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Suppose I have a phenyl Grignard and methyl benzoate.

If I react the two, can I expect benzophenone and a triphenylmethoxide ion as my products?

I ask because I know that Grignards react with ketones and aldehydes. So as the benzophenone is generated, couldn't more Grignard attack the benzophenone as well as the original reactant?

Expected products and mechanism shown below:

enter image description here

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Reaction of a Grignard reagent with an ester is a standard method for producing tertiary alcohols where at least two of the substituents (the "$\small\ce{R_2}$" group attached to the Grignard) are the same. Judicious choice of the starting ester allows for the preparation of a tertiary alcohol where all 3 substituents are the same. Alternately, you could react 3 equivalents of the Grignard with a cheap dialkyl carbonate (a "double" ester, if you will) to obtain a tertiary alcohol where all 3 groups are the same.

enter image description here

Typically a slight excess of Grignard reagent is used and the tertiary alcohol is produced after acid work-up. If an excess of Grignard is not used then a mixture (ugh!) of ester, intermediate ketone and alcohol may result.

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  • $\begingroup$ So I world need an acid workup after creating the anion? $\endgroup$ – Dissenter Nov 4 '14 at 16:55
  • $\begingroup$ If you mean the anion of the tertiary alcohol, then yes. $\endgroup$ – ron Nov 4 '14 at 16:57
  • $\begingroup$ Wait @Ron I'm confused, I'm not showing a grignard directly reacting with the anion $\endgroup$ – Dissenter Nov 4 '14 at 16:58
  • $\begingroup$ Oh, I thought the reactant in the top line of your drawing was a carboxylate anion. I guess that's not a minus sign but rather a bond where you just didn't draw the alkyl group on the end. $\endgroup$ – ron Nov 4 '14 at 17:02
  • $\begingroup$ yes, that's a methyl group on that oxygen. $\endgroup$ – Dissenter Nov 4 '14 at 17:25
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Yes, that is exactly what happens. And since ketones are typically more reactive than esters, you should expect that the tertiary alcohol (after workup) will be the predominant product.

In order to access the ketone, add the Grignard to a nitrile, which will give an imine (after protonation) that can be hydrolyzed to a ketone. Alternatively, add the Grignard to an aldehyde and oxidize the resulting alcohol to the ketone.

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