Polarity of organic compound

[freedom856]

i was wondering the polarity of ethyl acetate and nitrobutane, which one is more polar?
As i face this question in test(liquid chromatography using normal phase and reverse phase)
Asking for the elution order, i am not sure which one is more polar? Can anyone answer my question? Also, if i face this kind of question again, what can i do? Thank you

[chemisttree]

I would say that the nitroalkane is more polar as it can form an aci type structure.

R-CH2-NO2 <-------> R-CH=NO2H

[sjb-2812]

I would say that the nitroalkane is more polar as it can form an aci type structure.

R-CH2-NO2 <-------> R-CH=NO2H

Well, so can the ester CH3CO2Et <-> CH2=C(OH)OEt, so I doubt that's the full story. Perhaps take a reasonably polar compound and see what kind of an Rf you get on TLC with pure ester and nitroalkane each as solvent, as a first approximation?

[freedom856]

Sorry.It is not TLC. It is HPLC. i do remember the question does not state what solvent is used

[sjb-2812]

I appreciate what you're saying, but you can get a measure of solvent polarity from Rfs on TLC (I know it's a different stationary phase). Imagine running a TLC in pure hexane, your compound will hardly move as the interaction with the silica (or other phase, e.g. alumina) and your compound is much greater than between compound and solvent. Now, if you run the same compound in EtOAc, or DCM, or similar, then the Rf would be greater as the polarity is greater..? (Or am I totally confused here?)

[freedom856]

your answer is correct if i run TLC once and i can find the answer, but the problem is i only sit in the classroom and answer this question. How can i run TLC to determine their polarity? i really request a way to solve this kind of problem.

[Jack the Stri]

My guess would be nitrobutane as it's actually two ions (notwithstanding resonance structures). Ethyl acetate might have one less carbon moiety, but the hydrophilicity of two charged atoms outweighs the hydrophobicity of one methyl group.

[chemisttree]

Well, so can the ester CH3CO2Et <-> CH2=C(OH)OEt, so I doubt that's the full story. Perhaps take a reasonably polar compound and see what kind of an Rf you get on TLC with pure ester and nitroalkane each as solvent, as a first approximation?

Trust me. It's enough of the story to get the question right. TLC will give you the same ranking but how will you visualize the spot?

Bottom of page 88. (http://books.google.com/books?id=HNpMvao75tkC&pg=PA88&lpg=PA88&dq=nitroalkanes+polarity&source=bl&ots=3u3cpMxaSK&sig=A_4mCVUo2hIXv-Ad3j00LPerkxo&hl=en&ei=MzX0TOPZI4X7lwf0_p2EDQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBMQ6AEwAA#v=onepage&q=nitroalkanes%20polarity&f=false) Some things you just have to remember.

[Galap]

I would say the nitro compound is more polar because NO2 is a very strong electron drawing group.