soma
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chromatography - polarities
In reading about chromatography, one site says:
Polarity Ranking of the Functional Groups:
(most polar first)
Amide > Acid > Alcohol > Ketone ~ Aldehyde > Amine > Ester > Ether > Alkane
However, a different site says:
Elution sequence: An approximate elution sequence, based broadly on polarity, for compounds from fastest to the slowest is hydrocarbons, olefins,
ethers, halocarbons, aromatics, ketones, aldehydes, esters, alcohols, amines and acids.
I would have thought that the more polar compounds would elute 1st (fastest)?
In any case, amines are listed as pretty non-polar, but acids are pretty polar, so how could the acids be the slowest preceded by amines?
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PirateDocBrown
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Chromatography media are oxides, with strong affinity for compounds with high polarity.
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byko3y
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It's okay for the author of another book to have no idea what he talks about. Acid is not much polar - acid is protic. Dichloromethane is just as
polar, as alcohol or acetic acid.
Eluotropic value, which is affinity of the solvent to the static phase, might be measured. For silica, on my opinion, basicity promotes absorption and
dispersion forces promote desorption, the rest is insignificant. Eluotropic values
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DraconicAcid
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The more polar compounds will elute slowest, because they stick most strongly to the silica.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Alice
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Would be nice to link your sources if possible.
About your question. No, less polar compounds will elute faster for silica and alumina, but the other way round for reversed-phase chromatography
(hydrophobic stationary phase). But what does polar mean? Molecular dipole moment, polar surface area, or hydrogen bond donors and acceptors, lewis
acidity/basicity?
Looking at the first polarity ranking:
| Quote: | | Amide > Acid > Alcohol > Ketone ~ Aldehyde > Amine > Ester > Ether > Alkane |
Does an amine elute faster than a ketone, or aldehyde? Probably not. However, the context of this ranking would be interesting.
The second ranking resembles more what would be expected for silica gel chromatography, although it remains an estimate.
[Edited on 29-5-2017 by Alice]
[Edited on 29-5-2017 by Alice]
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byko3y
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Context is absolutely irrelevant. The author clearly is a douchbag, there's tons of such books written. Go read Stahl or Sherma or some other author
with practical experience.
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Alice
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Source: Polarity of Organic Compounds
As I suspected, the ranking has nothing to do with column chromatography. 
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Metacelsus
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From personal experience, ionizable compounds such as amines and carboxylic acids elute much more slowly than anything else. In these cases,
triethylamine (for amines) or acetic acid (for carboxylic acids) needs to be added to the mobile phase to ensure that the molecule is largely in the
neutral form. For zwitterionic compounds like amino acids, reverse-phase chromatography works best.
A rough guide to choosing eluents for TLC:
-Purely aliphatic compounds, very nonpolar purely aromatic compounds -> hexanes
-Purely aromatic compounds, aliphatic ethers -> 5% v/v ethyl acetate in hexanes
-Methoxy- or halo- substituted aromatic compounds, ketones -> 10% v/v ethyl acetate in hexanes
-Esters, amides, alcohols -> 25% v/v ethyl acetate in hexanes
-Carboxylic acids -> 20% ethyl acetate and 5% acetic acid v/v in hexanes
-Amines -> This really varies by pKa and other properties of the molecule, but use polar solvent and add 5% v/v triethylamine
There will be exceptions to this list, but these eluents are a good starting point for optimization (aim for Rf 0.3). Do lots of tests by TLC before
trying to run a flash column!
[Edited on 5-30-2017 by Metacelsus]
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soma
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Thanks.
The 2nd reference I quoted was from http://www.columbia.edu/cu/chemistry/ugrad/hssp/EXP_8.html
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