Reaction of acetaldehyde with grignard reagents

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Offline adianadiadi

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Reaction of acetaldehyde with grignard reagents
« on: 17/10/2013 20:18:41 »
We know that Grignard reagent, GR is strongly basic and can abstract a proton from alcohols. This reaction is not possible with NaOH i.e., it cannot abstract proton from alcohols. So GR is stronger base than NaOH

We also know that sodium hydroxide is just enough to abstract a proton from active methyl group of acetaldehyde to carry out aldol reaction.

Now my point is, if we use Grignard reagent, acetaldehyde should undergo aldol reaction instead of Grignard reaction.

Any comments?
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Re: Reaction of acetaldehyde with grignard reagents
« Reply #1 on: 19/10/2013 14:41:20 »
That's a good question.
I think that part of the answer is that the aldol reaction may also happen, but not very much. The yields for syntheses are usually not 100%.
However I think the reason that most of the aldehyde doesn't get deprotonated is that the solvent for Grignard reactions is usually ether which isn't very polar and so it doesn't encourage the formation of ions in the way that alcohol or water would.
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Offline chiralSPO

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Re: Reaction of acetaldehyde with grignard reagents
« Reply #2 on: 23/10/2013 15:37:15 »
Deprotonation is a competing side reaction in the addition of grignard and organolithium reagents to carbonyl compounds. Aldehydes are very reactive at C because it is very electrophilic and there is an easy, clear trajectory for attack. For most aldehydes the nucleophilic attack at C is faster than the deprotonation. Ketones are much more protected from nucleophilic attack, especially if one or both substituents are bulky. In many cases, ketones give unsatisfactory yields of the desired tertiary alcohol because the grignard reagent acted as a base instead. For these cases, it can be advantageous to use an additive like CeCl3, which activates the ketone and promotes the desired reaction.

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Offline adianadiadi

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Re: Reaction of acetaldehyde with grignard reagents
« Reply #3 on: 24/10/2013 04:34:21 »
Thank you very much.
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