Acidity and basicity of some compounds

[pisluca99]

Why do some compounds, such as amides, ketones, aldehydes, ethers, behave neither as acids nor as bases in water? Does it depend on the difference between the pKa/pKb of water and the pKa/pKb of the compounds themselves?

 

[Borek]

To some extent your guess is correct, very weak acids/bases "hide" behind water, but broadly speaking being an acid or base is in no way a must. Compound can be (almost) perfectly inert when it comes to acid/base reactions, no matter how you define them.

 

[pisluca99]

To some extent your guess is correct, very weak acids/bases "hide" behind water, but broadly speaking being an acid or base is in no way a must. Compound can be (almost) perfectly inert when it comes to acid/base reactions, no matter how you define them.

yes, essentially I was thinking about why, for example, an amine behaves as a base, therefore it actually increases the pH of the water when inserted here, while a ketone, even if it has free electron doublets on oxygen and theoretically, even if it can behave as a base it does not modify the pH, therefore it behaves like a "neutral" compound, i.e. it does not participate in acid-base equilibria.
Or, even considering an amide, in theory it can behave as both an acid and a base, but in reality it does not..

 

[snorkack]

yes, essentially I was thinking about why, for example, an amine behaves as a base, therefore it actually increases the pH of the water when inserted here, while a ketone, even if it has free electron doublets on oxygen and theoretically, even if it can behave as a base it does not modify the pH, therefore it behaves like a "neutral" compound, i.e. it does not participate in acid-base equilibria.
Or, even considering an amide, in theory it can behave as both an acid and a base, but in reality it does not..

The reason is pK_a range. A ketone is a considerably weaker base than water, so it does not modify the pH of water. The pKa of H₃O⁺ is -1,7, but the pKa of protonated ketones is in the range of -6 to -10. Amides are far stronger acids than amines, but still weaker than water. They are also about as strong bases as water.

 

[pisluca99]

The reason is pK_a range. A ketone is a considerably weaker base than water, so it does not modify the pH of water. The pKa of H₃O⁺ is -1,7, but the pKa of protonated ketones is in the range of -6 to -10. Amides are far stronger acids than amines, but still weaker than water. They are also about as strong bases as water.

so what you mean is that since the ketone protonated on oxygen is a stronger acid (lower pKa) than the hydronium ion, then water will be a stronger base than the ketone, so accepting any proton would be the water. However, since there are no protons on the ketone's oxygen, no reaction takes place. Similarly, water is a stronger acid than the hydrogens of the CH2/CH3 groups of the ketone, so water would tend to transfer a proton to these groups, but since there are no free doublets, absolutely no acid-base reaction takes place . However, if amides are less strong acids than water, this means that water can actually deliver a proton to nitrogen, liberating OH-. So should an aqueous solution of amide be basic?

Reference: https://www.physicsforums.com/threads/acidity-and-basicity-of-some-compounds.1048810/

 

[symbolipoint]

Why do some compounds, such as amides, ketones, aldehydes, ethers, behave neither as acids nor as bases in water? Does it depend on the difference between the pKa/pKb of water and the pKa/pKb of the compounds themselves?

They do not because they do not. Presence of hydrogen ions (hydronium ions?) is done in water. I think I am a little off here, since there are some acids in an anhydrous form, like "Glacial" Acetic Acid, and like Oleum. But some of the other general classes of compounds you mentioned by themselves, not dissolved in water , may not form loose hydrogen ions. (I did not yet read what Borek said.)

 

[snorkack]

However, if amides are less strong acids than water, this means that water can actually deliver a proton to nitrogen, liberating OH-. So should an aqueous solution of amide be basic?

