[wakeborder556]

Hi, I am an undergrad currently in honors organic 1.  In our lab we were asked the question: If two compounds have the same RF (.87, if that matters) value under identical conditions does that show they are identical structures? 

My initial thoughts were that they can be different compounds due to some sort of ceiling effect of the polarity of the solvent.  I'm pretty sure they can be different compounds but I am having a hard time coming up with an explanation.  Any help or hints would be greatly appreciated.

Thanks,
Mitch

[KritikalMass]

Think you are over-thinking. If they have the same R_f under identical conditions then that would strongly indicate 

If this were not the case then there would be no rapid drug testing (which is mainly TLC)  because people would be suing saying that the R_f of cannabinoids or whatever for each particular test was variant. But the TLC testing is always done under "standard" conditions. Therefore people fail drug tests and can't argue with the results cuz THC always produces the same TLC results under standard conditions, i.e. it has it's own unique R_f.

Additionally, what would be the point in calculating R_f under standard conditions if it didn't indicate something specific?

[sjb]

I'd perhaps argue the other way, in that different Rfs would indicate different compounds, but the same may indicate the same.

After all, depending on your precision, there are a lot more than say 100 compounds in existence (you have quoted an Rf with an accuracy of 1% with 0.87. You'd need other methods to confirm or deny identity, be they different solvent systems for TLC, or things like IR / MS / NMR / etc... I doubt there is just one test carried out in rapid drug testing, to use the example, but a whole bank of assays and the like.

[Borek]

If this were not the case then there would be no rapid drug testing (which is mainly TLC)  because people would be suing saying that the R_f of cannabinoids or whatever for each particular test was variant. But the TLC testing is always done under "standard" conditions. Therefore people fail drug tests and can't argue with the results cuz THC always produces the same TLC results under standard conditions, i.e. it has it's own unique R_f.

Rf depends on solvent, stationary phase and substance. For many pairs of substances it must be possible to select such a combinations of stationary phase and solvent that their Rf is identical. However, at the same time, for specific substances it is possible to select conditions in such a way that you can be almost 100% sure you know what you have detected. I am more then sure that conditions they use for drug testing were carefully selected to minimalize possible false positives; at the same time I am more than sure that it is possible to select these conditions in such a way that false positives would be a norm.

Note that solvent doesn't have to be a single substance. Imagine you have stationary phase and two substances. Imagine now that when using water Rf1 > Rf2, but when using ethanol Rf2 > Rf1. Obviously there will exist such a concentration of ethanol in water for which Rf1 = Rf2.

[KritikalMass]

I'd perhaps argue the other way, in that different Rfs would indicate different compounds, but the same may indicate the same.

After all, depending on your precision, there are a lot more than say 100 compounds in existence (you have quoted an Rf with an accuracy of 1% with 0.87. You'd need other methods to confirm or deny identity, be they different solvent systems for TLC, or things like IR / MS / NMR / etc... I doubt there is just one test carried out in rapid drug testing, to use the example, but a whole bank of assays and the like.

I think you are over-thinking it also. It's organic chemistry 1. If they are going to be getting into solvent systems, IR/MS/NMR in ochem 1 over a simple TLC then that is some really advanced ochem 1.

So tell me what is the point in calculating R_f under standard conditions? Are you saying they calculate retention factors mainly to confirm that two compounds are not the same?

And most rapid drug testing is not done with a whole bank of assays. They will have you urinate in a cup and run it on a TLC right in front of your face. If you fail it they don't ship it out to have an NMR or IR done on it, they fire you or send you back to prison or revoke your probation or whatever they decide to do.
http://www.rapiddrug.com/
http://www.rapiddetect.com/
http://www.americanbiomedica.com/products/rapiddrugscreen.html

[azmanam]

I agree with sjb.  Equal retention times does not necessarily indicate identical compounds.  There are a few steps in the total synthesis problem I'm working on where the product of my reaction has the same Rf as the starting material of the reaction. 

For example, at one point in the synthesis, I do a 1,4-reduction of an alpha-beta unsaturated ketone.  Fortunately, the reaction is finished in an hour, but I wouldn't know by TLC, because the SM and pdt have the same Rf.  (btw, I can quench a small aliquot and see my enone peaks disappear in the NMR, if I want confirmation).

Point is, identical Rf does not guarantee identical compounds.

[Markov]

KM, rapid drug screens are not TLC techniques. The stick they would dip in your urine sample right in front of you is not a TLC analysis. I suppose you have more experience of TLC than of urine tests? 

There are some THC analysis kits available, which are TLC-based, but these are not to test urine... (google for "cannalyze")

 

[wakeborder556]

Different substances could have the same RF value correct?  I'll just put it could be the same compound but there is also a chance that another compound could have the same RF value and that further testing such as NMR (which were doing in lab today) and other more precise techniques would have to be used to determine if they are the same structure of not.

[Dan]

At a crude level, you could think of TLC as a measure of polarity - so if you have two compounds of very similar polarity, they could could be indistinguishable by TLC.
As Azmanam was saying, I've also had a few reactions where the product has the same R_f as the starting material. I've also had cases of two diastereoisomeric products that formed in a reaction being indistinguishable by TLC.
Unless you use a chiral stationary phase, enantiomers will have the same R_f and they don't have the same structure.

[sjb]

I think you are over-thinking it also. It's organic chemistry 1. If they are going to be getting into solvent systems, IR/MS/NMR in ochem 1 over a simple TLC then that is some really advanced ochem 1.

May well be, yes. I'm not even sure what you mean by OChem I, suffice it to say that I remember characterisation methods being mentioned prior to university, and indeed, used on occasion. But that's a different thread, and does not affect my conclusion.

So tell me what is the point in calculating R_f under standard conditions? Are you saying they calculate retention factors mainly to confirm that two compounds are not the same?

Perhaps look into false positives, or similar?

[a student]

so if someone face to this problem ( I mean the similar RF for two diffrent compounds ) how should separate that two compounds from each other, or in a reaction how he can find that the reaction has been completed or not?
I really confused

[azmanam]

you can try different solvent systems.  sometimes compounds will show different Rf values in different solvent systems (ethyl acetate:hexanes, acetone:hexanes, ether:hexanes, methanol:methylene chloride, etc)

if they show any separation, you can run a column where the compounds have Rf 0.2 or even 0.1  Along with lengthening the column, that might give you enough theoretical plates to get some separation.

How to know when rxn is done?  If the functionality changes dramatically, you might get away with using a stain that colors compounds differently based on functionality (vanilin sometimes does that) and you can tell by color when the rxn is done.  if it's not too moisture sensitive, you can take 100 uL or so and do a mini-extraction in a vial to wash out some salts then run an NMR or IR to see if diagnostic peaks in the SM have disappeared.