Solutions, solvents and solutes

Question

In all the chemistry books and eBooks it says that "like dissolves like"... but when it comes to some molecules like ether, cellulose, vitamin A and $\ce{CO_2}$, it confuses me.

If we take ether, it is true that it will not form hydrogen bonds with water when it dissolves, but they have polar bonds between $\ce{O}$ and $\ce{C}$ and it's not in a linear form like $\ce{CO_2}$ where the dipoles cancel out.

Vitamin A also has $\ce{-OH}$ group but it still is hydrophobic. In some book, I read that this is because it has more hydrophobic molecules or that they have more $\ce{C-H}$ bonds than $\ce{-OH}$. If it is so, octadecanol and nonadecanol also have way more $\ce{C-H}$ bonds than $\ce{-OH}$ groups, so why is it still soluble? $\ce{CO_2}$ is dipolar — the dipoles cancel out, so why is it still soluble? Cellulose also has a bunch of $\ce{-OH}$ but still it is soluble. I am very confused by this.

Answer 1

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When you make assumption for guessing the solubility of the molecule in some solvent you should consider the whole molecule, his structure and his size.

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We can start with the ether, in the below figure (from wikipedia the one at right ) is shown a diethylether molecule.

                           

You can see that is quite a non-polar molecule (charge 0,green, in the most part of the surface) so this explains why has a low solubility in a polar solvent like water. Same thing for Vitamin A:

                           

The most part of the molecule is not polar. Only one $-OH$ can't make the difference. And so is for 1-Octadecanol that has a solubility in water of $1.1 \times 10^{-3} mg/L$ and Nonadecyl alcohol (1-nonadecanol) seems something like $4.8 \times 10^{-3} mg/L$ both up to my references have very low solubility in water (where did you find that were not?). For the $CO2$ is true what you have stated it has not a very strong polarity (but in fact it has a little bit!) because is a linear molecule.

                                       

This last example let me introduce to an important factor that influence the solubility: molecule size. Little molecule are more easily solvatated by solvent molecule, this bring us to the case of cellulose:

                  

Cellulose is very hydrophilic and so should be soluble in water, but this doesn't happen because cellulose is a polymer (in figure is show his monomer) that can have a molecular weight very hight. Polymers may swell but can't be solubilize if their chains are bonded with chemicals bonds until the bonds are break.