I'm really confused about the polarity and solubility of salicylic acid. Since there is carbon and two other types of atoms, it should be polar, right? However, some people online say it is non-polar.
Also, is it soluble in water?
$\begingroup$
$\endgroup$
2
-
$\begingroup$ I'm going to bed, but some hints, before I come back to answer (if no one else does): You should think of the shape, as well as the atoms themselves. Types of atoms aren't the only important factors in predicting polarity. An example is Carbon dioxide. And if a substance is polar, it'll assumingly dissolve in water, but that's a rule of thumb. $\endgroup$– M.A.R.Commented Mar 1, 2015 at 21:50
-
$\begingroup$ The shape tells me it is polar: not symmetrical at all. Also, since there is carbon and hydrogen PLUS oxygen, it should be polar. However, I don't think it dissolves in water well, so maybe non-polar? $\endgroup$– NoobCoderCommented Mar 1, 2015 at 22:23
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
2
I think that, first and foremost, the misconception "dipole = polar substance = soluble in water" needs to be pointed out. Molecules of salicylic acid obviously have non-zero dipole, like hexene, for example, but suspecting hexene of being soluble in water would be a folly.
Now how to tell if a substance is soluble? Salicylic acid has polar groups (carboxylic and phenolic), but the benzene ring reduces polarity of the substance. One could say carbon atoms win 7:3 with oxygens, but it's still not bad :)
PubChem says the solubility of salicylic acid is about 2 g/L. Kiper’s Chemister gives more info: it dissolves much better in hot water and its solubility in methanol (which is quite polar) is good, and in benzene (non-polar) bad.
Generally, it seems to be somewhere in the middle between strongly polar and strongly non-polar compounds.
-
$\begingroup$ So would you say it has dipole dipole forces or not? $\endgroup$ Commented Mar 1, 2015 at 23:05
-
$\begingroup$
$\endgroup$
Salicylic acid has just the right arrangement of two OH groups to do intramolecular hydrogen bonding. The ability to do hydrogen bonding usually makes a molecule MORE polar, but intramolecular hydrogen bonding ties up polar groups so that their polarity is partially cancelled out.
This reduces the dipole moment of the molecule, so that its overall polarity is lower than otherwise expected. Salicylic acid is only slightly soluble in water: it takes about one liter of water to dissolve just 2 grams of salicylic acid at room temperature (https://pubchem.ncbi.nlm.nih.gov/compound/Salicylic-Acid#section=Solubility)
This might be why some of your sources list this molecule as non-polar. It is certainly polar, but its intramolecular hydrogen bonding leads to some non-polar behavior.
Without the intramolecular hydrogen bonding, the dipole moment of the molecule is more than 2 debyes, but with the intramolecular hydrogen bonding the dipole moment of the molecule is only 0.7 debyes. Look for the word "polarity" on page 2709 of the article I cite below:
Citation: Hill, A., & Wang, F. (2023). Intramolecular O···H Hydrogen Bonding of Salicylic Acid: Further Insights from O 1s XPS and 1H NMR Spectra Using DFT Calculations. The Journal of Physical Chemistry A, 127(12), 2705-2716. https://doi.org/10.1021/acs.jpca.2c08981
