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.

  • $\begingroup$ Cellulose is not soluble.. Maybe you should change your question title to something like this "Why solubilities of ether, cellulose, vitamin A and CO2 are these." No one with your same doubts can find your question with a title so generic... $\endgroup$ – G M Jan 11 '14 at 10:04

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.

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

                           enter image description here

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:

                           vitamin a electrostaic potential with Avogadro

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.

                                       carbon dioxide electrostaic potential with Avogadro

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 with Avogadro

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.


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