid sudden boiling of
solvent.
Then he used too little of the wrong solvent
And instead of cooling the reaction as instructed he HRATED it to reflux, say 10 C above the stated reaction temperature.
And he continued heating intermittently for several hours, while the instruction was that this reaction would be completed in this solvent at 25 C in
30 minutes.
So what we have is a chemist who can't seem to follow a simple procedure.
It is no wonder this reaction ran away. BTW if you had bothered to read my posts on previous page you would have seen that I know the word "exotherm"
very well.
I say this fellow screwed the pooch and apparently, in every orifice.
I say that this reaction is very likely to be perfectly safe in competent hands at any scale, if the procedure is adhered to, and proper precautions
are taken. This chappie did this in a residence, refluxing ether that did not need to be refluxed, heating when he was supposed to be cooling.
He is lucky he is alive.
BTW I have been doing organic chemistry for 45 years. If you knew my name you could find it in JOC and C.A. So keep your snide condescension to
yourself. It is uncalled for.
This matter is closed. There is no mystery.
You want to argue thermodynamics, when what happened here was that the thread author skipped, forgot, misunderstood, or ignored avery specific warning
that said "Take this step or you will have a runaway with diethyl ether."
He did NOT take the required, step, he did the worst possible thing and heated what should have been cooled.
End of story.
Still want to argue thermodynamics and mechanisms?
How about basic prudent lab practice and familiarity with the pertinent literature he himself cited?
The runaway does not occur with solvents THF, acetonitrile, CCCl4, CHCl3, DCM, etc etc. ONLY with Et2O. Yet he elected to use Et2).
A self inflicted wound!
And he had better face up to his own arrogance or he will learn nothing from this affair. Nothing.
[Edited on 27-11-2007 by Sauron]
Sic gorgeamus a los subjectatus nunc.
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mitesh
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@ Sauron and jam640
Dear Sir(s),
I do get your point and agree fully with the discussions, however, my point was, why should one feel guilty after having done something wrong (which i
don't consider wrong as yet) unintensionally. I may have made mistake in gathering the situation, in which case my apologies.
As per the precautions, only precaution accessible in my lab is an ill maintained fume hood (so be it!). By accidents, i mean instances when my
reaction flask shattered into pieces, however, as has been pointed out not all of them were explosions, some were mechanical shocks too. But that is
not my point u see!
Only thing i would still stress upon is, when i am working with chemistry, it should be only thing to demand my attention. But i may feel so 'coz i'm
still a student. I may not stick to it while i'll have to set up my lab in my home!
Regards,
M.
Knowledge should be free! If it\'s not, let\'s make it.
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Sauron
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A thorough reading indicates that as I suspected these reactions were performed by the authors at a 10 mmol level. Even then, they cautioned that the
reaction in Et2O needed cooling to hold it to 25 C or sudden boiling of the solvent would occur.
The required amount of solvent in a typical run was 10 ml. A direct scaleup to half a mol would call for 500 ml. The thread author used 300 ml. A
contributing factor in the subsequent excursion from the flask.
The best combination of short rxn time and yield was Et2O FOR CYCLIC KETONES. The best solvent for acyclic ketones was CCl4 @ 80 C.
Which is what I advised as solvent for NBS early in this thread. Succinimide floats on CCl4.
Reaction time in both cases was 30 minutes.
I suspect that the unnecessary prolonged heating was a factor. Heating of ANY kind was contraindicated, as cooling was stated as necessary for Et2O
solvent.
This is, again, a compounded, exacerbated chemist error.
A man drives up a mountain. The speed limit is 35. There's a dangerous curve up ahead. He accelerates to 90 and drives off the cliff.
Do we need a "scientific discussion" of vectors and classical mechanics?
Or do we say "Sumbitch was speeding at 55 mph over the limit." ?? Is that an ad-hominen attack, or simply an obvious conclusion?
[Edited on 27-11-2007 by Sauron]
Sic gorgeamus a los subjectatus nunc.