No, it means that aqueus solution of an amide´s salt would be not merely basic but strongly basic and the amide´s salt should hydrolyze completely.
Like
NaCl+H₂O <<< NaOH+HCl - starting on the right, reaction goes to completion - starting from the left, no reaction, so no effect on pH
NaOCOCH₃ +H₂O <<> NaOH+HOCOCH₃ - starting from the left, this is an equilibrium reaction, so sodium acetate hydrolyzes at a minority but detectable amount and makes water weakly basic
NaNHCOCH₃+H₂O >>> NaOH+H₂NCOCH₃ - starting from the left, acetamide sodium salt hydrolyzes completely, so starting from the right, no reaction
If you react acetamide water solution with an excess of a very strong base, like NaNH₂, the statement "amide is less strong acid" means that first it is water that reacts
HOH+NaNH₂>>>NaOH+NH₃
and only after water is exhausted will the amide (a weaker acid) react with excess of strong base
CH₃CONH₂+NaNH₂>>> CH₃CONHNa+NH₃

 

[pisluca99]

No, it means that aqueus solution of an amide´s salt would be not merely basic but strongly basic and the amide´s salt should hydrolyze completely.
Like
NaCl+H₂O <<< NaOH+HCl - starting on the right, reaction goes to completion - starting from the left, no reaction, so no effect on pH
NaOCOCH₃ +H₂O <<> NaOH+HOCOCH₃ - starting from the left, this is an equilibrium reaction, so sodium acetate hydrolyzes at a minority but detectable amount and makes water weakly basic
NaNHCOCH₃+H₂O >>> NaOH+H₂NCOCH₃ - starting from the left, acetamide sodium salt hydrolyzes completely, so starting from the right, no reaction
If you react acetamide water solution with an excess of a very strong base, like NaNH₂, the statement "amide is less strong acid" means that first it is water that reacts
HOH+NaNH₂>>>NaOH+NH₃
and only after water is exhausted will the amide (a weaker acid) react with excess of strong base
CH₃CONH₂+NaNH₂>>> CH₃CONHNa+NH₃

Thanks!
So, following the same reasoning:
R-CONH2 + H2O
pKa 17 --- 15,7
Water has a lower pKa, so it Is water that behaves as an acid:

R-CONH2 + H2O = R-CONH3+ + OH-

Since OH- Is an enormously stronger base than R-CONH2 and since R-CONH3+ Is an enormously stronger acid than H2O, the equilibrium is greatly shifted to the left, so that the pH is almost practically unchanged.
The same goes for any other compound. Right?

 

[snorkack]

Thanks!
So, following the same reasoning:
R-CONH2 + H2O
pKa 17 --- 15,7
Water has a lower pKa, so it Is water that behaves as an acid:

R-CONH2 + H2O = R-CONH3+ + OH-

The reactions
R-CONH₂ =R-CONH^- + H⁺ pKa 17
and
R-CONH₃⁺=R-CONH₂+ H⁺
are different reactions with different pKa. But we know tha yes, what is important is that R-CONH₃⁺ is far stronger acid than H₂O.

 

[pisluca99]

The reactions
R-CONH₂ =R-CONH^- + H⁺ pKa 17
and
R-CONH₃⁺=R-CONH₂+ H⁺
are different reactions with different pKa. But we know tha yes, what is important is that R-CONH₃⁺ is far stronger acid than H₂O.

Therefore it is not correct to compare the pKa to understand who acts as an acid and who as a base, given that pKa=17 refers to the reaction:
R-CO-NH2 + H2O = R-CO-NH3+ + OH-
And pKa=15,7 refers to:
H2O + H2O = H3O+ + OH-, right?

therefore, amides return a solution whose pH is practically unchanged simply because the basic or acid hydrolysis equilibrium is considerably shifted to the left (given the extremely small pKa/pKb) and the amide is ionized in a negligible amount.
Conversely, for a carboxylic acid, the acid hydrolysis equilibrium is always shifted to the left, but less than in the case of the amide (given its pKa), so the dissociation becomes more consistent and for this reason a variation of pH.
If instead we consider the basic hydrolysis of a carboxylic acid, we would fall again in the case of the amide, because the pKb of a carboxylic acid is very low. Quite right?

Last doubt.
If we had an acid-base reaction that takes place between two species of which we do not know which acts as a base and which as an acid, how can we predict from which species to which species the proton travels if it is not correct to use the relative pKa?

PS. My book (Bruice) uses pKa to determine who acts as acid. But actually It doesn't make much sense if theese pKa refer to different reactions.