Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: sharbeldam on May 06, 2021, 05:28:38 AM
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If one does tlc , which one would have a higher rf value? trans or cis stillbene, i know the more polar should have the lower rf value, but they are isomers, and the internet gave me 2 opposite answers :/ my thought is that cis stilbene should be less polar because it looks more symmetrical, but can someone confirm? also should it be easy to separate the two since one is a solid and the other is a liquid?
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They have same bp so they must be very similar in polarity. Hard to say if they separate on tlc.
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I understand that but one is more polar because of its geometry, I don’t get which, is it trans because it’s less symmetrical?
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Are their dipole moments known?
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my thought is that cis stilbene should be less polar because it looks more symmetrical
one is more polar because of its geometry, I don’t get which, is it trans because it’s less symmetrical?
Do you know any molecule with center of inversion that have non-zero dipole moment?
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What dipole ? It’s all C-H
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@sharbeldam Electric dipole of molecule.
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So wait if I understand you right, do I look vector wise? In cis it’s left and right so there is a dipole, but in trans they cancel each other ?
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I say there is more dipol in the cis but if this was significant it should result in higher bp?
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They have same bp so they must be very similar in polarity. Hard to say if they separate on tlc.
Melting points differ by over 100°, so they must be quite different ;)
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Can someone confirm that cis is more polar ?
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I think that the story is made slightly more complex by the conformation of cis-stilbene in the ground state. I have skimmed one article that indicated that a planar conformation is not possible.
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@Babcock_Hall Obvious that cis isomer cannot be planar, but does it change anything? Symmetry of trans isomer speaks for itself.
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So I get that trans isomer is less polar that's why it's less soluble in cold absolute ethanol, but the question is, since cis stilbene is liquid... shouldn't it be easier to separate the isomers? why do we need crystalization and not just filtration?
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They have same bp so they must be very similar in polarity. Hard to say if they separate on tlc.
Melting points differ by over 100°, so they must be quite different ;)
Thats another matter, it does not mean one os more polar then the other.
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So I get the fact now that cis is more polar, but my inquiry that remains in this synthesis.
if you allow me though, in a specific step of this lab, we get trans/cis isomers and the side product TPPO, so basically two solids and a liquid (cis isomer), to get the cis isomer, can we just used filtration to get the liquid which is the cis isomer, and then we are left with two solids, one is polar and the second is not which is easy to seperate because of the difference in polarity?
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Its not that easy, the liquid stilbene is a good solvent for the solid stilbene so you get mixture. If you use benzylmagnesium chloride and react this with benzaldehyde, then heat with mild acid I think you get pure trans. If you use Wittig you will always get mixture I think. The TPPO can be removed by crystalization if Wittigis used. You can use modifications of Wittig to get pure E-stilbene:
https://en.wikipedia.org/wiki/Wittig_reaction
Also if you have a solution of the cis and add some Pd(0) you will probably get pure trans in time.
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It is difficult to separate in TLC or HPLC, you can try SFC separation