Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: qw098 on November 07, 2011, 03:26:52 PM
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Hi Guys,
I synthesized triphenylmethanol in the lab three weeks ago. I first made the Grignard reagent phenylmagnesium bromide from the reaction of bromobenzene and magnesium. I then reacted my phenylmagnesium with methyl benzoate to form triphenylmethanol.
I took a look at my NMR, and my lab professor told me that besides triphenylmethanol there were four other compounds present in this crude mixture.
I have posted by NMR spectra. Could anyone give me a slight clue on where I should start to tackle this problem to find what four other compounds are still present in my crude mixture?
Would maybe one of them still be my methyl benzoate, the starting material, and perhaps some phenylmagnesium bromide as well?
Thanks guys!
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You'll need to isolate each product separately. It will be too difficult to determine the side reactions when they are still all mixed together.
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That's what I thought too Mitch!
But my professor said that I should still be able to tell what four other products I expect to see in my crude product based on my general knowledge of the reaction taking place and based on the posted NMR.
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Any remaining phenylmagnesium bromide should have been quenched from work-up. What would the product after work-up be?
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Well the product after workup would be triphenylmethanol, no?
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If solvents count as a "compound", then I guess I see some ether left.
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How do you know this nox?!
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Triplet ~1.25, quartet ~3.5
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Well the product after workup would be triphenylmethanol, no?
Yes! :)
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Will your excess phenylmagnesium bromide survive your workup conditions? If not, what will it become?
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Will your excess phenylmagnesium bromide survive your workup conditions? If not, what will it become?
No, it wouldn't. It would turn into PhCOOH (Benzoic acid) after workup!
So I know I would have Benzoic acid, ether, and some other compounds present.
Would I possible have some Biphenyl left as well because the Grignard reagent coupled with itself?
I know a Grignard Reagent (PhMgBr in my case) can have some side reactions such as reaction with CO2, H2O, and O2 as well as self-coupling like I mentioned earlier to produce biphenyl. In the self-coupling reaction a biphenyl would be produced, while for the reaction of PhMgBr with CO2 after work-up I would get Benzoic acid (PhCOOH). Then the reaction of PhMgBr with H2O after work-up I would get a phenyl peroxide (PhOOH). Then the reaction of PhMgBr with O2 after work-up I would simply get a benzene ring.
So my compounds present in my crude product would be benzoic acid, ether, biphenyl, PhOOH (Whatever this is called), and a simply benzene? And triphenylmethanol as well of course ;)
Thanks!
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Not quite - actually, the phenylmagnesium bromide reacts with water to give benzene during the workup procedure. It forms the phenol or phenyl peroxide with oxygen, and the benzoic acid with CO2.
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Not quite - actually, the phenylmagnesium bromide reacts with water to give benzene during the workup procedure. It forms the phenol or phenyl peroxide with oxygen, and the benzoic acid with CO2.
Yes, I mixed them up when stating what would form what. Thanks a lot for the aid! :)
I guess to identify these products in my NMR, I'd simply have to take a look and see where a typical OH chemical shift would be seen and try to see if my NMR was affected. I'd have to do likewise for the functional groups present in a carboxylic acid and then for the simple benzene/biphenyl compound.
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What other intermediates might you have? Surely all three phenyl groups didn't jump together in one step...
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Honestly... I can't think of anything else!
Maybe I still had some methyl benzoate, bromobenzene, and phenylmagenesium bromide if not all of it reacted. So, methyl benzoate, triphenylmethanol, bromobenzene, phenylmagnesium, benzoic acid, phenol, benzene, and biphenyl were all in my crude product?!.
What do you mean by "Surely all three phenyl groups didn't jump together in one step..."
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Honestly... I can't think of anything else!
Maybe I still had some methyl benzoate, bromobenzene, and phenylmagenesium bromide if not all of it reacted. So, methyl benzoate, triphenylmethanol, bromobenzene, phenylmagnesium, benzoic acid, phenol, benzene, and biphenyl were all in my crude product?!.
What do you mean by "Surely all three phenyl groups didn't jump together in one step..."
What's the mechanism for the reaction?
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Please see it below, I drew it on my piece of paper :)
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Great, so what's the compound top right?
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It would be Benzophenone :)
So Benzophenone as well would be in my crude material! Wow, so there are a lot of different compounds in my crude material!!
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It would be Benzophenone :)
So Benzophenone as well would be in my crude material! Wow, so there are a lot of different compounds in my crude material!!
There could well be! That's one of the things that makes organic chemistry and especially reaction workups interesting...
My guess would be that the compounds your professor expected you to recognize as possible impurities would be those that you knew had to be in your reaction at one point or another, like the starting materials, intermediates, solvents, and workup products, rather than the side products which you might or might not expect to find based on your conditions, like the CO2 and O2 coupling with the grignard, or the grignard self-coupling. So at a minimum, you might expect to see ethyl ether, methyl benzoate, bromobenzene, benzene, benzophenone, and your product. That's five extra compounds instead of four, but maybe he wasn't considering solvent as an extra compound?
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Ahh I see!
Well I did do a rotoevap... so my solvent (diethyl ether) should have been gone, but evidently based on my NMR I still have some left!
I guess I'll take a look at my crude NMR and compare it with the reference NMR's of the compounds I believe are still present in my mixture.
Thanks again Fledarmus et al.! Brilliant stuff. I just love it when I learn and understand what is going on. Nothing better in the world! :)
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Thanks qw08, we all had something to learn from this problem.