Aldol Questions

[w4nt2baDDS]

I wrote up 6 equations and believe the first three are correct but am not sure about the last three. Can someone let me know where I've gone wrong on any of them?

On numbers 4,5,6 would the -O groups all grab an Hydrogen from water and leave an -OH group floating around? If that did happen.. then wouldn't the -OH group re-attack a carbonyl or basically restart the reaction all over again....

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[tbogdan89]

I attached an example which hopefully will help. In 1-3 you almost have it, just make sure that you form the Alpha-Beta unsaturated bond, I think the bond is one carbon off right now.

And I think 4-6 is the Claisen Condensation? So the key is is that the O-R that leaves actually does deprotonate the alpha carbon between the two carbonyls which is why 2 alpha protons is necessary for a claisen to work. Every step until the second deprotonation is reversible. That is why there is usually an acid or h20 workup afterwards, to protonate the enolate.

 

[w4nt2baDDS]

make sure that you form the Alpha-Beta unsaturated bond, I think the bond is one carbon off right now

Oh, you're right. I actually added an extra Carbon which is shifting the alkene to the left.

On the last step of your drawing, does a base attack the alpha hydrogen or does the beta OH group attack the alpha hydrogen and condensate off? Also, is heat or an acid required for this step or would it still happen (albeit slower) without heat or an acid?

And I think 4-6 is the Claisen Condensation?

I see. Yeah 5-6 use esters and I can see them more like Claisen Condenations.

Question 4 is kind of different though, it doesn't use an ester. I'm not sure if that means it would react any differently... I have it forming an acetal type structure.. not sure if that's remotely right.

Oh! Interesting, that helps me figure out what happens to the reactive -(O-R) groups in 5 and 6. So they attack one of the alpha hydrogens and then water or acid reprotonates it to stop an equilibrium reversal, nice.


Appreciate the help!

 

[FeralisExtremum]

Reaction 4, unlike the others, would first form a carboxylate (of the molecule on the left) rather than the enolate of the molecule on the right. The base would deprotonate that carboxylic acid.

Beyond that...maybe the carboxylate ion could act like a nucleophile and then attack the carbonyl carbon of the ketone on the right to produce what you've written...but I can't say I've seen that reaction before and I would think due to the stabilization, a carboxylate ion is a very weak nucleophile (and a ketone would not make the best electrophile to begin with). Most likely, reaction 4 would just end up with the same ketone you started with and a benzoate ion.