Polarity of Functional Groups AAMC FL2

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sus2000
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Polarity of Functional Groups AAMC FL2

Post by sus2000 » Fri Apr 19, 2019 4:37 pm

Hi,

For the below question, the answer was A, however, on page 199 of our Chem/Phys book, it says the polarity of ketones is higher than alcohols. If that was true, B would have been the correct answer. Can someone clarify?

Thanks,
Shree

Four organic compounds: 2-butanone, n-pentane, propanoic acid, and n-butanol, present as a mixture, are separated by column chromatography using silica gel with benzene as the eluent. What is the expected order of elution of these four organic compounds from first to last?

A.n-Pentane → 2-butanone → n-butanol → propanoic acid
B.n-Pentane → n-butanol → 2-butanone → propanoic acid
C.Propanoic acid → n-butanol → 2-butanone → n-pentane
D.Propanoic acid → 2-butanone → n-butanol → n-pentane
Solution: The correct answer is A.

This is an Organic Chemistry question that falls under the content category “Separation and purification methods.” The answer to this question is A. The four compounds have comparable molecular weights, so the order of elution will depend on the polarity of the molecule. Since silica gel serves as the stationary phase for the experiment, increasing the polarity of the eluting molecule will increase its affinity for the stationary phase and increase the elution time (decreased Rf). It is a Reasoning about the Design and Execution of Research question because you must reason about the features of a research design to identify a causal relationship.
NS_Tutor_Will
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Re: Polarity of Functional Groups AAMC FL2

Post by NS_Tutor_Will » Sun Apr 21, 2019 9:12 am

Thanks for the question!

This is really a case of the AAMC explanation not being as complete as it could be. You are correct that B lists them in order of increasing polarity. And, yes, ketones and aldehydes are more polar than alcohols. BUT, because the butanol has the ability to hydrogen bond, it'll actually elute more slowly than the ketone. Technically, the elution speed depends on intermolecular interactions between the two phases and not just on polarity. So, both are true! Carbonyls are more polar, but alcohols will elute more slowly.

I hope this helps!
sus2000
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Re: Polarity of Functional Groups AAMC FL2

Post by sus2000 » Mon Apr 22, 2019 9:50 am

Got it, so to be clear that means that aldehydes and ketones cannot hydrogen bond usually? But we see hydrogen bonding among nucleotide base pairs...and secondary structure of amino acids?
NS_Tutor_Will
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Re: Polarity of Functional Groups AAMC FL2

Post by NS_Tutor_Will » Mon Apr 22, 2019 11:43 am

Aldehydes and ketones can participate in hydrogen bonds but only as acceptors. Alcohol's (and water) can act as both acceptors and donors, so they can more readily engage in h-bonding.
sus2000
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Re: Polarity of Functional Groups AAMC FL2

Post by sus2000 » Tue Apr 23, 2019 12:30 pm

Ahhh, okay that makes so much more sense! Thank you.

If I want to strengthen my grasp on some of these basic chem concepts, what would you recommend? Will science Q book have problems like that?

My test is in 3 weeks...so im really trying to tailor my studying as much as possible for the biggest score improvements.

Thank you Will
NS_Tutor_Will
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Re: Polarity of Functional Groups AAMC FL2

Post by NS_Tutor_Will » Wed Apr 24, 2019 8:10 am

The science QBook will have some questions like that, so you can dip into that a bit. I'd say the best approach is to be critical in choosing the problems to do and not just blindly do 1-50 or something like that. Instead, skip around, go from question to question, and do those that you think would be useful.

I also think the content review videos can be useful for improving with some of the concepts, as well as Khan Academy videos.

With three weeks left, I think focusing on the AAMC material is also a good idea, since that's prep material straight from the writer of the test.

I wouldn't focus too much on content for the sake of content. Try to couch it in practice and then do any relevant targeted review based on whatever questions you are missing.

I hope this helps and good luck!!
sus2000
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Re: Polarity of Functional Groups AAMC FL2

Post by sus2000 » Wed May 01, 2019 3:35 pm

Thanks, Will!

Question on when electrons are removed from the s subshell.

I thought the d orbital would prefer a half-filled or fully filled orbital so I chose answer b below but I noticed for this question below that Co (II) has the 2 electrons removed from its s subshell. Do we always remove from the s subshell first? How do we know when electrons should be first removed from the s orbital versus the d orbital

What is the electronic configuration of the Co(II) center found in vitamin B12?

[Ar] 3d7
[Ar] 4s2 3d5
[Ar] 4s2 3d7
[Ar] 4s2 4d5
Solution: The correct answer is A.

This is a General Chemistry question that falls under the content category “Atoms, nuclear decay, electronic structure, and atomic chemical behavior.” The answer to this question is A. Co(II) is a dication and is formed from the atomic element by the loss of two 4s electrons. As a consequence, only seven 3d electrons remain in the valence shell. It is a Scientific Reasoning and Problem Solving question because you are asked to bring together theory and evidence to draw a conclusion.
NS_Tutor_Will
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Re: Polarity of Functional Groups AAMC FL2

Post by NS_Tutor_Will » Thu May 02, 2019 7:58 am

Good question!

You are right that there are circumstances when half-filled orbitals are preferred, but for the most part the electrons will be removed from whichever orbital has the highest principal quantum number.

If both the s and the d orbitals can reach a half-filled state, however, that would be preferable. Elemental chromium [edit: I initially indicated this was true for chromium (II), but it's actually elemental chromium], for example, ends up as 4s1 3d5, instead of 4s2 3d4, preferring to have both orbitals in their half filled state which is preferable. This has often been taught as a stability that goes along with half filled orbitals, but it looks like it's better to think about it as an energetically preferred state compared to the alternatives (interesting reading: https://eic.rsc.org/section/opinion/fiv ... 33.article). In any event, if both the s and the d can be half filled, that'd be preferable, but otherwise the electrons will typically be lost from the orbital with the highest principal quantum number.

I hope this helps and good luck!

edit:

P.S. to add one more thing that my colleague, Mathias, added:

The rule for these sorts of situations is that electrons are added to the lowest available energy level and removed from the highest possible. This makes sense, since the ionization energy will be lowest for the highest energy level and electron affinity highest for the lowest energy level.

h/t Mathias!
sus2000
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Re: Polarity of Functional Groups AAMC FL2

Post by sus2000 » Thu May 02, 2019 4:03 pm

That really helps clarify, thank you so much!
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