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pisluca99
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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 said: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.
The reason is pKa 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 said: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 said:The reason is pKa 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.
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.)pisluca99 said: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?
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.pisluca99 said: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?
Thanks!snorkack said: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
The reactionspisluca99 said: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-
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:snorkack said: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.