Mechanism for methylbenzoate, help

[justinbaker]

Methylbenzoate reacted with excess phenylmagnesium bromide, forms the salt of triphenylmethanol.

I need help figuring out this mechanism.

 

[maverick280857]

PhMgBr rxns are pretty straightforward as are most Grignard Reagent rxns. What you should know is that the PhMgBr reagent is polar with a +ve charge on Mg and a negative one on Ph, or I should say it behaves this way in reactions. In case of a -C=O you simply shift the pi electrons towards the carbonylic oxygen and show the attack of the nucleophile Ph- on C and that of (MgBr)+ on the oxygen I think. Here however you have a benzoate (ester) group, vis. PhCOOMe. So this: http://chemistry2.csudh.edu/rpendarvis/carb-enolate.html#estergrig will be useful I think.

Cheers
Vivek

 

[justinbaker]

maverick, i think i understand how to do it now. but what it the structure of the salt. i know i could get the entire mechanism if i only knew the structure. thanks for your help

 

[justinbaker]

anyone know what the structure is?

 

[movies]

I think it would be Ph3COMgBr, but I'm not sure. These salts hydrolize easily in the presence of water so you very rarely isolate them.

 

[chem_tr]

Hello, triphenylmethanol has the structure Ph₃COH. It is probably formed from the nucleophilic attack of phenylmagnesium onto positive carboxy carbon.

In the first step, Ph₂C(OMgBr)-OMe is formed, then with somewhat acidic hydrolysis, it converts to Ph₂C(OH)-OMe. Acidic attack continue onto methoxy group especially, due to its more basic nature; so after a series of methanol and water eliminations, we can get Ph₂C⁺. This may also be attacked by phenyl anion from another phenylmagnesium bromide molecule, to give Ph₃C⁺ cation. This, when exposed to water, eventually may yield Ph₃C-OH and H⁺.

I think this mechanism is plausible, of course more logical ones may be proposed.

Regards, chem_tr

 

[movies]

Well, Grignards won't work if you have any acidic protons around, so I think that after the first step of chem_tr's mechanism it would collapse to the ketone (benzophenone) and MeOMgBr. The ketone could then be attacked by another equivalent of the Grignard reagent to form the triphenylmethoxide-MgBr salt. Final hydrolysis would give triphenylmethanol. So the oxygen in the final product would come from the carbonyl of the starting material, not from the water. However, it is possible for the oxygen to exchange with water from the workup, since the triphenylmethyl cation is highly stabilized. I am not familiar with any experiments that test this, but it would be easy enough to perform with oxygen-18 labeling.

 

[chem_tr]

Yes, there are some uncertain points in my mechanism, and I agree with you, Movies, about ¹⁸O labelling to clear the reactions.

 

[GCT]

Why don't you try observing the mechanism of the grinard reaction yourself? The first reaction involves an addition-elimination mechanism while the second simply involves addition to create the alcohol. It's pretty simple.