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Author Topic: COOH Very Acidic  (Read 5797 times)

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fledarmus

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Re: COOH Very Acidic
« Reply #15 on: March 01, 2012, 02:18:09 AM »

I am firmly in fledarmus's camp on this. The acidity of an acid is directly related to the basicity of it's conjugate base. If you can argue that the conjugate base is more stable, and thus less basic, it follows by definition that the acid is more acidic.
I disagree with this. If this were true, then a ketone and its enol should be equally acidic if their acidity were determined by the stability of their identical conjugate bases.

"Directly related to", not "determined by". As I said previously, "Both the product stability and the starting material instability contribute to the energy difference between product and starting material, and it is that energy difference that determines the relative proportions of the two in an equilibrium." You have given an example where the product of the two dissociations is identical, but the starting materials are not; in this case, the differences in energy are determined by the relative stabilities of the starting materials.

Whether the larger energy difference is due to a higher energy starting material (as most enols would be in relation to most ketones), or to a resonance, electronic, or inductive stabilization of the products (as a carboxylic acid would be to an alcohol), it is the DIFFERENCE in energy between starting materials and products which determines the extent of dissociation. Stabilizing the product and destabilizing the starting material have the same effect - they both increase the difference in energy, and increase the relative concentration of dissociated material. The starting material doesn't need to "know" what the product is going to be - if there is enough free energy available, the association and dissociation reactions of products and starting materials will each proceed at their own pace, and an equilibrium will be established between products and starting materials which depends only on the difference in energy between them.
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orgopete

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Re: COOH Very Acidic
« Reply #16 on: March 01, 2012, 02:52:59 AM »

… it is the DIFFERENCE in energy between starting materials and products …

That was my point. Tell me why the enol has the energy difference?
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qw098

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Re: COOH Very Acidic
« Reply #17 on: March 01, 2012, 08:02:26 AM »

My "simple" question has turned into quite the discussion!

I am learning sooo much this is awesome! Thanks guys! :)
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Dan

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Re: COOH Very Acidic
« Reply #18 on: March 01, 2012, 12:49:29 PM »

The acidity of an acid is directly related to the basicity of it's conjugate base. If you can argue that the conjugate base is more stable, and thus less basic, it follows by definition that the acid is more acidic.
I disagree with this. If this were true, then a ketone and its enol should be equally acidic if their acidity were determined by the stability of their identical conjugate bases.

Your example does not refute the fact that acidity of an acid is directly related to the basicity of its conjugate base and vice versa. This relationship reflects the equilibrium of deprotonation of an acid, and protonation of its conjugate base by the reverse reaction.

You have neglected an important aspect of the relationship, and that enolates are ambident bases. Here is my take on it:

The ketone is less acidic than the enol. I absolutely agree with that.

While the same enolate is the conjugate base of both acids (the ketone and the enol), the fact that the enol is more acidic than the ketone just shows us that the enolate is more basic at C than at O. The direct relationship between the acid and it's conjugate base is not thrown into question.

The pKa of the enol is directly related to the basicity of the enolate by O protonation (the reverse of enol deprotonation).

The pKa of the ketone  is directly related to the basicity of the enolate by C protonation (the reverse of ketone deprotonation).

The deprotonation reactions are different, and are not equally favourable. The reverese reactions are also different, and are not equally favourable. The more favourable the deprotonation, the less favourable the reverse protonation.

Apples and oranges.
« Last Edit: March 01, 2012, 02:29:26 PM by Dan »
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orgopete

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Re: COOH Very Acidic
« Reply #19 on: March 01, 2012, 05:41:49 PM »

The pKa of the enol is directly related to the basicity of the enolate by O protonation (the reverse of enol deprotonation).

The pKa of the ketone  is directly related to the basicity of the enolate by C protonation (the reverse of ketone deprotonation).

The deprotonation reactions are different, and are not equally favourable. The reverese reactions are also different, and are not equally favourable. The more favourable the deprotonation, the less favourable the reverse protonation.

Quote from: orgopete
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.

I had hoped that because the ketone and enol can form the same enolate, the difference in pKa cannot be due to the resonance stabilization. It must be due to the difference in the pKa of the ketone and enol themselves.

As a device, I suggested to my students that you can predict the site of reaction of an ambident anion (a resonance effect) by an analogy to a boxer. The boxer (electrons) with the longer reach will be most likely to hit you. When you compare the resonance structures, why might the electrons on carbon extend further than those of oxygen?
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Dan

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Re: COOH Very Acidic
« Reply #20 on: March 01, 2012, 09:06:48 PM »

You could argue on the basis of C vs O electronegativity.

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Kran

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Re: COOH Very Acidic
« Reply #21 on: March 02, 2012, 09:48:18 AM »

Just one more example:

Peroxyacetic acid: O-H bond length = 1.10A, pKa = 8.2
Acetic acid: O-H bond length = 0.97A, pKa = 4.8

Higher inductive effect in peroxyacetic acid can be seen in the longer (so weaker) O-H bond. But pKa is higher becouse conjulgated base is not stabilized by ressonance.
« Last Edit: March 02, 2012, 10:06:08 AM by Kran »
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orgopete

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Re: COOH Very Acidic
« Reply #22 on: March 21, 2012, 02:27:39 AM »

I thought I was done here, but the xanthine solubilities post got me thinking and then the additional post by Kran. I have written an additional blog in which I argue stereoelectronic control virtually precludes a resonance effect.

