Carbonyl electron donating group: less acid?!?!

[JDHK]

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Watching Chad's, and really confused about acidity of alpha carbon or hydrogen. In carbonyl, take for example comparing a ketone and aldehyde. Which is more acidic? Methyl group is electron donating, which makes it less acidic apparently but the logic really makes no sense to me.

 

[ChrisM07]

The removal of an H from the methyl group would make the enolate ion. Essentially, it is resonance stabilized. Removing the aldehyde H is NOT stabilized and not acidic.

 

[JDHK]

The removal of an H from the methyl group would make the enolate ion. Essentially, it is resonance stabilized. Removing the aldehyde H is NOT stabilized and not acidic.

I was trying to go that route too, but from what I learned from chad earlier, the more stabilized the conjugate base will mean less basic, meaning more acidic. But in this case, aldehyde is more acidic than the ketone, so we would be wrong.

 

[ChrisM07]

Hmmm. I guess I'm stumped too.

 

[Illfavor]

Watching Chad's, and really confused about acidity of alpha carbon or hydrogen. In carbonyl, take for example comparing a ketone and aldehyde. Which is more acidic? Methyl group is electron donating, which makes it less acidic apparently but the logic really makes no sense to me.

Deprotonation creates an anion, a region of high electron density. Methyl groups are electron donating, meaning they increase electron density. This makes a base more reactive, less stable, more basic and less acidic. Therefore, an aldehyde is more acidic than a ketone because it has fewer methyl groups. Make sense?

 

[JDHK]

Deprotonation creates an anion, a region of high electron density. Methyl groups are electron donating, meaning they increase electron density. This makes a base more reactive, less stable, more basic and less acidic. Therefore, an aldehyde is more acidic than a ketone because it has fewer methyl groups. Make sense?

So I guess I can memorize that fact, but I know it won't last long unless I can logically reason that fact. I can't atm =(..what does that mean (the highlighted region)? is there any intuitive way to explain that?Yea it's a donating group, and so the carbonyl will have a larger density of electrons around it right? Bases in the sense that it will donate electrons by lewis definition, and it has more electron density to give away? is that what it means?

 

[ChrisM07]

How come then, in the picture that I posted, the aldehyde H is not removed with the strong base?

 

[Illfavor]

So I guess I can memorize that fact, but I know it won't last long unless I can logically reason that fact. I can't atm =(..what does that mean (the highlighted region)? is there any intuitive way to explain that?Yea it's a donating group, and so the carbonyl will have a larger density of electrons around it right? Bases in the sense that it will donate electrons by lewis definition, and it has more electron density to give away? is that what it means?

If it has "too many" electrons, it wants to react to get rid of some. The best way to do that is to "donate" them as a Lewis base.

How come then, in the picture that I posted, the aldehyde H is not removed with the strong base?

You said yourself, the single aldehyde H is not resonance stabilized and is nowhere near as stable as the enolate. The question here was why is a ketone less stable...and that's due to the abundance of electron density.

 

[JDHK]

Got it. Thank you!

 

[JDHK]

The removal of an H from the methyl group would make the enolate ion. Essentially, it is resonance stabilized. Removing the aldehyde H is NOT stabilized and not acidic.

I think we have a misunderstanding. I was asking ketone and aldehyde overal acidity comparison and I think you're comparing the acidities of the hydrogens in the pic you put up?

 

[ChrisM07]

My bad. Sorry guys.

Guess it's too late for this 😳