Solubility trends in acids and bases

[F.Underwood]

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Hi

I understand that solubility and polarity are highly dependent. But now I am trying to figure out how acidity/basicity plays a role in solubility. For example, why are amide and nitro groups not soluble in dilute aq acid or base?

Thanks

 

[chenzt]

Like dissolves like applies to polarity, not acidity/basicity. A non-polar base will never dissolve in polar acid/base. Amide and nitro compound, isn't really that polar, especially when the R group is long and fatty.

 

[inasensegone]

Like dissolves like applies to polarity, not acidity/basicity. A non-polar base will never dissolve in polar acid/base. Amide and nitro compound, isn't really that polar, especially when the R group is long and fatty.

Heheh... long and fatty.

Hi

I understand that solubility and polarity are highly dependent. But now I am trying to figure out how acidity/basicity plays a role in solubility. For example, why are amide and nitro groups not soluble in dilute aq acid or base?

Thanks

First of all... I'm not sure what you mean by "solubility and polarity are highly dependent?"

Solubility, as chenzt pointed out, has more to do with like dissolves like, than anything else. For example, a highly polar substance will likely be quite soluble in a highly polar medium like water. Similarly, a nonpolar substance (like fats) will likely be quite soluble in a non-polar medium like oil (or liquid butane or whatever).

Also, amides are VERY weak bases. Nitro groups are actually electron withdrawing when bonded to a molecule so they would increase the acidity of a substance if anything, however the presence of the nitro group alone does not make it an acid.

The question in the picture you posted is asking about the likely functional groups present on the unknown compound. Well if the compound is soluble in both an acid and a base, it may very well have aspects of both on the molecule (an amine and a carboxylic acid). This is similar to the amphipathic qualities of the phospholipid bilayer. The negatively charged phosphate group makes it hydrophilic and water soluble, and the hydrophobic tails (fatty acid tails) make the inner portion of the bilayer nonpolar. In the case of your question, which two functional groups would make it soluble in an acid and a base? A base and acid functional group respectively.

So far I've noticed quite a few questions from AAMC that are a variation on this. Bottom line is that an acid is very soluble in a basic liquid, and a base is very soluble in an acidic medium. Let's take solid Ca(OH)2 and aqueous HCl acid for example. Normally, Ca(OH)2 isn't super soluble in water like NaOH is... however if you put it in aqueous HCl, some of it dissolves, the OH + H combine to form water, and then the equilibrium shifts according to Le Chatelier's principle to keep dissolving more and more:

Ca(OH)2 + 2HCl ---> 2 H2O + Ca ion (aq) + 2Cl ion(aq)

 

[chenzt]

Ca(OH)2 + 2HCl ---> 2 H2O + Ca ion (aq) + 2Cl ion(aq)

This is a reaction, not solvation is it? Acids dissolve in bases (or vise versa) because the species reacts via protonation/deprotonation and becomes charged in the aq solution, which is perfect for water to dissolve. Amide and Nitro can't readily take in or give up a hydrogen, therefore it can't become a charged molecule and the tiny polarity it has alone isn't enough to dissolve it.

 

[inasensegone]

This is a reaction, not solvation is it? Acids dissolve in bases because the specie is charged in the aq solution, which is perfect for water to dissolve. Amide and Nitro doesn't have a hydrogen to readily give up, therefore it can't dissolve on the tiny polarity it has alone.

Yes true... however the same can be applied to an amine:

RNH2 + HCl ---> RNH3+(aq) + Cl- (aq)

Or a carboxylic acid:

RCOOH + OH- ---> RCOO- (aq) + H2O

I guess it would be more accurate to say that the solids are dissociating into the liquid... however in the AAMC question, the above reactions are what are actually taking place.

EDIT: Ahhh I see what you mean... to clarify for the OP: the amine is being protonated to its conjugate base and is therefore made positive, polar, and therefore more soluble in the polar medium. The Carboxylic acid is dissociated into its carboxylate, and is therefore made more polar and therefore more soluble.