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jam640
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Sauron: Yes, of course it was my fault. First and foremost because I did not take the necessary safety precautions. However I do not necessarily agree
with you on the other points.
In my book there is a vast difference between an exothermic reaction heating a low boiling point solvent to reflux (which was there reason why I used
a condenser) and a reaction that obviously "sleeps" for several hours and then in a matter of seconds react explosively.
As I stated previously the reaction had been tried out two times earlier on millimolar scale. The first with 50 mmoles in 20 mL of diethyl ether which
was stirred at room temperature for 3 hours. There was no apparent heat generation and the solvent did not start to boil. The yield was miniscule.
The second time I used 30 mL of diethyl ether, and after standing at several hours when the reaction had not gone to completion I decided to gently
heat the mixture to see if the reaction would proceed (although somewhat violently) which it did and yields were very good.
Hence from my experiments I drew the conclusions that under my ambient conditions the reaction needed external activation and when I performed it on a
larger scale I first allowed it to "work it's magic" followed by very gentle heating to reflux for short periods of time (with external cooling always
ready).
However, anyone who actually has tried to repeat a literature synthesis knows that even very small variations can alter the outcome greatly. Heck,
I've witnessed reactions that didn't proceed at all in December but worked flawlessly in July. Most likely because of a difference in ambient
temperature. Chemistry is not black or white.
The article I reffered to does not report bromination of butyrophenone, where the most similar compound is 1-tetralone. A compound which they
brominated in diethyl ether.The acyclic ketones bear very little resemblence to butyrophenone. The other article I refered to reports the bromination
of several acetophenones in diethyl ether. I do not agree that the choise of solvent was extremely unwise. And as trilobite says exothermic reactions
are always risky to scale up.
There is a reason it's called research.
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Sauron
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You maybe want a lecture on the psychology of denial?
You want to maintain that you proceeded prudently and properly and this just happened out of the blue?
Then you haven't learned a thing.
You chose the wrong solvent. Et2O was for cyclic ketones. CCl4 was advised for acyclic ketones.
Et2O was used at 25 C. With CYCLIC ketones. And needed cooling to avoid just the kind of sudden boilup you achieved.
CCl4 was used at reflux. 80 C. That alone ought to have tipped you that acyclic ketones require harsher conditions than cyclic ketones.
You used an inadequate amount of solvent. 40% less than a direct scaleup. That compounded your error.
So, in hindsight, it is PERFECTLY clear that you erred in choosing Et2O, you erred in heating this reaction mix in this solvent, and you erred in
using too little solvent.
Take responsibility for your own mistakes. No one wants to excoriate you for being fallible.
By your own account you got very poor results first time round at 5X the lit. scale (but 60% less solvent than advised) at 25 C for 3 hours. Per the
lit., reaction ought to have completed in 30 min. At that point, you ought to have reread the paper more carefully, and the proper conclusion would
have been to go to CCl4 and 80 C.
You chose to stick with Et2O and got better results but according to your own account, somewhat violent reaction.
Red warning lights and klaxons ought to have been going off.
But instead you charged on and scaled up 10X from there (50X from lit.) and got your runaway.
If you are surprised, well, no one else is, or ought to be.
No, the literature reaction did not exemplify butyrophenone. But it DID acetophenone, the 2-carbon homolog, and did so in CCl4 at 80 C for 30 min and
NO sudden boilup, and with a high yield.
BTW I bet this reaction would run just fine in a pressure vessel at 80 C in Et2O. With butyrophenone, acetophenone or propiophenone.
THF works, requiring 60 minutes, but yield is poorer, and no risk of sudden boilup. Not sure what temp they used with THF.
[---------------------------------------------
If I were wanting to prep this monobromoketone, I would select a method that did not involve ammonium acetate catalysis, to avoid all this hassle.
I would, if possible, work in CCl4.
I would use NBS or Br2. NBS is more convenient but if you have septa and syringe then chilled bromine (to reduce fumes) is not too hard to handle.
