共役塩基の安定性が原因か。
問題 Which -OH in vitamin C contains the acidic proton? 1 2 3 4
jce.divched.org/jcedlib/qbank/collection/conceptests/structur
問題２
web.pdx.edu/~wamserc/C334F08/E1ans
プロトンを放出した後の共役塩基が共鳴構造を持ち、負電荷を非局在化させて安定化できるためということか。プロトンの移動によって不安定なジケトンに互変異性１，２と１，３が生成可能。
ビタミンＣの酸性
p.bunri-u.ac.jp/lab23/esumi/S4_ans
chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter_07%3A_Organic_compounds_as_acids_and_bases/Section_7.3%3A_Structural_effects_on_acidity_and_basicity
Conversely, acidity in the haloacids increases as we move down the column.
In order to make sense of this trend, we will once again consider the stability of the conjugate bases. Because fluorine is the most electronegative halogen element, we might expect fluoride to also be the least basic halogen ion.
But in fact, it is the least stable, and the most basic! It turns out that when moving vertically in the periodic table, the size of the atom trumps its electronegativity with regard to basicity. The atomic radius of iodine is approximately twice that of fluorine, so in an iodine ion, the negative charge is spread out over a significantly larger volume:
Re: ascorbic acid
curvedarrowpress.com/wpblog/?p=128
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?
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.
chemthes.com
s10.lite.msu.edu/res/msu/botonl/b_online/library/newton/Chy251_253/Lectures/AcidBase/pK
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.
＊ There are four hydroxyl groups on this molecule – which one is the most acidic? If we consider all four possible conjugate bases, we find that there is only one for which we can delocalized the negative charge over two oxygen atoms.