Aldol Condensation Dehydration

[oceanblue5841]

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Hey guys, I know that the nucleophile or the alpha carbon can lose a hydrogen along with OH, but when does it happen? I've been having problems, knowing when the major product is the alcohol form or the unsaturated product form.

 

[PooyaH]

Whenever the last step of the reaction is 'heat', then you will have dehydration of alcohol and you'd end up with the unsaturated form. But whenever there's only base present, then you'd see alcohol in the final product.

 

[LaughingGas]

Hey guys, I know that the nucleophile or the alpha carbon can lose a hydrogen along with OH, but when does it happen? I've been having problems, knowing when the major product is the alcohol form or the unsaturated product form.

After aldol addition, you get either B-hydroxyaldehyde or B-hydroxyketone right? .
The pattern I use is that a) if you see Carbon Attaching OH is two single bonds away from the Carbonyl Carbon, b) acid or base with heat, would result in aldol condensation (after you get B-hydroxyaldehyde or B-hydroxyketone, in acidic condition, no aldol addition is possible)
What I mean with "a)" is : HO - C ---[1st single bond]--CH2 ----[2nd single bond]----C(=O) ----- H or R
If you draw the structure you will see what I mean.
The product obviously, becomes alpha, Beta- unsaturated.
Hope that is helpful