[qw098]

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!

[Mitch]

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.

[Goala]

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.

[Mitch]

Any remaining phenylmagnesium bromide should have been quenched from work-up. What would the product after work-up be?

[qw098]

Well the product after workup would be triphenylmethanol, no?

[nox]

If solvents count as a "compound",  then I guess I see some ether left.

[qw098]

How do you know this nox?!

[nox]

Triplet ~1.25, quartet ~3.5

[Mitch]

Well the product after workup would be triphenylmethanol, no?

Yes!

[fledarmus]

Will your excess phenylmagnesium bromide survive your workup conditions? If not, what will it become?

[qw098]

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!

[fledarmus]

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.

[qw098]

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.

[fledarmus]

What other intermediates might you have? Surely all three phenyl groups didn't jump together in one step...

[qw098]

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..."