[kashya]

Hi there,

Wondering if you guys could point me in the right direction with this one.

I have two compounds, one with aLogP of 2.1 and another of 1.7. I'm asked if I partitioned each between Octanol and water, which compound would have a higher concentration in the water layer?

Am I right in thinking I need to examine then which will have higher solubility in water? I know the details of the two compounds/structure etc, do I need to examine physical structure in terms of hydrophobicity, molecular mass..? I'm at bit of a loss!

The two compounds are Pantoprazole and Omeprazole for the curious.

Thanks!

[mjc123]

What is the definition of aLogP?

[kashya]

ALogP is the partition coefficient which is the ratio of a compound’s concentration in octanol to its concentration in water when the phases are at equilibrium

[mjc123]

Then isn't the answer to your question obvious?

[kashya]

Well I realize the mathematics would indicate that the compound with value 2.1 would have the lower concentration in water IF both compounds had the same concentration in octane, however there is nothing there to suggest that

Am I missing the point entirely or overthinking the question?

[kashya]

Say for example:

Omeprazole aLogP = 4. The ratio of solute in octane to solute in water is 8/2
Pantoprazole alogP = 3 The ratio is 12/4 or it could be 6/2.. or 0.3/0.1

So the values tell me nothing. Where do I look? Structure? Trying to determine what exactly this question wants me to do

[mjc123]

Well, I can only assume they mean the same total amount of each compound in solution (e.g. 10 mmol in 500mL octanol/500 mL water). Otherwise you are right, it depends how much you have and the question is unanswerable. If you have a lot of the less water-soluble one, there will be more of it in the water than the other, if there is only a little of that.

[Furanone]

Say for example:

Omeprazole aLogP = 4. The ratio of solute in octane to solute in water is 8/2
Pantoprazole alogP = 3 The ratio is 12/4 or it could be 6/2.. or 0.3/0.1

So the values tell me nothing. Where do I look? Structure? Trying to determine what exactly this question wants me to do

If aLogP = 4 the ratio is not 8 to 2 and if aLogP = 3 then the ratio is not 12 to 4 or 6 to 2, since it is in a logarithmic scale.

An aLogP of 0 means 50% of the compound partitions in the aqueous phase and 50% in the hydrophobic (octanol) phase.
An aLogP of 1 means 90% of the compound partitions in octanol and 10% in water.
An aLogP of 2 means 99% of the compound partitions in octanol and 1% in water.
An aLogP of 3 means 99.9% of the compound partitions in octanol and 0.1% in water.
An aLogP of 4 means 99.99% of the compound partitions in octanol and 0.01% in water.

Likewise..
An aLogP of -1 means 90% of the compound partitions in water and 10% in octanol.
An aLogP of -2 means 99%  of the compound partitions in water and 1% in octanol.
An aLogP of -3 means 99.9%  of the compound partitions in water and 0.1% in octanol.
An aLogP of -4 means 99.99%  of the compound partitions in water and 0.01% in octanol.

[sjb]

An aLogP of 0 means 50% of the compound partitions in the aqueous phase and 50% in the hydrophobic (octanol) phase.
An aLogP of 1 means 90% of the compound partitions in octanol and 10% in water.
An aLogP of 2 means 99% of the compound partitions in octanol and 1% in water.
An aLogP of 3 means 99.9% of the compound partitions in octanol and 0.1% in water.
An aLogP of 4 means 99.99% of the compound partitions in octanol and 0.01% in water.

Minor point, but an aLogP of 1 means 90.9090...% of the compound partitions in octanol and 9.09090...% in water. Slight difference and not really relevant to this question.

[pgk]

The question can be put, in another way:
LogX = 2.1
LogY = 1.7
Comparing X and Y, which one has the higher numerical value?
How do these higher or lower numerical values, reflect on the partition coefficient?
Which is the relationship of between the partition coefficient and the water solubility?
Which is the relationship of between the partition coefficient and the hydrophobicity?

[pgk]

For your own education:
Water solubility (S) is not only a question of logP, but also is a question of melting point (mp) because the crystal lattice must be destroyed during solvation, too.  The relationship between the water solubility and the partition coefficient, follows the equation:
LogS = a – bLogP + c(mp)
Where a,b and c are constants and their values depend on the rigidity or the flexibility of the molecule
(Both Pantoprazole and Omeprazole are rigid molecules.Why?).
Please, search the literature in order to find the units of the equation and the values of the a,b,c constants.
Question 1: The above equation is valuable for non-ionized molecules. By which modification, this equation can be valuable for ionized molecules?
Question 2: The modified equation would be pH-depended or not?

[snorkack]

For your own education:
Water solubility (S) is not only a question of logP, but also is a question of melting point (mp) because the crystal lattice must be destroyed during solvation, too. 

And a question of boiling point as well! Obviously even highly miscible solutes must have finite solubility at any conditions where they are not liquid.

At 1013 mbar, hydrogen fluoride boils at 19,51 Celsius.
What is the solubility of hydrogen fluoride in water at 20,00 Celsius?
And what s octanol/water partitioning coefficient of HF?

[pgk]

Solubility is a question of vapor pressure, rather than boiling point.
The term “octanol/water partitioning coefficient” is applied to compounds that are soluble in n-octanol. If a given compound is not soluble in octanol but soluble in another organic solvent that is not miscible with water, the hypothetical  “octanol/water partitioning coefficient” can be calculated, as follows:
logP (o/w) = alog P(solvent) ± b
The negative or positive value of the b constant depends on the proton donor or proton attractive character of the compound's molecule.
Hydrogen fluoride is gas and therefore, the term “octanol/water partitioning coefficient” is not applicable. For gases and highly volatile compounds, the partition coefficient is identical with the (dimensionless) Henry’s law constant. Consequently, the water solubility of gasses and highly volatile compounds, follows the equation:
S = p/k
Where, p is the vapor pressure at a given temperature and k is the (dimensionless) Henry’s law constant.
That’s why the water solubility of gasses increases, by decreasing the temperature.
For further reading on the issue:
Albert Leo, Corwin Hansch, David Elkins: “Partition coefficients and their uses”, Chemical Reviwes, 71(6), 525–616, (1971)
http://pubs.acs.org/doi/abs/10.1021/cr60274a001
http://www.bioparadigma.spb.ru/files/Leo-1971-Partition.coefficients.and.their.uses.pdf
Exceptionally, hydrogen fluoride is miscible with water at any proportion, due to formation of strong hydrogen bonds.
---HOH----HF---HF----HOH----
In practice, hydrogen fluoride forms “polymers” in water (HF)n, where the hexamer is  predominant (HF)6.

[pgk]

Question 1: Apart the oxygen and the fluorine, another element forms hydrogen bonds. Which one?
And which derivative gas of that element therefore, ought to be miscible with water at any proportion?

[pgk]

Question 2: Hydrogen bond plays an important role in life. Therefore, water is the solvent of the cellular apparatus, in terrestrial fife. Thus, apart the water and the hydrogen fluoride which compound could also be a hypothetical solvent of the cellular apparatus, in extraterrestrial life?