I think you might investigate the usual catalysts AIBN and dibenzoyl peroxide. Both have their hazards but you do not need much of them.
Or, return to NH4OAc. In CCl4 you will not have any problems. It lacks the fire hazard of ether, there will be no sudden boilup, the higher bp
precludes any difficulty with reflux using conventional condenser and cooling water.
I would not skimp on amount of solvent, and you really ought to be done in 30 minutes.
[Edited on 28-11-2007 by Sauron]
Sic gorgeamus a los subjectatus nunc.
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Bronstein
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Just be glad you did this experiment with butyrophenone and not with propiophenone. I have experienced both the alpha-bromoketones, and the one from
propiophenone is much much worse. In fact I think alpha-bromobutyrophenone was fairly pleasant to work with. Every time you brominate propiophenone
you swear: "Never again!!".
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Sauron
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How does the a-bromopropiophenone compare to the corresponding acetophenone ("phenacyl bromide") which is a military tear gas? Far worse than the
chloride (CN, older standard police tear gas prior to the advent of CS).
I think a lab prep of a-bromoacetophenone can be found in Sartori's book. (The War Gases) in forum library.
Sic gorgeamus a los subjectatus nunc.
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syntelman
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As elemental bromine readily halogenates acyclic ketones such as butyrophenone in diethyl ether at either 0C or room temperature (keeping the addition
of bromine at a very moderate level) I doubt that the solvent was the major source for concern here. However it is very unwise to heat exothermic
reactions, but hey we all made our fair share of mistakes.
I experimented with NBS to brominate various ketones in DEE some years ago, both without any catalyst and catalyzed by HBr and concentrated sulphuric
acid. Although all attempts failed, bromine release was very evident and increased over time but halogenation didn't go to completion and the yields
were miniscule. I never witnessed any runaways but all attempts were quenched and worked up after a few hours (I do not have my lab notes with me at
the moment so no exact numbers).
My guess is as already said that the problem is the keto-enol tautomerism that with bromine is autocatalytic with the release of HBr. I suspect that
the freed succinimide interacts with any present acid and what we see is an increase in bromine concentration until all NBS is consumed and all
available succinimide is protonated, whereupon any further liberated HBr instead enolizes the ketone and at that moment we have a very high
concentration of bromine leading to a violent chain reaction.
If you decide to re-attempt the reaction I would try using AlCl3 as a catalyst and see if it works. Otherwise bromine is not as hard to work with as
you might suspect, BUT it should never be used outside a fume hood (as with a-haloketones so...).
Take it safe, and I hope your health has returned to normal! 
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Reflux_catalyst
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Quote: Originally posted by mitesh  | Dear jam640,
Chemistry is all about taking risks. I have never consulted any MSDS till date for my reactions.
Regards,
M. |
Don't ever say this again. You have no business propagating this garbage. You will go blind or die one day, and it will be your fault.
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Mailinmypocket
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Just FYI: The last time mitesh was online was back in 2007.
Note to self: Tare the damned flask.
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CuReUS
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this is an alternative reagent for NBS.maybe this accident can be prevented by using this
http://www.sciencemadness.org/talk/viewthread.php?tid=61512#...
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Dennis A
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I came across this while searching for related material hopefully its of some use to somebody and avoids accidents rather than causes them i have felt
pain like the OP from such reactions ... The reaction is carried out on acetophenone but this of coarse can be substituted the above post has nothing
to do with alpha brominations
Can someone tell me what role silica gel has in the reaction ?
