Answers to all questions should be written in complete sentences. 1. Why did you

Question

Answers to all questions should be written in complete sentences.

1.       Why did you observe different Rf values for the three development solvents tested in your TLC analysis of your compound? Explain.

2.       Why is the Rf of your compound greater in ethyl acetate than in hexane?

3.       A student performs two melting-point determinations on a crystalline compound. In one determination, the capillary tube contained a sample 1-2 mm in height, and the melting point range was found to be 141-142C. In the other determination, the sample height was 4-5 mm and the melting range was 141-145C. The reported melting range is 143C. Explain what may have caused the broader range for the second sample.

4.       Another student reported 136-138C for the melting range of the compound in question 3 and recorded that the rate of heating was 12C/min. Further analysis of the compound did not reveal any impurities. Explain the low melting range.

5.       When 2-propanol was used as the developing solvent, two substances moved with the solvent front (Rf = 1) during TLC analysis on a silica gel plate. Can you conclude that they are identical? If not, what additional TLC experiment(s) would you perform?

Explanation / Answer

1. Retention factor (Rf) of substance is measure of how much dissolved substance is retained on the stationary phase. And this retention of substance is dependent on solubility of substance in the solvent, or polarity of solvent. More polar substances dissolves well in more polar solvents, similarly less polar substances dissolves well in less polar solvents. Thus, if polarity of three different solvent is differs significantly then the Rf value of particular substance will also differ significantly if TLC is developed in those solvents.

2. Solvent eluting strength is dependent on the strength of absorbtion on the absorbent (stationary phase). Generally, absorbants are highly polar and hence more polar solvent will have more eluting strength. Ethyl acetate is more polar solvent than hexane, so the Rf value in ethyl acetate will be greater than in hexane.

3. Weight of sample (amount of sample) is directly proportional to height of sample in capillary tube. In first run if height was 1-2mm and in second run height was increased almost double to 4-5mm, means the amount of substance also increased. Melting range also increase as amount of substance increases.

4. Melting also dependant on rate of heating. Higher the rate of heating, lower the melting point range. Hence, if heating rate is increased melting range would decrease.

5. 2-Propanol is very polar solvent and both spots are moving with solvent front. There is a possibility that both compounds are different. To confirm this, we need to run TLC for both compound in Hexane, mixture of Hexane:Ethyl acetate (10:90, 30:70, 50:50, 70:30, 90:10), 100% ethyl acetate, then mixture of ethyl acetate:isopropanol (10:90, 30:70, 50:50, 70:30, 90:10) to see if there is a separation. In all these runs if both compounds have same Rf value then we can confirm both are same compound, but if it is different in even one system then these compounds are different.

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