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Topic: Predict Rf values  (Read 26607 times)

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Offline christina

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Predict Rf values
« on: October 14, 2007, 07:06:36 AM »
consider a sample that is a mixture of biphenyl, benzoic acid, and benzyl alchohol. The sample is spotted on a TLC plate and developed in a methylene chloride cyclohexane mixture.

Predict the relative Rf values for the three components in the sample.
For assistance use the table 12.3 on page 676.

~ Well on pg 676 in my organic chem book it's talking about organic layer seperation and on table 12.3 on page 683 note that it talks about common signs that a solution is dry.
Basically the wrong pages...

I have no clue how to predict relative Rf values for these compounds especially since it's using a mix of 2 solvents which I haven't tried in the lab. I've used either or but not both and not with these compounds either.

I would think it would depend on the polarity of the substances but that would also require me to look at the mix of the solvent which both have different polarities so technically I would think that it would only be feasible to go and find the positions such as this would go first and second and so on but are they asking for numbers?? Also I'm confused as to the solvent combination..I really don't know what would be the effect it would have since they don't say how much they used..but if they don't say I'm guessing it would be equal...

I really need help on this...  :'(
« Last Edit: October 14, 2007, 09:42:01 AM by ~christina~ »
:)

Offline macman104

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Re: Predict Rf values
« Reply #1 on: October 14, 2007, 02:24:39 PM »
I would agree that there is no way you could guess actual numbers, and I'm going to assume that they just want 1st, 2nd, 3rd, even though the question reads differently.  As far as the solvent mixture is concerned, DCM and hexane should give you a moderately polar solution.

Do you have the correct edition of the book?  I would guess that benzoic acid will be the most polar, followed by benzyl alcohol, then biphenyl.  So benzoic acid will move the least on the plate.

Offline christina

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Re: Predict Rf values
« Reply #2 on: October 14, 2007, 04:16:21 PM »
unfortunately I don't have that edition of the book since everyone has the newest version but for some reason they never changed it...


Thanks for your help :)
:)

Offline Dolphinsiu

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Re: Predict Rf values
« Reply #3 on: October 15, 2007, 04:26:33 AM »
Yes, you are right! Polarity of solvent does affect the degree of transition of all components in your sample. Different combination of the solvent used will cause different separation of components in your sample. I have done this experiment before, I recommend you to read

Doyle MP, Experimental Organic Chemistry, New York: Wiley, 1980, pp.86-92

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