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Why is benzoic acid well soluble in benzene (and other unpolar solvents, see e.g. wikipedia). What causes the hydrogen bonds in the solid to break? Something (i.e. polar and ionic substances) is soluble in water because of the hydration enthalpy with water, but i can see no similar cause for a solubility in benzene!

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    $\begingroup$ Are you saying benzoic acid dissociates in benzene? $\endgroup$ – Ed V Feb 13 at 17:42
  • $\begingroup$ @EdV So says wikipedia. en.wikipedia.org/wiki/Benzoic_acid $\endgroup$ – Karl Feb 13 at 23:07
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    $\begingroup$ @Karl The wiki article gives pKa values for benzoic acid in water and DMSO, but benzoic acid also dissolves in hexane and carbon tetrachloride. No pKa values are given for those. $\endgroup$ – Ed V Feb 13 at 23:15
  • $\begingroup$ @EdV Ah, sorry. My eye read "dissociates", but my brain got "dissolves". ;) I meant the solubility lists in the box on the right. Nothing in benzene can take the proton, so no. $\endgroup$ – Karl Feb 13 at 23:19
  • $\begingroup$ chemistry.stackexchange.com/questions/1289/… $\endgroup$ – Mithoron Feb 14 at 16:51
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Solubility is one of the most complicated phenomena in chemistry. The "why" part of science is extremely difficult to answer in general. As a result one can ask one millions questions as to why iodine is soluble is benzene? What is so unique about benzoic acid? So many organic acids dissolve in benzene including acetic acid. Even a trace of water is soluble in benzene. Everything dissolves everything, to a very very small extent (depends on your limit of detection).

So the point to keep in mind is that although benzoic acid has a carboxylate group, it is a very hydrophobic acid and it literally floats on water. Similarly, you can sublime benzoic acid easily, so the intermolecular bond in benzoic is apparently not that strong.

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  • $\begingroup$ So does the benzoic acid dissociates like that in water or does the benzene break the intermolecular force( van der waal) force of the benzoic acid molecules to distribute it i.e to dissolve it. $\endgroup$ – Shahroze Feb 14 at 13:55
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    $\begingroup$ benzoic acid cannot dissociate in benzene. If it did, it the solution would have some electrical conductivity (that is how you can check if a molecule is dissociating in a solvent, by measuring their electrical conductivity). So benzene must be overcoming the intermolecular attraction among benzoic acid "molecule". $\endgroup$ – M. Farooq Feb 14 at 16:29
  • $\begingroup$ But i can't understand how benzene is able to seperate the molecules of benzoic acid. What's the mechanism behind this? $\endgroup$ – Shahroze Feb 14 at 16:35
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    $\begingroup$ At a molecular level do not try to think in a literal way, as shown in animations that molecules are coming close, they pulling apart benzoic acid. Most of these animations on the false until and unless someone has done a real quantum calculation. The rest are cartoons. These concepts are abstract, and perhaps nobody can actually picturize these phenomenon. $\endgroup$ – M. Farooq Feb 14 at 17:05
  • $\begingroup$ I must say I strongly disagree to the sentiment in this answer. If I cannot think of a good reason why some substance A is well soluble in some other substance B, then either A is indeed not very soluble in B, or I haven´t been thinking hard enough. $\endgroup$ – Karl 2 days ago
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I would guess that benzoic acid forms dimers or trimers connected by hydrogen bonds in nonpolar solvents. To the outside, these only show the phenyl rings.

The colligative properties of those solutions would clearly show how many individual particles they contain. I´ll leave the literature research for you. ;)

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  • $\begingroup$ The water Vs benzene distribution equilibrium shows forming a dimer, as I remember a physical chemistry lab task. $\endgroup$ – Poutnik Feb 14 at 2:09

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