Aldol Condensation Double bond formation and Haloform Reaction reactants

[diroten]

I've been looking at the two reactions in the title, and I have a question about each:

1) How are you supposed to determine which carbons the double bond forms between after the
dehydration of the aldol into the enal? I know one of them is always the carbon that was attached
to the oxygen before it leaves, but what about the second carbon?

2) According to my book, the haloform reaction only occurs with methyl ketones. Why can't it also
occur with acetaldehyde? Acetaldehyde also has alpha carbons that can be deprotonated for halogenation.

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[Hadi7183]

1) The carbon-carbon double bond is formed where it can stabilize the product (enal) through conjugation with the double bond of the carbonyl group. This is why it is formed between alpha-beta carbons.

2) As far as I know, haloform test should be positive for acetaldehyde, as it has a methyl group next to the carbonyl group. Acetaldehyde is actually the only aldehyde that is positive for the iodoform test.

 

[diroten]

Oh, so then the double bond can only form between the alpha carbon of both the hydroxyl-carbon and the carbonyl?

 

[Hadi7183]

The double bond is formed between alpha-beta carbons WRT the carbonyl group.

 

[diroten]

That's what I meant, but the way you said it is more clear. Thanks!

 

[Hadi7183]

Not a problem!

 

[chemtopper]

I've been looking at the two reactions in the title, and I have a question about each:


2) According to my book, the haloform reaction only occurs with methyl ketones. Why can't it also
occur with acetaldehyde? Acetaldehyde also has alpha carbons that can be deprotonated for halogenation.

Haloforms are CHX3 so you need minimum three hydrogens on alpha carbon from carbonyl carbon for this reaction to take place.Each Hydrogen is first replaced by OH- as acidic proton and a double bond is created between Carbonyl carbon and alpha carbon .On this double bond X2 is added by electrophilic addition mechanism.This will happen till all three hydrogens are replaced by X group and finally H+ is added from the solution to give CHCl3.If you follow mechanism carefully you will see that presence of CH3 group on C=O is must to get CHX3(haloform).Acetaldehyde does show haloform test because it is also CH3CH=O