[Jazzified]

I am stuck on four problems involving Grignard synthesis (somewhat conceptual questions), and was hoping I can receive some help here.

1. Why is it unwise to allow the solution of the Grignard reagent to remain exposed to air for an unnecessary period of time even if it is protected from moisture by drying tubes?

2. What is the solid that forms during the addition of phenyl magnesium bromide to methyl benzoate (step before triphenylmethanol)?  I know the structure of the product, but I don't know exactly how to name it.  The product has a carbon bonded to three phenyls, and the fourth bond is to OMgBr.  Any idea on how to name this?

Lemme know if you're able to assist me with answering these questions.  Thanks!

[macman104]

1)  Grignard reagents will react with atmospheric air to produce an alcohol (ROH) where the R group is the carbon structure attached to the MgX.

[Jazzified]

Thank you for your response.  Any idea on how to name the product in #2?  I can't seem to find any nomenclature rules for that product.

[macman104]

I suppose this could be right....ChemDraw tells me it's "Magnesium bromide triphenylmethanolate".  But really I would just call it the magnesium (or maybe Magnesium Bromide?) salt of the final product, because in the next step, you quench the reaction to remove the MgBr, and are left with OH, ya?

[alphahydroxy]

Actually, I think the desired alcohol product (triphenyl methanol, or trityl alcohol or something like that) is unlikely to precipitate from the reaction mixture.

I think the solid you are looking for is the other product from a grignard reaction...

[m3lm4k]

I wonder why Grignard reactant won't abstract acidic alpha-hydrogen ?

[alphahydroxy]

I wonder why Grignard reactant won't abstract acidic alpha-hydrogen ?

There isn't actually any acidic protons in methyl benzoate - you're probably just confused by the nomenclature! The carbonyl group is actually on the benzylic carbon. Actually, the grignard probably wouldn't deprotonate the homologous compund, preferring instead to react with the carbonyl in a 1,2- sense.

[m3lm4k]

I know that here there are no acidic alpha hydrogens here. I wonder - in general - why Grignard reagent won't react in such way. E.g. with acetone or acetophenone where acidic hydrogens are present and carboanion is really strong base. To sum up, I know that mechanism but I wonder why it goes that way.

[alphahydroxy]

I know that here there are no acidic alpha hydrogens here. I wonder - in general - why Grignard reagent won't react in such way. E.g. with acetone or acetophenone where acidic hydrogens are present and carboanion is really strong base. To sum up, I know that mechanism but I wonder why it goes that way.

In general terms, it's to do with the balance of nucleophilicity and basicity. Grignard reagents will abstract acidic hydrogens if they are basic enough. For example, Leo Paquette synthesised a range of beta lactams using this strategt, by using t-butyl magnesium chloride to remove NH protons and form the corresponding magnesium amides. However, it's not a commonly employed methodology, as there are other stronger and less nucleophilic bases which can be used instead (e.g. LDA or KHMDS).

[m3lm4k]

OK. I am really glad for your answer.

[alphahydroxy]

Glad it helped !

Just another thing - you'll note that I mentioned that Paquette used t-butyl magnesium chloride as a base. This is more basic than, say, MeMgBr due to it's increased steric bulk which essentially make it more difficult for the molecule to be a nucleophile.

However, this is all relative, and there are many instances where it will readily react as a nucelophile rather than a base - again ,as with many things in chemistry, it's all about the balance!