They appear to use the NBS upto 3 times after the original reaction so thats some saving of reagents
2.1. Typical experimental procedure
The α-bromination reaction was carried out using acetophenone (1200 mg, 10 mmol), N-bromosuccinimide (2136 mg, 12 mmol), 10% (w/w) silica gel (120
mg) in 10 mL of methanol at reflux conditions until the disappearance of the substrate. (Note: 2136 mg of N-bromosuccinimide was added portion wise
i.e. 356 mg for each time in six portions). The progress of the reaction was monitored by TLC. The reaction mass was filtered after the completion of
the reaction as per TLC and the catalyst was collected for reuse. The filtrate was concentrated under vacuum. Double distilled water was added to the
reaction mixture and quenched with aqueous sodium thiosulfate and the product extracted with dichloromethane (Caution: Severe burning sensation of
eyes was observed during the work-up process). The layers were separated and the organic layer was collected and washed thrice with distilled water (3
× 50 mL). The collected organic layer was dried over anhydrous Na2SO4, filtered and concentrated. The obtained crude product was purified by column
chromatography over silica gel (60-120 mesh) using n-hexane-EtOAc (99:1 ratio).
With the aim of studying the recycling of the catalyst, the isolated catalyst was washed with ethyl acetate (5 mL) after its filtration from the
reaction medium, collected and dried in vacuum at 70 ℃ to a constant weight. Subsequently it was reused for the abromination of acetophenone and
achieved 95%, 86% and 83% yields of product (2a) for first, second and third reuse of catalyst respectively. All products gave spectroscopic data in
agreement with the literature [15, 21, 27, 28, 29, 30].
The method is also very practical for scale up in process development. We attempted large scale (100 gram scale) synthesis of 2-bromo-1-phenylethanone
2a and obtained fruitful results with isolated yields ranging from 93% to 96%.
2.2. General experimental procedure for large scale (100 g scale) preparation
The α-bromination reaction was carried out using acetophenone (100 g, 0.832 mol), N-bromosuccinimide (213.6 g, 1.2 mol), 10% (w/w) silica gel (10 g)
in 1 L of methanol under reflux conditions until the disappearance of the substrate (Note: 213.6 g of N-bromosuccinimide was added portion wise i.e.
35.6 g for each time in six portions). Excess N-bromosuccinimide (+0.1 mol) was added to complete the reaction as per TLC. The reaction mass was
filtered, after the completion of the reaction as per TLC and the catalyst was collected for reuse. The solvent was removed and double distilled water
was added to the reaction mixture and was quenched with aqueous sodium thiosulfate. The product was extracted with dichloromethane. (Caution: Severe
burning sensation of eyes was observed during the work-up process).
The layers were separated and the organic layer was collected and it was washed with distilled water for three times (3 × 1 L). The collected organic
layer was dried over anhydrous Na2SO4 and filtered, concentrated and purified by recrystallization using n-hexane (300 mL). The resulting yield of
pure α-brominated ketone, i.e. 2- bromo-1-phenylethanone (2a) was 94%. The same experiment was repeated twice and the yields were 93% and 96%,
respectively. The reproducibility of the protocol was thus confirmed with the consistent isolated yields of desired product 2-bromo-1-phenylethanone
(2a).
http://html.rhhz.net/zghxkb/20140145.htm
Regards
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Melgar
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I did this with elemental bromine at some point. Added it to acetophenone in some solvent or other (acetic acid IIRC, not sure of the concentration).
The literature I was following stressed adding it slowly, which I did. The red color slowly disappeared at first, but disappeared almost instantly
with later additions. The later additions also heated the mixture considerably. I'm 99% sure that HBr was forming and lowering the pH, which
increased the reaction rate. NBS could help here, since it combines with HBr to release Br2, similar to TCCA and HCl, if I'm not mistaken. Still
would recommend adding it in portions and keeping an eye on temperature. Also, would mandate an effective fume hood. That shit made me weep into a
towel for what seemed like hours, and could have been hours because I couldn't open my eyes enough to check the time.
I don't know why OP used an ammonium salt here, of all things. It just seems to be asking for trouble. Granted, that was 11 years ago, so I hope
he's still alive and well.
The first step in the process of learning something is admitting that you don't know it already.
I'm givin' the spam shields max power at full warp, but they just dinna have the power! We're gonna have to evacuate to new forum software!
