[adianadiadi]

We know that Grignard reagent 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?

[Dan]

Hint: Grignard can theoretically deprotonate the aldehyde or add to it. Consider now NaOH: NaOH can also undergo an addition reaction with an aldehyde, but the product of this reaction is not observed - can you think why?

Hint: Kinetics vs theermodynamics

Also:

This reaction is not possible with NaOH i.e., it cannot abstract proton from alcohols. So GR is stronger base than NaOH

This is false. With a pKa of ~17, acetaldehyde has a similar acidity to most alcohols. Ethanol (pKa ~16) is actually more acidic and hence easier to deprotonate than acetaldehyde.

Acid-base equilibria are significant in both cases.

NaOH + ROH H₂O + RO^-
NaOH + CH₃CHO  H₂O + CH₂=C(ONa)H

[orgopete]

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

If you made a slow addition of a Grignard reagent to acetaldehyde, what reagents are present at the midpoint of the reaction. How does this compare if acetaldehyde is added to the Grignard?

[adianadiadi]

If you made a slow addition of a Grignard reagent to acetaldehyde, what reagents are present at the midpoint of the reaction. How does this compare if acetaldehyde is added to the Grignard?

Thanks for the reply. Slow addition might lead to enolate??