Dear Sir. Concerning your issue about the significance of Rf value of organic compounds in medicinal chemistry. The Rf value is a physical constant for an organic molecule and can be used to corroborate the identity of a molecule. The Rf for a compound is a constant from one experiment to the next only if the chromatography conditions below are also constant: · Solvent system · Adsorbent · Thickness of the adsorbent · Amount of material spotted Since these factors are difficult to keep constant from experiment to experiment, relative Rf values are generally considered. "Relative Rf" means that the values are reported relative to a standard, or it means that you compare the Rf values of compounds run on the same plate at the same time. The larger an Rf of a compound, the larger the distance it travels on the TLC plate. When comparing two different compounds run under identical chromatography conditions, the compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate. Conversely, if you know the structures of the compounds in a mixture, you can predict that a compound of low polarity will have a larger Rf value than a polar compound run on the same plate. The Rf can provide corroborative evidence as to the identity of a compound. If the identity of a compound is suspected but not yet proven, an authentic sample of the compound, or standard, is spotted and run on a TLC plate side by side (or on top of each other) with the compound in question. If two substances have the same Rf value, they are likely (but not necessarily) the same compound. If they have different Rf values, they are definitely different compounds. Note that this identity check must be performed on a single plate, because it is difficult to duplicate all the factors which influence Rf exactly from experiment to experiment I think the following below links may help you in your analysis:

http://www.globalresearchonline.net/journalcontents/v18-1/18.pdf

http://www.flash-purification.com/why-is-tlc-rf-important-for-flash-column-chromatography-optimization/

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/retardation-factor

Thanks

Dear Sir. Concerning your issue about the significance of Rf value of organic compounds in medicinal chemistry. The Rf value is a physical constant for an organic molecule and can be used to corroborate the identity of a molecule. The Rf for a compound is a constant from one experiment to the next only if the chromatography conditions below are also constant: · Solvent system · Adsorbent · Thickness of the adsorbent · Amount of material spotted Since these factors are difficult to keep constant from experiment to experiment, relative Rf values are generally considered. "Relative Rf" means that the values are reported relative to a standard, or it means that you compare the Rf values of compounds run on the same plate at the same time. The larger an Rf of a compound, the larger the distance it travels on the TLC plate. When comparing two different compounds run under identical chromatography conditions, the compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate. Conversely, if you know the structures of the compounds in a mixture, you can predict that a compound of low polarity will have a larger Rf value than a polar compound run on the same plate. The Rf can provide corroborative evidence as to the identity of a compound. If the identity of a compound is suspected but not yet proven, an authentic sample of the compound, or standard, is spotted and run on a TLC plate side by side (or on top of each other) with the compound in question. If two substances have the same Rf value, they are likely (but not necessarily) the same compound. If they have different Rf values, they are definitely different compounds. Note that this identity check must be performed on a single plate, because it is difficult to duplicate all the factors which influence Rf exactly from experiment to experiment I think the following below links may help you in your analysis:

http://www.globalresearchonline.net/journalcontents/v18-1/18.pdf

http://www.flash-purification.com/why-is-tlc-rf-important-for-flash-column-chromatography-optimization/

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/retardation-factor

Thanks

Dear Isam,

Thanks for elaborating Rf value with respect to analysis but i want to correlate with ADMET properties of organic compounds. Can you highlight some points accordingly, based on your experience?

I think it is unlikely that there is a correlation between Rf values and ADMET properties of organic compounds generally. There could be such a correlation between a homologous series of compounds (e.g. methyl, ethyl, propyl derivatives) because both the Rf value in a specific chromatography system reflects the same physical property (hydrophobicity, probably) that influences one or more ADMET property. However, because ADMET properties include a wide range of interactions between a compound and a complex biological system, any such correlation would be limited in scope.

It is difficult to pinpoint ADMET properties (absorption, distribution, metabolism, excretion and toxicity) by TLC & HPLC alone.

The relatively easy TLC technique is used as a fast test to find out polarity & degree of purity of single organic compounds and the number of compounds in an organic mixture. Some of our postgraduate students used TLC to monitor the progress of organic reactions successfully.

As the " sciencing.com" pointed out : " HPLC offers a quick, automated and highly accurate method to identify certain chemical components in a sample, but it can be costly, complex and doesn't work for all samples.".

There is also one unfortunate drawback that sometimes pops up with TLC Rf determination: impurities present can affect (usually, push up) bands, so for adequate comparison one needs a combined reference+reaction band in order to see a shift (or lack of thereof). Also, eluent solutions generaly have low boiling points, which means that a complex system's composition changes in time. So, Rf is not a very precise parameter as published data on a substance may or may not be distorted by some factor.

I have read all the beautiful answers. But, for me, the basic and beginner's question as to "why we need to go for calculating Rf value of a sample?" is not yet answered. Suppose, I have unknown A, B, C samples. What do I do with the Rf value I have calculated?

The Rf value allows you to compare the position of bands in your sample to the position of standards, in order to decide whether the band is made of the same thing as the standard. You can also compare your result to the results of others who have used the same solvent system.

When measuring Rfs, it is important that the chromatography be stopped before the solvent rises all the way to the top of the plate.

Thank you Adam sir. However, I feel, I still missed my answer. Without any known standard and solvent system, when we go for separating a completely unknown sample, how Rf value will help me? Theoretically I know, my Rf value may range from 0 to 1.

Dear Ananta,

First why do you check the TLC? The answers is to check how many spots are there in your crude/pure material. The Rf value is nothing but it is a relative position of your compound with respect to the solvent front. This may change according to the chosen solvent system.

The Rf value gives you the idea of approximate polarity of the compound.

I think this is OK for now.

In addition to other significance of running in TLC, what I now understand is that TLC helps one to determine which mobile phase or solvent will be most suitable for a particular sample or spot/chemical. Separation of different compounds is a function of mobile phase solution which can never be selected without calculating its Rf value.

Enlighten me with better input.