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carbon85
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More information on the related topic however this arises questions for me i hope somebody more educated on such topics can elaborate further
I understand the use of acid catalyst,s to help initiate and speed up the reaction P-TsOH/GAA/HBr and many more
Below is a quote from this link >https://www.sciencedirect.com/science/article/pii/S1350417701001080
Substituted acetophenones react with N-bromosuccinimide (NBS) and p-toluenesulfonic acid (p-TsOH) in the presence of ultrasound in methanol at
35±2°C to give α-bromoacetophenones in high yield. In the absence of ultrasound the reaction takes place at the boiling point of methanol (65°C)
and takes longer time. The reaction does not take place in the absence of p-TsOH thermally or sonically. However the reaction is possible under
photochemical conditions in the absence of p-TsOH. The best solvent for the reaction was found to be methanol
Its stated they could only get a reaction to take place without the aid of photochemical and ultrasound conditions when the reaction was heated to the
BP of MeOH not sure how forgiving that would be to certain substituted ketones
I quote " The reaction does not take place in the absence of p-TsOH thermally or sonically "
Yet in my above post they use no acid catalyst with high yield instead using silica gel can anybody enlighten me as to why ?
Which method is more favourable ?
Then onto this > https://www.academia.edu/14523990/Regioselective_%CE%B1-Brom...
Which uses montmorillonite K-10 clay not exactly OTC to my knowledge never the less some good reading for sure
EDIT in response to melgars comment unseen by the time i posted
Thank you for your input i see no role ammonium acetate playes in the reaction either but the OP for sure is a fool he would have been extremely lucky
to come out without any permanent damage !! I have suffered deeply from such reactions rubber gloves and swimming goggles defiantly make things alot
easier IMO
I dont mean to hate im sorry if i apear that way i would rather make others more aware of dangers and come up with a solution ie a proven reaction
than can safely be performed by anyone with some common sense
He chose to carry the reaction out in diethyl ether when many alternatives were available it " specifically " stated cooling was needed when diethyl
ether was used yet he was hell bent on using it not only that he left the reaction "unattended" and "wedged" a septum needle in the top of his refulx
condenser and paid for it deeply .
I mean what the hell was it so hard to have some tubing even garden hose lead out the window from his condenser or into scrubber ?
I mean when you have many alternatives solvent wise to perform a reaction in and you pick the most volatile out of them for a " exothermic " reaction
and then restrict its only means of it releasing its byproducts/gases to what 1ml or less?
At that rate its pretty much a sealed glass bomb full of super heated flammable solvent fumes and irritating halogenated ketone an extremely foolish
act what could only be worse was if innocent people were effected by this
Regards
[Edited on 15-6-2018 by carbon85]
[Edited on 15-6-2018 by carbon85]
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Loptr
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After a few small trials some years ago I found bromine source + AcOH/p-TsOH and a catalytic amount of H2SO4 to work quite nicely. The bromine sources
that I tried were DBDMH, NBS, and bromine itself.
I have also tried similar reactions in methanol, acetonitrile, and toluene. I believe that I also ended up with benzyl bromide formation--didn't
follow up.
I had the most trouble with methanol.
[Edited on 15-6-2018 by Loptr]
"Question everything generally thought to be obvious." - Dieter Rams
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carbon85
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Please explain why you had the most trouble with methanol as some people swear by it for such reactions having never tried it myself leaves some
questions ,
Did you experience uncontrollable reactions ?
What size reactions were you performing ?
What was the concentration of the ketone in methanol ? This has a effect on rate of reaction as i,m sure you know i tend to double the volume of
solvent to substrate
DCM has always been favourable without the aid of acid catalysts the reaction is almost instant every time only a few times slight warming with
friction was required however bigger reactions wont be so forgiving and will need a kick start so to speak AlCl3 etc
DCM its good because its non flammable easily stripped from the reaction and easily recycled because of its low BP not to mention the low cost of it
hell its even OTC in some products depending where you are on this god forsaken planet however it is considered a carcinogen and "frequent" exposure
to heated vapour,s shouldn't be taken lightly
DMSO works a treat when using CuBr2(ll) suppose it would work with NBS but i can only speculate
Regards
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