Re: acetic, peroxyacetic, and resonance effects
I find this data in accord with what one should expect for an inductive effect. For example, hypochlorous acid (HOCl) has a pKa of 7.5 and hydrogen peroxide, also not resonance stabilized, has a pKa of 11.6. These pKa’s are in the range of peroxyacetic acid and could have been expected. I would characterize an acetate as a weaker electron withdrawing group than chloride. The poster also stated that the acidity was reduced in peracetic acid because it was not resonance stabilized. Although I agree that peracetic acid is not resonance stabilized, I disagree that resonance is why acetic acid is a stronger acid. Note the pKa for pyrrole, 17.5. Even though a pair of non-bonded electrons are present on the nitrogen atom, the newly formed electrons of the anion are not participating in the resonance structure. The anion electrons are orthogonal to the pi-electrons, yet the pKa is much lower than a simple amine (~35).

The poster made a second point about bond length and bond strength. Bond strength arguments are frequently made in a rather casual manner. If you were to find bond strength data, it refers to homolytic bond strength. If homolytic bond data were used, then a completely different and incorrect prediction of acidities would result. Acidity is a heterolytic bond cleavage. Secondly, the bond length data does agree with heterolytic bond strengths. Here is why. If you compare the bond lengths of CH4, NH3, H2O, and HF, HF has the shortest bond. It also is the most acidic. The key to understanding acidity is the proton-electron pair distance. We cannot measure that directly, but if the inverse square law applies (and it does), then the greater the proton-electron pair distance, the weaker the bond. Therefore, we know as the electrons are pulled closer to the nucleus, they are pulled away from the proton resulting in a weaker bond. Additionally, the bond length tells us the distance between the proton and oxygen nucleus in this case. The charges of both are the same, therefore the closer the proton is to the nucleus, the greater the repelling force. This repelling force is complimentary an increased proton-electron pair distance and results in the increase in acidity. 
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Babcock_Hall

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Re: COOH Very Acidic
« Reply #23 on: March 21, 2012, 03:37:32 AM »

I would consider the acidity of ethanol versus ethanethiol.  If inductive effects were the entire story, ethanol would be more acidic, but ethanethiol is more acidic by several orders of magnitude.  The same is true of phenol and thiophenol.  I would explain this by invoking the larger volume of sulfur over oxygen, which allows the negative charge to spread out more.

Also, one example from bioorganic chemistry comes to mind with respect to resonance.  The alpha hydrogen of an amino acid is not very acidic.  However, when one makes an imine with pyridoxal phosphate, the alpha hydrogen is easily removed.  One can draw five resonance structures that delocalize the electron throughout the pyridinium ring. 
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orgopete

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Re: COOH Very Acidic
« Reply #24 on: March 21, 2012, 08:35:46 AM »

This topic won't go away. It is interesting how many different ways we can argue inductive effects are not inductive effects.
I would consider the acidity of ethanol versus ethanethiol.  If inductive effects were the entire story, ethanol would be more acidic, but ethanethiol is more acidic by several orders of magnitude.  The same is true of phenol and thiophenol. 
Ethanthiol is more acidic. Sulfur is more electron withdrawing (see discussion here). It is an inductive effect.
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I would explain this by invoking the larger volume of sulfur over oxygen, which allows the negative charge to spread out more.
Invoking this argument is only necessary if one believes oxygen is more electron withdrawing than sulfur. This Gaussian sphere argument is questionable. Electrons are negative. There are 18 electrons surrounding sulfur and 10 around oxygen. I do not conclude the electron density surrounding sulfur is less. None the less, this kind of argument is paradoxical to the facts.

I much prefer a simple Coulomb's Law model. The further a proton is from an electron (pair), the weaker the attraction. The larger a nuclear charge, the greater the repulsion. The greater the nuclear charge, the greater the attraction to its electrons. Sulfur has a greater nuclear charge, greater electron pull, and greater nucleus-proton repulsion. (I concede this simple model assumes minimal electron-pair interactions. As atoms become larger, it becomes more complicated. I believe the additional interactions are similar to or the same as the resonance effects we see in benzene.)
Quote
Also, one example from bioorganic chemistry comes to mind with respect to resonance.  The alpha hydrogen of an amino acid is not very acidic.  However, when one makes an imine with pyridoxal phosphate, the alpha hydrogen is easily removed.  One can draw five resonance structures that delocalize the electron throughout the pyridinium ring.
 
If you take an amino acid, convert the amino group from sp3 to sp2, the acidity is increased. I thought I pointed that out.

Take pyrrole (pKa 17.5), make the anion, draw the resonance structures of the anion. I get one (of the anion). The non-bonded electrons are orthogonal to the pi-electrons.
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