I got another flag.
A poster gave a really good problem though, ascorbic acid. See the post here. What follows is my post. (I have edited my post on the forum as it was too long. Below is what I had written.
Let me go through this backwards.
Re: acetaldehyde v hydroxyethylene
Because they share the same resonance structure as the enolate, the acidity must be due to the energy levels of the aldehyde and enol. I argue an OH is more acidic than a CH is an inductive effect. This is true whether resonance is present or not.
I also concede that I am drawn to using a different definition for inductive and resonance than even I am accustomed to. The process of thinking about the different terms and what they mean has had that effect. I have suggested to students that p-nitrophenol is a stronger acid than meta due to resonance of the phenolate. It is easier to show this with resonance structures as the p-nitro-group will participate in the resonance structures.
If you seek consistency, then little problems may arise. If you wish to argue that an enol is more acidic due to resonance, then how does one explain the differences in acidity of a carboxylic acid, a carboxamide, and a methyl ketone with resonance?
I use resonance to explain the energy difference where pi or non-bonded electrons can interact. You can see the effect in reactivity or energy measurements. Increasing the number of electrons I argue increases the resonance effect. You can see this spectroscopically with UV and IR.
Compounds can have resonance and inductive effects. If you were to ask, which had a greater resonance effect, an amide or a carboxylic acid, I would argue an amide. I would argue the decrease in carbonyl stretching frequency shows how the electrons of nitrogen are interacting with the carbonyl group. Reactions of neutral amides occur on the oxygen (or at least the productive ones). The reaction of trimethyloxonium tetrafluoroborate give imidates, acylation to give nitriles, and acid catalyzed hydrolysis are consistent with a reaction on the oxygen. Resonance is consistent with an increase in electron density on oxygen.
Cyclohexenone forms an alpha anion due to greater acidity, if equilibrated forms the gamma-anion, resonance stabilized form. Conjugated enones are more stable than non-conjugated ones. I see these as a resonance effect, the manner in which pi-electrons interact with one another. Their interaction may stabilize an anion. Acidity is generally an inductive effect. A carboxylic acid is more acidic than an amide even if an amide were more resonance stabilized. A ketone uses what some have referred to as a non-resonance effect. There may be some resonance examples, but I have not found any that convinces me the reason for acidity is resonance.
Re: ascorbic acid
Ascorbic acid is a good problem to examine. I really like this example. It is data. I do not doubt the values. Because organic chemistry is really about electron movements, it is always intriguing to learn about how and what factors result in their movement and effects. Ammonia is an inductive electron withdrawing group to form an amide anion, but a resonance donating group in aniline.
What should the values for ascorbic acid be? Acetoacetate is about 11, acetylacetone is about 9, 3-oxobutanal is about 6, dimedone is about 5. I drew 3-oxobutanal as 4-hydroxybut-3-en-2-one as I believe that is its predominant form. This is the vinylogous acid I was originally noting. In that instance, I was arguing that although a greater number of resonance structures can be drawn for the anion, its pKa was less than a carboxylic acid. Even though resonance exists, separating the OH group from the more electron withdrawing C=O results in lower acidity. I think increasing conjugation will make an enol/aldehyde behave more like an isolated enol-aldehyde.
Why is ascorbic acid more acidic than dimedone or acetoacetate and which OH is more acidic? As pointed out, it could have been either one. “A” is closer to the electron withdrawing C=O group. “B” bears the resonance effect noted earlier. What if there wasn’t a “B”-OH? Which carbon of an enone is more electron deficient? The beta carbon is. Is this resonance or inductive? It actually is both. The electrons can interact to deliver the pi-electrons toward the carbonyl group. Because they can interact does not lead me to argue the carbon should be electron deficient. The electron withdrawing property of the oxygen does. The actual acidity appears to be a combination of effects. The dimedone is lower than acetylacetone, so a ring probably increases the acidity. The OH at “A” may also increase the acidity slightly.
What is probably more interesting is the second pKa. Why should the pKa of “A” be so low? Presumably, all of ascorbic acid becomes an enolate as the pH increases. The enolate places a negative charge alpha to the OH as well. Presumably, our resonance isomers do not reveal the true electron withdrawing character of the vinylogous enolate to accept another negative charge, because 11 is surprisingly acidic for this dianion.
We are not using resonance and inductive in the same manner. I am trying to model the physics of interactions. There are four forces of nature, the strong, weak, gravity, and electromagnetic. It is only electromagnetic that applies to organic chemistry. Chemists argue energy. “Why does an energy difference exist?” “What is the force responsible for the energy difference?” If you have a car on a hill and one in the valley, the car on the hill has more (potential) energy due to gravity. I see electron withdrawing effects to be responsive to nuclear charge and inversely to the square of distance. That is the easy one. If conjugation or resonance results in a lower energy state, then conjugation reflects an attractive force between electron pairs, but I didn’t just say that. What force is acting to reduce the energy state of conjugated dienes compared to non-conjugated ones?
My “complaint” was a simple one. I do not agree that a proton is acidic because you can draw a resonance structure of its conjugate base.