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What is the order of the following compounds from more polar to less polar?

  • Hydrochloric acid,
  • Methanol,
  • Hexane,
  • Petroleum benzene

I know that $\ce{HCl}$ has the largest polarity. Hexane I guess has the less. I guess the order should be like:
$\ce{HCl}$ > Methanol > Petroleum benzene > Hexane

Is this correct? I'm not sure of the place of petroleum benzene.

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    $\begingroup$ While "petroleum ether" confusingly means a mixture of volatile liquid hydrocarbons, "petroleum benzene" doesn't seem to be a specific solvent, and appears to just be benzene. Am I wrong? $\endgroup$
    – Nicolau Saker Neto
    Jun 13, 2015 at 11:54
  • $\begingroup$ I agree. There is no solvent named petroleum benzene. Both hexane and benzene are not polar at all and I do not think it make sense to compare. $\endgroup$
    – Coconut
    Jun 13, 2015 at 14:38
  • $\begingroup$ @NicolauSakerNeto Well, in my university’s organic lab, petroleum benzine (Petroleum Benzin) and petroleum ether (Petrolether) were confusingly two names for the same thing. Maybe carolina is at some German university and mistranslated Benzin? $\endgroup$
    – Jan
    Jun 13, 2015 at 14:55
  • $\begingroup$ @Jan . Im not in German university.but in our language,benzin has only one letter more than benzene,so it is possible there was a typing mistake in the test.and about HCl,it was written Hydrochloric acid,so it is "eq" $\endgroup$
    – carolina
    Jun 13, 2015 at 16:12

2 Answers 2

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The concept of solvent polarity is commonly used to describe solvent effects. For example, the Glossary of Terms Used in Physical Organic Chemistry (IUPAC Recommendations 1994) uses the following general definition (which was originally proposed by Christian Reichardt) of the term polarity:

When applied to solvents, this rather ill-defined term covers their overall solvation capability (solvation power) for solutes (i.e. in chemical equilibria: reactants and products; in reaction rates: reactants and activated complex; in light absorptions: ions or molecules in the ground and excited state), which in turn depends on the action of all possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute. Occasionally, the term solvent polarity is restricted to nonspecific solute/solvent interactions only (i.e. to van der Waals forces).

However, the polarity of a solvent is not a physical quantity. Therefore, it is difficult to express quantitatively. Several scales exist that involve various physical properties in order to provide a measure of a solvent’s polarity.

In particular, the relative permittivity $\varepsilon_\mathrm{r}$ is often used as a quantitative measure of solvent polarity:

Hexane: $\varepsilon_\mathrm{r} = 1.8865$
Benzene: $\varepsilon_\mathrm{r} = 2.2825$
Methanol: $\varepsilon_\mathrm{r} = 33.0$
Water: $\varepsilon_\mathrm{r} = 80.100$

(values at 20 °C taken from “Laboratory Solvents and Other Liquid Reagents”, in CRC Handbook of Chemistry and Physics, 97th Edition (2016), William M. Haynes, ed., CRC Press/Taylor and Francis, Boca Raton, FL.)

By way of comparison, the empirical $E_\mathrm{T}(30)$ scale is based on the solvatochromism of Reichardt’s dye (Betaine 30):

Hexane: $E_\mathrm{T}(30) = 31.0\ \mathrm{kcal/mol}$
Benzene: $E_\mathrm{T}(30) = 34.3\ \mathrm{kcal/mol}$
Methanol: $E_\mathrm{T}(30) = 55.4\ \mathrm{kcal/mol}$
Water: $E_\mathrm{T}(30) = 63.1\ \mathrm{kcal/mol}$

(values taken from here)

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Note that petroleum benzene is nothing but benzene itself. The order must be HCl > Ethanol > hexane > benzene. It is because benzene is a perfectly symmetrical molecule and dipole moment vectors cancel each other completely.

But hexane is not that symmetric and hence there is a dipole moment due to difference in electronegativity of C and H atoms (but it is almost negligible).

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    $\begingroup$ However, benzene is capable of dissolving cations much better than hexanes because of it’s polarisability (think $\pi$ system). So I’m pretty sure it could therefore be considered more polar. $\endgroup$
    – Jan
    Jun 13, 2015 at 14:57
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    $\begingroup$ Benzene is able to dissolve cations more efficiently because it has more electron density for cations to attack.But that electron density is symmetrical in space so it does not provide dipole moment.Dipole moment is a vector quantity on the other hand electron density is scalar.Dont confuse between them. ;-) $\endgroup$
    – user247855
    Jun 13, 2015 at 15:01
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    $\begingroup$ Don't confuse permanent dipole moment and polarity, though ;) 1,4-Dioxane in its trans-configuration is also point symmetric thus has a permanent dipole of $0$ yet is rather polar. $\endgroup$
    – Jan
    Jun 13, 2015 at 15:05
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    $\begingroup$ That is what i am trying to say.Polarity is different from polarisability.HCl is most polar because it has got a permanent dipole moment. Theoretically A single oxygen atom is not polar,but will still attract cations, cause it is polarisable $\endgroup$
    – user247855
    Jun 13, 2015 at 15:10
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    $\begingroup$ I agree with @Jan. There is only little correlation for a molecule to have dipole moment and being polar. The simplest and most prominent example is carbon dioxide, which is very polar, but has no dipole moment whatsoever. I am pretty certain, that the +/- charge separation is larger in benzene, than it is in hexane. Your ordering is therefore wrong. $\endgroup$
    – Martin - マーチン ♦
    Jun 13, 2015 at 15:19

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