[afung22]

Hi there,

I've been practicing some questions that the prof has given us. And I'm still confused on the mechanisms. Here's one question. How are you suppose to know to either deprotonate the alpha hydrogen or to attach a hydrogen from NaOH to the oxygen on the carbonyl group? (I attached a picture of the problem)

Thank you!

[discodermolide]

Hi there,

I've been practicing some questions that the prof has given us. And I'm still confused on the mechanisms. Here's one question. How are you suppose to know to either deprotonate the alpha hydrogen or to attach a hydrogen from NaOH to the oxygen on the carbonyl group? (I attached a picture of the problem)

Thank you!

Look up aldol reactions.

[fledarmus]

NaOH is a base - my first step in a reaction mechanism probably wouldn't be using it to protonate something.

[orgopete]

I don't think more knowledge is a detriment. Acetone has a small percentage of hydrate in water. Formaldehyde has a high percentage. The addition of water does not lead to further chemical reaction per se. If you added ¹⁸O labeled water to acetone, the water would exchange the ¹⁶O with ¹⁸O. If you added NaOH (or HCl), you could accelerate the exchange. This should tell you that this reaction is taking place, but it is not altering the structure.

For the problem in question, it isn't that NaOH is not adding to the carbonyl group, it does add. However, this addition is not leading to a new product. If ¹⁸O labeled water were present, then you could measure that this reaction were taking place.

If formation of an enolate were to occur by removal of the alpha proton, that would enable a new reaction to take place. As it turns out, this new reaction can take place and so we know that enolization must also have occurred to explain its formation. When one writes the reaction mechanism, one generally does not describe reversible events that do not give new products. It is the kinetically smaller reaction that leads to the products so that is what is written.

It is also important that a non-enolizable aldehyde (benzaldehyde) is used to produce a mixed aldol product. Enolization would enable a competing reaction path. Furthermore, the success of this reaction depends on how the reaction is carried out. If one added the ketone to hydroxide and then added benzaldehdye, the enolate would form in the presence of unenolized ketone. This would lead to self-condensation. If benzaldehyde is present with the ketone or if the ketone is added last to the benzaldehyde/NaOH mixture, then formation of the enolate would take allow reaction with the aldehyde.