Aldol addition vs. base-catalyzed aldehyde hydration???????!!!

[dentalplan]

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I had a question about the first part of the aldol condensation (the aldol addition) and as it relates to base-catalyzed hydration of an aldehyde, for example.

So anyways, let's use an aldehyde as an example.
Both reactions are gonna use OH- in their first step. Both reactions use water in a later step. In the aldol addition, the OH- attacks the alpha hydrogen of the aldehyde. HOWEVER, in base-catalyzed hydration...the OH- attacks the carbonyl carbon (nucleophilic addition)......and not the the alpha hydrogen.

My question is -- Why the discrepancy???????? What's the difference between these two reactions that would explain why the OH- wants to attack the alpha hydrogen in one reaction, but a carbonyl carbon in the other reaction?

Hope someone one even knows what I'm talking about

These are obviously completely different reactions involving aldehydes and ketones.....but I just can't figure out why the OH- has this discrepancy of preference between these 2 reactions.

peace

 

[doc3232]

Can you write out the reactants? I know reactions well, but not their names.

 

[dentalplan]

For example.....

Adding OH- and water to butanal, or any aldehyde.....



I'm looking for the difference between the two reactions I mentioned above with respect to added OH- !!!

Please can anyone help

 

[nixon13]

so lets say you start out with acetone. Always assume two moles unless its a mixed aldol. So one mole will stay the way it is (as acetone) and the other will lose the carbonyl oxygen and turn into an OH. Then the carbon with the OH will attach to the carbon next to the carbonyl on the acetone. Thats your product. Now if there is heat added, the OH and a H on the carbon next to the carbonyl will dehydrate and you will lose H2O and form a double bond. So with heat, your end product should have a carbonyl and a double bond. Without heat, your product has a carbonyl and an alcohol.

 

[dentalplan]

so lets say you start out with acetone. Always assume two moles unless its a mixed aldol. So one mole will stay the way it is (as acetone) and the other will lose the carbonyl oxygen and turn into an OH. Then the carbon with the OH will attach to the carbon next to the carbonyl on the acetone. Thats your product. Now if there is heat added, the OH and a H on the carbon next to the carbonyl will dehydrate and you will lose H2O and form a double bond. So with heat, your end product should have a carbonyl and a double bond. Without heat, your product has a carbonyl and an alcohol.

actually....the carbonyl oxgen will not turn into an OH .....don't know where you got that from.

Acetone is a bad example b/c ketones don't even really react by way of aldol addition. But let's take butanal for example.
The carbonyl oxygen turns into an O- ........ and the beta carbon will bear an OH group after the aldol addition reaction is done.


Either way....what you said was good and all...but it's sort of out of touch with what my question was getting at. Read my original post. My original question also had to do with a little reaction called "hydration of aldehydes."

Please. ...if anyone out there can help me i will be eternally grateful.

I will restate my question if anyone is still confused: "Why does the OH- attack the alpha hydrogen in aldol addition, but in OH catalyzed hydration of aldehydes...the OH- nucleophilicly attacks the carbonyl carbon?"

 

[ongmeister]

It is because the alpha carbon is the acidic carbon. thus it would remove that hydrogen to an aldol condensation.

 

[dentalplan]

It is because the alpha carbon is the acidic carbon. thus it would remove that hydrogen to an aldol condensation.

so butanal wouldn't undergo base-catalyzed hydration in the presence of OH- and water, but an aldol addition instead?

 

[atlanta213]

I do not understand what you guys are talking about. doesn't it always bind to alpha hydrogen? I never see it binds to carbonyl directly...😕

 

[dentalplan]

I do not understand what you guys are talking about. doesn't it always bind to alpha hydrogen? I never see it binds to carbonyl directly...😕

thats what im saying...... in base-catalyzed (OH- catalyzed) hydration of aldehydes........OH- will attack the carbonyl carbon......the final product will be a geminal diol.


but....in aldol addition, the OH- attacks the alpha carrbon....so thats why im confused....i dont know why OH does different things in (seemingly) identical conditions.

so far no one has explained to me this discrepancy; any one out there good with organic chem?

 

[atlanta213]

thats what im saying...... in base-catalyzed (OH- catalyzed) hydration of aldehydes........OH- will attack the carbonyl carbon......the final product will be a geminal diol.


but....in aldol addition, the OH- attacks the alpha carrbon....so thats why im confused....i dont know why OH does different things in (seemingly) identical conditions.

so far no one has explained to me this discrepancy; any one out there good with organic chem?

like you said, geminal diol is attacking carbonly carbon (nucleophilic addition)

I understand that , but aldol.....

 

[orthodocs12]

if there are alpha hydrogens, such as in aldehyde and ketone, they will undergo aldol, since the carbanion is very stable.

 

[khmaiDDS]

thats what im saying...... in base-catalyzed (OH- catalyzed) hydration of aldehydes........OH- will attack the carbonyl carbon......the final product will be a geminal diol.


but....in aldol addition, the OH- attacks the alpha carrbon....so thats why im confused....i dont know why OH does different things in (seemingly) identical conditions.

so far no one has explained to me this discrepancy; any one out there good with organic chem?

In order to get a geminal diol as the product you are dealing with only one equivalent of aldehyde or ketone. And in order to get this "geminal diol" (or hydrate) you must use acid (H30, HCl, etc..) and water (H20). The H20 in this case will attack the carbonyl with the protonation of the Oxygen on the carbonyll and will eventually reduce to OH-.

In the case of the aldol you will need 2 equivalance of aldehyde, 2 equiv. of ketone, or one aldehyde and ketone. Here you can use OH- (as you mentioned) and heat or H30 and heat.
Since OH is such a strong base it will remove the alpha proton directly to create the enolate.

So, I guess that the confusion is that OH- does not add directly in "base-catalyzed (OH- catalyzed) hydration of aldehydes," but it is the H20 that gets turn into OH crating the geminal diol.

Sorry, I hope I didn't confuse this matter even more.