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I have a doubt about solubility: a molecule is polar when it has a dipole moment that is ≠ 0. Very often it is said that like dissolves like, that is, that polar substances dissolve in polar solvents, and apolar substances dissolve in apolar solvents. But this is not always true: for example, 1 - hexanol is polar (its dipole moment is ≠ 0), but it is not soluble in water. This means that polarity does not imply water solubility. That said, what do you rely on to roughly predict solubility?

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    $\begingroup$ Also organic substances are better dissolved in organic solvents and water definitely isn't a organic solvent. $\endgroup$
    – Volpina
    Apr 4, 2023 at 18:28

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Being polar in context of whole molecules or chemical bonds means rather significant than nonzero dipole moment, while nonpolar means rather small than zero dipole moment.

Comparing dipole moments makes sense for molecules of similar sizes. Consider there are polar=hydrophilic atomic groups and nonpolar=lipophilic atomic groups. Dominating group characters determines overall hydro/lipophility of molecules.For n-alkanecarboxylic acids, hydrophilic group is the same, but there are growing lipophilic n-alkyl chains, progressively turning character from hydrophilic (formic acid) to lipophilic (stearic acid).


A dipole moment does have the strong chemical meaning, but it cannot be taken mechanically as the only decisive parameter. One has to consider the molecular context. You may think about nonpolar parts of molecules to be effectively attracted to nonpolar solvents while polar parts to polar solvents. With the polar part attraction significantly stronger, so nonpolar lipophilic parts must be bigger/longer to get to fuzzy boundary between lipophilic and hydrofilic overall behaviour. CO2 has zero dipole moment, but has symmetrical polar bonds, so it has quite good water solubility.

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  • $\begingroup$ so, generally, it is correct to evaluate the ratio between polar and non-polar fractions in a molecule. With this, It Is possible to evaluate if a molecule is effectively polar or non polar and where It Will be soluble. This is regardless of the presence or absence of a dipole moment, which is probably more important for estimating the polarity/solubility of small molecules, such as chloroform, water, carbon dioxide or CCl4, etc.. $\endgroup$
    – Luckenberg
    Apr 6, 2023 at 17:25
  • $\begingroup$ @Luckenberg It can have a polar end and a nonpolar end, like classical soap, or most of surfactants. $\endgroup$
    – Poutnik
    Apr 6, 2023 at 17:38
  • $\begingroup$ What are you referring to? (In relation to my comment). $\endgroup$
    – Luckenberg
    Apr 6, 2023 at 17:49
  • $\begingroup$ @Luckenberg "...It Is possible to evaluate if a molecule is effectively polar or non polar..." $\endgroup$
    – Poutnik
    Apr 6, 2023 at 19:17
  • $\begingroup$ oh sure, we have to evaluate the balance between polar and non-polar Groups in a molecule. So, does what i wrote in the comment above make sense (more or less)? $\endgroup$
    – Luckenberg
    Apr 6, 2023 at 21:39
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The longer the chain of carbon atoms, the less influence on polarity the alcohol moiety has. Methanol, ethanol, and 1-propanol are all miscible. 1-butanol is soluble ~70g/L, 1-hexanol: ~6g/L and 1-heptanol: ~0.1 g/L.

Each additional carbon link in the chain reduces solubility by a factor of more than 10.

However, add some more alcohol groups, and solubility increases. For example, 1,3-butanediol is miscible with water, and 1,6-hexanediol is soluble at 500g/L.

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  • $\begingroup$ So polarity does not depend (exclusively) on the dipole moment of a molecule, but more on the number of polar groups that appear in it and, therefore, more on the ability of the same to form hydrogen bonds? So, a molecule can have a dipole moment = 0, but still be water soluble (and polar). Probably the dipole moment has a more "physical" than "chemical" meaning (?) $\endgroup$
    – Luckenberg
    Apr 4, 2023 at 22:02
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    $\begingroup$ The diols with alcohol groups at both ends have negligible polarity, but are more soluble than those with single alcohol group... Your conclusion seems reasonable. $\endgroup$ Apr 5, 2023 at 18:18

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