Hydroxide as a leaving group-Aldol

[brake4country]

Just out of curiosity, during an Aldol condensation, how is it possible for the hydroxyl group to leave? I thought that OH is a bad leaving group. Thanks in advance!

 

[AdityaDev]

Not exactly. Dehydration takes place. This is because of stability due to conjugation. The product formed is a beta unsaturated carbonyl compound and the C=O and C=C are in conjugation.
Also, the reagent is a concentrated base. Hence the OH- from base gets protonated by taking a hygrogen from beta carbon and this causes elimination of OH like E1-CB mechanism.

 

[brake4country]

Oh I see. My book did not specify concentrated base. I do see in the mechanism that the OH abstracts a proton from the alpha carbon, creating the C=C double bond. However, what happens when the OH is kicked off? Is it stabilized by its cation counterpart (i.e, Na+)?

 

[AdityaDev]

The β-unsaturated product is not formed instantaneously. It is not truly a part of the reaction. The actual product is aldol.

There are 2 ways for getting β-unsaturated product:
1)acid catalyzed (using dil. sulphuric acid)
2)base catalyzed(requires heating)

Both the reactions involve formation of water molecules.
You know acid catalyzed dehydration. In the second reaction, OH is eliminated only after the formation of carbanion, while in the first case, OH is eliminated before the formation of carbocation.

what happens when the OH is kicked off? Is it stabilized by its cation counterpart

You are talking about the acid catalysed dehydration. Once the OH is eliminated as H20 from the γ-Carbon, then according to saytzeff rule, a double bond is formed between β-carbon and γ-carbon.

 

[brake4country]

I finally understand now. Thank you for explaining this! My book did not clarify the role of the γ hydrogens but this makes more sense.