Solubility

[reising1]

Advertisement - Members don't see this ad

How can I remember this knowledge, or must I simply memorize it?

Carboxylic acids are soluble in dilute base.
Amines are soluble in dilute acid.
Amides and nitro compounds are not soluble in dilute acid or base.

 

[navho77]

If given a choice between those compounds, it should be reasonable that carboxylic acids are soluble in dilute base because they are weak acids and amines should be soluble in dilute acid because they are weak-moderately strong bases. Amides and nitro compounds, not having any especially strong proton donating/accepting/electron pair donating/electron pair accepting properties, would not be expected to interact with acids or bases to the same degree as carboxylic acids or amines.

 

[reising1]

So weak acids are soluble in weak bases and vice versa?

Are strong acids soluble in weak bases? Are strong bases soluble in weak acids? Are strong bases soluble in strong acids? And vice versa?

 

[reising1]

And also, why don't amides have proton donating/receiving properties? It seems like it would be a weak base because the negative charge could sit on the nitrogen and be in resonance with the oxygen from the carbonyl.

 

[navho77]

Yes, all combinations of acids/bases are soluble.

First, if amide were acting as a base, it would acquire a positive charge, not a negative charge (add a proton to the nitrogen and calculate the formal charge). I would say the reason this does doesn't happen is because the lone pair on the nitrogen is already in resonance with the carbonyl function. This possibility of tautomerism gives the lone pair less of an incentive to grab a proton, especially in comparison to an amine, which lacks the immediate resonance stabilization available to the amide.

 

[reising1]

Sorry, I meant, if it acted as an acid, it would have a negative charge, which could then resonate with the neighboring carbonyl oxygen. Why can't this happen?

 

[reising1]

Yes, all combinations of acids/bases are soluble.

Just to further clarify: all bases (weak or strong) are soluble with all bases (weak or strong) as well? (and same for acids?)

 

[navho77]

The issue is that the nitrogen is already holding onto its protons very tightly. Sure, if something with a very high pKa came along, it could theoretically steal a proton and there would be some resonance stabilization to provide some respite, but that would only occur in an extreme case. The amide function is just extraordinarily stable as is. That's why we need enzymes to efficiently break peptide bonds (acid hydrolysis doesn't get the whole job done thoroughly enough on its own).

 

[navho77]

This should be the case, at least for MCAT purposes.

 

[reising1]

That makes a lot of sense, when you brought up peptide bonds.

Are acids soluble with acids (regardless of strength) and same for bases?

 

[navho77]

That makes a lot of sense, when you brought up peptide bonds.

Are acids soluble with acids (regardless of strength) and same for bases?

Yes, this should be the case for MCAT purposes.

 

[reising1]

Great, thanks!