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wearelulu 2013-11-2 07:00 PM

trans-/cis- stilbenes

I did an experiment about thin-layer chromatography.
I used hexane as eluent, and silica gel plate.
I spotted the trans-/cis- stilbenes on the plate and let it to develop.

But strange result was come out.
The trans-stilbene (Rf~0.4) was at a lower position of cis-stilbenes (Rf~0.6)
I have repeated for several times, but the same results was come out.


I don't know why? Anyone can try to explain the reason?:smile_13: :smile_13: :smile_13:

jmlo 2013-11-2 11:45 PM

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This observation is normal. The published Rf data for cis- and trans-stilbene using the same TLC setup (i.e., silica plate, hexane eluent) are 0.27 and 0.20. Your numbers are a bit off but still reasonable.

The reason why the cis-isomer has a higher mobility (or in other words, larger Rf value) is due to its nonplanar structure.

wearelulu 2013-11-3 07:23 PM

cis-stilbene is non-planar? What are the reasons behind? How it related to tlc result? any reference website?

At first I think trans-stilbene is no net dipole moment it should move faster than cis.

Can you explain more?  thx



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jmlo 2013-11-4 10:06 AM

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[quote] cis-stilbene is non-planar? What are the reasons behind? How it related to tlc result? any reference website? [/quote]

The cis isomer of stilbene is not planar because of the steric interaction between two phenyl rings. The molecule is twisted a bit.

Reference: S. G. Levine, et al. Journal of Chemical Education, Vol. 65, 79 (1988).
                 K. Maeda, et al. Journal of the Chemical Society Perkin Trans II, 1282 (1980)

[quote] At first I think trans-stilbene is no net dipole moment it should move faster than cis.

Can you explain more?  thx [/quote]
It is true that less polar substances would stay longer in the non-polar mobile phase such as hexane.

The actual reason why cis-stilbene has a higher mobility than trans-stilbene in hexane is not fully understood. What I suspect is that the non-planar structure of the cis-isomer makes its adsorption on the silica plate less efficient than expected.
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