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Rf value on structural basis; Adv & Disadv of long TLC sheet

1. The relative movement of the substance is related to polarity of the substance. The TLC sheet is coated with highly polar silica gel and the solvent has a much lower polarity. Explain the relative Rf values on a structural basis.

2. Explain why leaves " change color" in the fall.

3. How does the Structure of chlorophyll compare with that of heme in mammalian blood?

What are possible advantages and disadvantages of using a longer TLC sheet?

Solution Preview

1. Explain the relative Rf values on a structural basis.

The identification of the compound is based on the Rf value of the compound, which is the ratio of the distance traveled by the compound to the distance traveled by the solvent. The Rf value is characteristic of the compound's polarity, weight and structure.

During elution, the molecules interact with the solid and liquid phases - different molecular structures have different interactions with these phases and thus elute at different rates up the plate and end up in different positions. A molecule can have different structures; let's say cis-trans isomers have different structures. This isomerism leads to change in polarity between two different structures and thus the interaction of the molecule with the solid and liquid phases.

The components in the mixture dissolve to varying extents in the mobile phase and are carried up the plate. Polar compounds tend to be strongly attracted to the coating on the plate and do not travel up the plate as rapidly as less polar molecules. The cis structure is polar, has lower Rf value ...

Solution Summary

The identification of the compound is based on the Rf value of the compound, which is the ratio of the distance traveled by the compound to the distance traveled by the solvent.

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