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I know from the literature that the following compounds have decreasing polarity: Rosmarinic Acid -> TMF -> Sinensetin and I guess comparing compounds of similar structure it is manageable to compare bond types, groups etc etc by which to order their polarity, but what about compounds with very different structures, groups and bonds, especially when you want to rapidly order 10's-100's of compounds.

My questions:

  1. Is there such a thing as an absolute measure of "polarity" for a compound by which other compounds can be compared/ordered?

  2. Depending on 2) is there a resource whereby you can lookup your compound to obtain its polarity?

I am asking this question from a Chromatography perspective. I would like to know the order of separation for a group of compounds and how to know what "polarity" of mobile phase to start my experimentations with.

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  • $\begingroup$ No. $\mathstrut$ $\endgroup$ Nov 10, 2020 at 15:50
  • $\begingroup$ In context of chromatography, It can be said in many cases that solvent A is "more polar" than solvent B wrt analyte C, and at the same time, solvent B is more polar than solvent A wrt analyte D. $\endgroup$
    – Poutnik
    Nov 10, 2020 at 19:51
  • $\begingroup$ There are many measures, none is definite. For example dielectric constant, Donor number Partition coefficients. $\endgroup$
    – Mithoron
    Nov 10, 2020 at 19:59
  • $\begingroup$ @Poutnik - I am looking at this from a chromatography perspective. I am interested in estimating the elution order of a group of compounds and the choice of mobile phase that I need to make. I will update my question to give this information. Maybe you could answer based on this new information? $\endgroup$
    – tomdertech
    Nov 11, 2020 at 7:59
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    $\begingroup$ The elution order cannot be easily theoretically predicted in general, except for big enough differences (which decrease with experience ) of analyte properties. It is near always combination of educated guess and experiment. For reverse HPLC on e.g. C18 columns, it can be interpolated from elution times of testing mobile phases MeOH+H2O, AN+H2O, THF+H2O, each mobile phase interacting with analytes in a different way. $\endgroup$
    – Poutnik
    Nov 11, 2020 at 8:13

1 Answer 1

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Polarity is an umbrella term which is popular among organic chemists. It is as vague as saying the weather is good today. "Good weather" may include, bright light, sunny, temperature, right humidity and so on. Now read your own statement

Rosmarinic Acid -> TMF -> Sinensetin and I guess comparing compounds of similar structure it manageable to compare bond types, groups etc etc

Since you correctly use etc. etc. ...yes polarity includes et cetra et cetra. Its usage is discouraged by IUPAC.

The correct term is dipolarity which is a measure of the dipole moment (you can look them up in tables). Even that dipole moment is of no good because you have plenty of conformers and each conformer has a different dipole moment (at least in the gas phase).

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  • $\begingroup$ It can be said polarity is multidimensional, as there are several phenomena that contribute in different proportions to the overall perceived "polarity". It is relative permittivity, dipole moment, polarisability, donor-acceptor properties, forming hydrogen bonds. It can be well illustrated by mobile phase choice in reverse HPLC, where 3 m. phases with the same elution time for main component (H2O + CH3OH/CH3CN/THF respectively ) have very different elution times for minor impurities, during determining the chromatographic purity ) $\endgroup$
    – Poutnik
    Nov 10, 2020 at 17:09
  • $\begingroup$ Yes, another source of confusion from polarity is the chromatographic usage. $\endgroup$
    – AChem
    Nov 10, 2020 at 19:43
  • $\begingroup$ goldbook.iupac.org/terms/view/P04710 $\endgroup$
    – Mithoron
    Nov 10, 2020 at 22:21

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