Solubility & pKa: Intuitively Determine Compound Dissolution?

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In summary, the speaker is asking if there is an intuitive way to determine what compounds will dissolve in what solvents based on their relative pKa values. They mention the example of an ester dissolving in H2SO4 but not in NaOH or H20. The speaker also notes that there is a difference between dissolving and ionization constants. They suggest that a general rule for determining solubility based on pKa may not exist and that the pKa values can give an idea of the strength of acid/base reactions. They provide resources for further help.
  • #1
thunderfvck
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Yes. Hello. I am new. THis is a nice forum. Very nice website. Wish I would have found it sooner.

Okay, my question.
Without having to rely on memorizing solubility rules (specifically for organic compounds), is there a intuitive way to determine what will dissolve in what based on the relative pKa's of solvent/solute? eg. a compound dissolves in H2SO4(pKa~-4), but not in NaOH(~40) or H20(~15). The compound is an ester (~25). Help? Or shall I have a nice warm bath with some MEMORIZING bubbles...!?

...mesmerizing...memorizing. Ahh...7 am and I still got it.
 
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Bath time --- sorry, a single, general rule would be too nice for mother nature to tolerate.
 
  • #3
There is a distinction between dissolving and ionization constants, for example when you are talking about a salt dissolving in water yet not leading to a change in pH. That is dissolving usually is relevant to water.

As you may know by now Pka is related to Ka which is the ionization constant commonly used for Arrehnius acids. Ka has to do with the equilibrium concentration of hydrogen ions. In this sense a base will have a high Pka. Thus in general you can say that a compound with a very low pKa will react with one with a high pKa since this is referring to a strong acid/base reaction.

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1. What is the difference between solubility and pKa?

Solubility refers to the ability of a substance to dissolve in a solvent, while pKa is a measure of the acidity or basicity of a compound. Solubility is affected by the chemical structure and polarity of a compound, while pKa is affected by the strength of its acidic or basic functional groups.

2. How does pKa affect solubility?

The higher the pKa of a compound, the more acidic it is and the less likely it is to dissolve in water, which is a polar solvent. Basic compounds with lower pKa values are more likely to dissolve in water. However, pKa is not the only factor that affects solubility, as other factors such as temperature and the presence of other solutes can also play a role.

3. Can you intuitively determine a compound's dissolution based on its pKa?

Not necessarily. While generally a higher pKa value means lower solubility, there are exceptions based on the specific chemical structure and other factors. It is always best to experimentally determine a compound's solubility rather than relying solely on pKa values.

4. How do I determine the solubility of a compound based on its pKa?

The best way to determine a compound's solubility is to perform a solubility test in the desired solvent. However, you can also make some assumptions based on the compound's pKa. If the pKa is significantly higher or lower than the pH of the solvent, the compound is likely to be insoluble. If the pKa is close to the pH of the solvent, the compound may be partially soluble.

5. Can solubility and pKa be used to predict the effectiveness of a drug?

Solubility and pKa are important factors to consider in drug development, as they can affect a drug's absorption, distribution, and effectiveness. However, they are not the only factors to consider and other properties such as stability, metabolism, and toxicity must also be taken into account.

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