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It is written in my textbook that :

Alkyl halides are used as solvents for relatively non polar compounds.

Alkyl halides are themselves polar with carbon having delta positive charge and halide having delta negative charge. If polar compounds are dissolved in them, then there will be more interactions between the compound and alkyl halide. The polar compound should dissolve in it.

What do they mean "relatively non polar compounds are soluble in alkyl halides" ? How will non polar compounds become soluble and why the textbook does not mention solubility of polar compounds?

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  • $\begingroup$ Alkyl halides are less polar than they seem. $\endgroup$
    – Ivan Neretin
    Mar 3, 2017 at 13:41
  • $\begingroup$ But "still" they are polar. How will non polar compounds be soluble in them at all? $\endgroup$
    – Arishta
    Mar 3, 2017 at 14:38
  • $\begingroup$ OK, they are even less polar than you seem to think now. $\endgroup$
    – Ivan Neretin
    Mar 3, 2017 at 14:53
  • $\begingroup$ I thought they were polar based on their electronegativity diffrence which is around 0.5 in Pauling scale. $\endgroup$
    – Arishta
    Mar 3, 2017 at 15:00
  • $\begingroup$ Like I said before, electronegativity is hardly a precise measure of anything. $\endgroup$
    – Ivan Neretin
    Mar 3, 2017 at 15:01

1 Answer 1

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Polarity is relative. Dipole moment (macro scale) and dielectric constant of the solvent (molecular scale) are probably the most decisive. Water has ε~80, ethanol 25 and DCM is "only" 9. By comparison toluene, benzene or tertachlormethane have ε~2. The last example also illustrates why differences in electronegativity are not enough. Being symmetric (Td) tetrachloromethane has dipole moment of zero, even if there is a big difference between electronegativity of carbon on chlorine.

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  • $\begingroup$ I understand that more is the dielectric constant, more is the force applied by the solvent molecules on the charged particles. But I don't understand how dielectric constant affects the process of dissolution? $\endgroup$
    – Arishta
    Mar 3, 2017 at 18:29

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