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Topic: TLC error  (Read 10865 times)

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Offline Sheryl

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TLC error
« on: October 14, 2006, 11:56:58 PM »
The question is:  A two-component mixture containing 1-octene and 1,4-dimethylbenzene gave only one spot with an Rf value of 0.95.  The solvent used was acetone.  What should be done to correct the error?

I know that both mixture compounds are nonpolar and acetone is polar so I'm assuming that a nonpolar solvent should be used but I don't know why.  The Rf value indicates that it went up the plate.  These compounds are not isomers so I don't know that having similar properties and so needing another eluting solvent would be the issue.

Can anyone enlighten me on this?

Thanks from Sheryl

Offline firassh2000

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Re: TLC error
« Reply #1 on: October 15, 2006, 04:32:12 AM »
U can use the mixture (Benzene:methanol-9.5:0.5) because that u should use mixture containing polar and nonpolar. ::)

Offline lavoisier

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Re: TLC error
« Reply #2 on: October 15, 2006, 08:06:28 AM »
Hi Sheryl,
the theory and practice of TLC show that when you use an eluent which is too 'strong' (i.e. too polar if you use a silica plate as the stationary phase) for the mixture you're trying to separate, you can get all the components to migrate together. It's not surprising at all.
So yes, the answer is that you must use a less polar eluent.

You say you don't know why. Again, the theory shows that your substances are subject to an equilibrium between the silica on the plate and the eluent. The stronger (more polar) the eluent, the more your susbtances will tend to stay in the eluent rather than to stick to the silica, and therefore the more they will migrate.
If you use a very strong eluent, your substances simply won't stick at all to the silica, and will be just mechanically transported by the eluent flow. This will eliminate their chemical differences, and ultimately they'll show the same Rf (about 1).
On the other hand, if you use a weaker (less polar) eluent, the substances will be able to stick a little to the silica, and there you have a chance of observing a different behavoiur, hence a different Rf.
Hope it helps.

PS: as an organic chemist, I'm sorry if my explanation will look over-simplified to some physical chemists out there, but you know, we are simple people, we don't want to solve systems of partial differential equations just to make tea in the morning.

Offline Sheryl

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Re: TLC error
« Reply #3 on: October 15, 2006, 02:44:48 PM »
Much thanks.  Your explanation was quite clear to me.

Sheryl

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