June 20, 2021, 08:07:45 AM
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Topic: Trans vs cis stillbene  (Read 667 times)

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Offline sharbeldam

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Trans vs cis stillbene
« on: May 06, 2021, 05:28:38 AM »
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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Offline rolnor

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Re: Trans vs cis stillbene
« Reply #1 on: May 06, 2021, 07:22:46 AM »
They have same bp so they must be very similar in polarity. Hard to say if they separate on tlc.

Offline sharbeldam

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Re: Trans vs cis stillbene
« Reply #2 on: May 06, 2021, 08:29:03 AM »
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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Offline Babcock_Hall

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Re: Trans vs cis stillbene
« Reply #3 on: May 06, 2021, 09:08:12 AM »
Are their dipole moments known?

Offline Orcio_Dojek

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Re: Trans vs cis stillbene
« Reply #4 on: May 06, 2021, 09:28:08 AM »
Quote
my thought is that cis stilbene should be less polar because it looks more symmetrical
Quote
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?

Offline sharbeldam

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Re: Trans vs cis stillbene
« Reply #5 on: May 06, 2021, 09:46:05 AM »
What dipole ? It’s all C-H
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Offline Orcio_Dojek

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Re: Trans vs cis stillbene
« Reply #6 on: May 06, 2021, 09:59:27 AM »
@sharbeldam Electric dipole of molecule.

Offline sharbeldam

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Re: Trans vs cis stillbene
« Reply #7 on: May 06, 2021, 10:07:11 AM »
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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Offline rolnor

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Re: Trans vs cis stillbene
« Reply #8 on: May 06, 2021, 10:24:51 AM »
I say there is more dipol in the cis but if this was significant it should result in higher bp?

Offline Borek

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Re: Trans vs cis stillbene
« Reply #9 on: May 06, 2021, 10:35:00 AM »
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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Offline sharbeldam

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Re: Trans vs cis stillbene
« Reply #10 on: May 06, 2021, 11:11:12 AM »
Can someone confirm that cis is more polar ?
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Offline Babcock_Hall

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Re: Trans vs cis stillbene
« Reply #11 on: May 06, 2021, 11:14:17 AM »
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.

Offline Orcio_Dojek

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Re: Trans vs cis stillbene
« Reply #12 on: May 06, 2021, 03:34:12 PM »
@Babcock_Hall Obvious that cis isomer cannot be planar, but does it change anything? Symmetry of trans isomer speaks for itself.

Offline sharbeldam

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Re: Trans vs cis stillbene
« Reply #13 on: May 06, 2021, 06:05:43 PM »
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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Offline rolnor

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Re: Trans vs cis stillbene
« Reply #14 on: May 06, 2021, 08:05:56 PM »
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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