I was wondering as to why, e.g. $\ce{CH3CH2CH2CH2OH}$ and $\ce{(CH3)3COH}$, do not have the same solubility in water, considering they both have the same molecular formula.

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    $\begingroup$ Of course it's different, like every property they have, only molecular mass is same. $\endgroup$ – Mithoron Nov 28 '17 at 16:04
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    $\begingroup$ What is attached to the C-O-H "part" has different surface areas. $\endgroup$ – MaxW Nov 28 '17 at 16:04
  • $\begingroup$ @Mithoron Yeah I do see why they're different. But I wanted to know why are they different? Like an actual reason $\endgroup$ – Hatem Chalak Nov 28 '17 at 16:19
  • $\begingroup$ While they have the same formula, the molecules have different shapes, which means that they are different in almost any important way with respect to how they interact with other entities. The electron cloud is different. The rotation and vibration modes are different. The reactivities are different. The 'actual reason' that they are different is because they are different. $\endgroup$ – Jon Custer Nov 28 '17 at 16:30
  • $\begingroup$ Your last question in comments is bordering on philosophical. They're different because they're not the same. We look different because we're not the same person. Not sure how useful it is to ask why... $\endgroup$ – Zhe Nov 28 '17 at 16:38

In the most general terms: Molecules consist of nuclei and a surrounding electron cloud. That cloud is dependent on molecular charge and geometry - so it will be different for two different isomers. It follows that its interaction with the nuclei and electron cloud of a third molecule, say, water, will be different.

More specifically, one will find that properties such as

  • polarizability (related to van-der-Waals forces)
  • bond dipoles (related to H-bonds and static dipole interactions)
  • effective electronegativity (related to H-bonds and static dipole interactions)

On a very basic level, any solvation involves the formation of solvent shells. When varying the shape and surface of the solute, those shells will change as well. One will then find shells that work well and others that work less well, influencing the solubility.


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