Hi, you need to choose the solvent or mixture of solvents in order to get the Rf value of your isolated compound or expected compounds e.g. in synthesis to be around 0.5 i.e. in the middle of TLC plate. There are solvents of different polarity. If the solvent has higher dielectric constant, it will be more polar. Mixture of polar and non-polar solvent will give the mixture with lower dielectric constant than the polar solvent, but higher than non-polar solvent.

Please tell me if you need further guidance.

Hi, to choose the correct molar ratio of solvents for TLC you have to have enough information of your compound's chemical structure. If it has more polar functional group such as amine, carboxylic acid, -OH ...groups compare to hydrophobic groups, you need to use polar solvent or mixture of polar/ non-polar. 

I suggest first start from non-polar solvent (n-Hexane), see whether your compound can move by this solvent or remain in base line. if you find that compound has stuck on base line, then add small amount of polar solvent to the TLC mobile phase to increase polarity.

Sometimes, you must use two separate mobile phase to see impolar products first, then  change the mobile phase ratios to polar one to see polar compound in your reaction.

Finding TLC mobile phase is very challenging and some times you have to do try and error.

cheers 

First, there is the so called polarity index, and is a kind of cualitative scale that depicts the polarity of many solvents but it´s a good point to start. In this scale the higher the polarity, the higher its index is. When making a mixture, polarity can be kind of related in base of mol proportion and the index of each component.

There is no actual method to choose a mobil phase, it´s a trail and error process, if you know something about your compounds, choose something of a likely polarity, low polarity compounds are well eluted in mobil phases of n-hexane/ EtOAc, medium polarity compounds go well with acetone, and dimethyl chloride combinations

http://www.perfinity.com/downloads/Solvent%20Miscibility%20and%20Polarity%20Chart.pdf

It is fairly common practice for people to maintain a collection of common TLC solvent combinations. The polarity chart given above is a good guideline for polarity.

However TLC and chromatography in general is about the relative affinity of the compounds for the mobile vs the stationary phase. Solvents and compounds have many more properties than just their polarity which can affect this.

Many mixtures of compounds will show better separation depending on the solvent system used. Often it is a matter of trial and error.

Polarity scales are used when trying to determine how polar a solvent is. These scales rank a solvent's polarity according to the polarity index, which is a measure of a solvent's relative polarity.

The polarity index increases with polarity, water having the highest value at around 9.0. Other values for some common solvents include 6.6 for methanol, 6.2 for acetic acid, 5.2 for ethanol, and 2.3 for toluene.

One way you could actually calculate a solvent's polarity is by using its dielectrict constant, which is used to measure how well the solvent can partly cancel the field strenght of the electric field of a particle added to it.

The higher the value of a solvent's dielectric constant, the more polar it will be.

For example, water's very strong polar nature is indicated by the value of its dielectric constant, which at 0∘C is 88. By comparison, solvents with dielectric constants of less than 15 are considered to be non-polar.

As far sa I know, the dielectric constant can predict the solvent's ability to dissolve ionic compounds.

Here's a link to a polarity chart for some common solvents:

http://www.perfinity.com/downloads/Solvent%20Miscibility%20and%20Polarity%20Chart.pdf

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https://socratic.org/questions/how-can-i-calculate-the-polarity-of-a-solvent