question about water solubility and carboxylic acids

[Meredith92]

In the first organic chemistry chapter in TBR, in passage three, there is a question about water solubility... I'm confused why something that is ionic/polar is more soluble in water than something that can hydrogen bond

20. Which of the following compounds is MOST soluble in water?
A) CH3CH2CO2H
B) CH3CH2CO2K
C) CH3CH2CH2CH2CH2CO2H
D) CH3CH2CH2CH2CH2CO2K

The answer key says:
To be soluble in water, a compound must be either charged or polar. Because choices B and D are ionic they are better in this regard than choices A or C. The organic tail is smaller in choice B, so it dissolves into water more readily than choice D. Pick choice B.


I understand that the smaller organic tail makes it dissolve better.. but why isnt A the better choice? Isnt something that can hydrogen bond more soluble in water?

I know that carboxylic acids can form dimers with each other.. but would this affect how well it dissolves in water??
I would greatly appreciate any help! Thank you in advance!

---

Members don't see this ad.

 

[BestDoctorEver]

In the first organic chemistry chapter in TBR, in passage three, there is a question about water solubility... I'm confused why something that is ionic/polar is more soluble in water than something that can hydrogen bond

20. Which of the following compounds is MOST soluble in water?
A) CH3CH2CO2H
B) CH3CH2CO2K
C) CH3CH2CH2CH2CH2CO2H
D) CH3CH2CH2CH2CH2CO2K

The answer key says:
To be soluble in water, a compound must be either charged or polar. Because choices B and D are ionic they are better in this regard than choices A or C. The organic tail is smaller in choice B, so it dissolves into water more readily than choice D. Pick choice B.


I understand that the smaller organic tail makes it dissolve better.. but why isnt A the better choice? Isnt something that can hydrogen bond more soluble in water?

I know that carboxylic acids can form dimers with each other.. but would this affect how well it dissolves in water??
I would greatly appreciate any help! Thank you in advance!

The explanation says because B is ionic...Will dissociate easily in water.

 

[DNA]

Think in terms of the actual charges. An ionic compound will dissociate to some extent in water leaving an actual charge on the remaining ions. Water on the other hand is a molecule that while polar, does not have an actual charge. Sure, the hydrogen is partially positive due to the pull of electrons from hydrogen to oxygen (partially negative). This like you said allows for hydrogen bonding which is a very favorable interaction. But ultimately, water is more attracted to the more charged species which in this case is the less hydrophobic ionic compound. Because they are more attracted physically, they are therefore more soluble when dissolved together.

In simple terms, ask yourself this:

Would the partial negative charge on oxygen be more attracted to the partial positive charge of a Hydrogen (on the alcohol chain), or would it be more attracted to an actual positive charge of a dissociated metal cation (on an Ionic Compound)? If it helps, think in terms of Coulomb's Law to determine which would be more attracted to each other (which would the electric force be stronger)?

 

[Meredith92]

Thanks that was very helpful! I think I had trained myself to always think of hydrogen bonding as the most powerful/ important intermecular force (esp in terms of boiling points) but I guess with solubility it's a bit different. In water having a charge is more helpful in terms of solubility than hydrogen bonding.