Fleaker - 26-7-2010 at 15:25
Awhile back, I was using some prepared 3M phenyl magnesium bromide in diethyl ether, supplied in the convenient Aldrich SureSeal bottle. It was a new
old bottle, that had sat unnoticed and unused in the reagent room for many, many years. I did not bother to titrate it, I merely cannula'd a bit to an
Erlenmeyer of methyl alcohol and saw it react fiercely as it should.
I then used as much as I needed for my reaction and left it to stir overnight.
Upon workup via flash column chromatography, I noticed a highly fluorescent band by accident (I was doing TLC and happened to wave the UV wand near
the column, and lo and behold!). It fluoresced using 354 nm light, quite brilliantly blue. It was the first to elute off the column using 20:1 hexanes
ethyl acetate (which yielded unsatisfactory separation, but that is another story!)
Anyway, I collected this fraction, evaporated off the solvent and it spontaneously crystallized to a white waxy compound having a unique smell. I
TLC'd it against phenol and bisphenol, likely impurities given my reaction, and it was not either. I will take a proton and carbon NMR soon for you
all to see.
It is an exceedingly potent fluorophore, for a gram of it will make 5L of solvent (it dissolves in hexanes, ethyl acetate, methanol, dichloromethane,
and toluene) glow very brightly. Any of the more experienced organic chemists know what this may be?
I now have quite a bit of it, since it cut the yield of my reaction in half, and was the chief product :-/
I can only think it's the Grignard at fault--I've done the reaction with fresh made grignard and it never happened. I've worked much with Grignards
and never seen such a thing. I can only assume it's some polyaromatic condensation or degradation product.
Any ideas?
[Edited on 26-7-2010 by Fleaker]
Eclectic - 27-7-2010 at 00:40
What was the substrate in your reaction? ie: What were you reacting the phenyl magnesium bromide with?
Paddywhacker - 27-7-2010 at 00:52
Bisphenol? ... you mean biphenyl. That is fluorescent, smells pleasant, and is consistent with phenyl magnesium bromide reactions.
Nicodem - 27-7-2010 at 06:06
In my experience old phenylmagnesium bromide solutions contain quite some biphenyl. Sometimes even fresh solutions contain some. That your side
product came out from the column first with 20:1 hexanes/ethyl acetate is consistent with the low polarity, but somehow I doubt biphenyl is that much
fluorescent. In another case where the arylmagnesium bromide was prepared in the lab, I also found the biaryl impurity as a side product after the
subsequent reaction workup and chromatography. The grignard was prepared from an aryl bromide with Mg powder in THF under ultrasound (I doubt
ultrasound or THF matters in this regard, but that's how I was making them) and the reaction was a simple addition on an ester, so no transition
metals were present to catalyse homocoupling of the grignard or crosscoupling with any residual aryl bromide. Furthermore, recently a colleague told
me that he also accidentally found out that comercial PhMgBr/THF contains the biphenyl impurity, so this must be a common problem. Yet, I never saw
this problem mentioned in the literature (though I did not specifically searched) and I still don't really understand how biaryls could form under
such conditions. Maybe via some radical reaction with O2? I have no idea.
12AX7 - 27-7-2010 at 13:43
Stibene is wonderful stuff, it would require at least a benzyl or ethyl group to make though.
Biphenyl comes to mind as a yellow impurity in these reactions. Heck, that was bottom level o-chem, you already know this..
Tim
chemrox - 27-7-2010 at 20:26
biphenyl doesn't flouresce like that. With phmgbr it is the product impurity of choice but its odor while distinctive, is not so strong and is
usually familiar. I think you need an FNG to spread the electrons around to get the flourescence. What do you know that also smells like it? What
FNG's are on that other compound? Could you make it from phmgbr and what you're using? Please describe the smell!
chemrox - 28-7-2010 at 22:16
another thought; if the other reagent was a phenyl alkanol you may have achieved a Mg analogue of something I've been trying to make with Ga. send me
email or pm
bbartlog - 29-7-2010 at 07:15
From the description I would think maybe you have some triarylmethane compound. There are a lot of those, though, and I don't know where the central
carbon/methane would have come from to make such a thing.
Fleaker - 29-7-2010 at 16:25
I did mean biphenyl not bisphenol (been working with that lately too).
I'll upload pictures shortly, and the NMR when I get a chance. NMR shows it's heavily aromatic, but there appears to be quite some alkyls in there
(which may be residual hexane from rotovapping) I'll pump down on it on the Schlenk line.
Eitherway, I'm curious about the mechanism of what has caused this.
These bottles were never open, and were pristine, just very old. The grignard was a clear brown.
I never get the fluorescent impurity when I make PhMgBr in THF or ether (I prefer THF as it has a higher reflux temperature, so a runaway is slightly
less severe 
)
DDTea - 2-8-2010 at 01:20
Fleaker:
You mentioned that your compound fluoresced under 354 nm light. http://webbook.nist.gov/cgi/cbook.cgi?Name=biphenyl&Unit... take a look at the UV-Vis spectrum for Biphenyl. Peak absorbance is at 245 nm,
and declines pretty rapidly after that. You were using a very non-polar solvent system, so I would suspect that your observed absorbance would be
close to 245nm (whereas a polar solvent system would cause a red shift).
Also, you mentioned TLC'ing it against biphenyl and phenol and that it wasn't either, so that's another indicator against the biphenyl suggestion.
You mentioned that the NMR shows some alkyl component and that it could be residual hexane. I'm going to suggest anisole as your mystery aromatic
compound, although I'm curious how that would be formed. I'd suspect the Schlenck Equilibrium would have something to do with it:
2 RMgX <---> MgX2 + MgR2 (If you're going to be pedantic, yes, I know that is technically a resonance arrow!)
Then again, in a solution diethyl ether, the equilibrium should favor the RMgX.
Anyhow, what is the ratio of alkyl protons to the aromatic protons in the NMR?
[Edited on 8-2-10 by DDTea]
Fleaker - 12-8-2010 at 18:22
The fluorescence at 280 was weak. My UV wand has both 280 and 354 nm settings if I remember correctly. It's the "black light" low energy setting that
got this to fluoresce.
I never bothered to have a crack at the structure, mostly I saw a bunch of aromatic protons and a lot of aliphatic stuff as well. It must be some sort
of coupling product, as this doesn't happen when I use "home-made" PhMgBr.
DougTheMapper - 2-11-2010 at 22:05
Call me daft, but this has a striking resemblance to the fluorescence of quinine at about the same wavelength.
It's also extremely potent... maybe a side reaction?
[Edited on 3-11-2010 by DougTheMapper]
mnick12 - 3-11-2010 at 20:18
I doubt its quinine, quinine is quite a complex and I think it would be amazing if it formed from some grignard by-products. Could it possibly be some
sort of polyaromatic or anthracene type compound, I know many of these tend to be very fluorescent when in solution. Cool stuff though
DJF90 - 4-11-2010 at 05:32
Anthracene fluoresces green under a blacklight IIRC, so that would rule that out. Quite possible for it to be another polyaromatic hydrocarbon though,
maybe pyrene or chrysene?