In: Chemistry
3. The
Rf of a compound = Distance travelled by the compound/ Distance travelled by the solvent front
Rf values are strongly dependent on the nature of the adsorbent and solvent system. In general, low polarity compounds have high Rf value and high polarity compound have low Rf value. The stronger the compound is bound to adsorbent, slower it moves up TLC plates. Non-polar compound move up the plate most rapidly.
Starting material consist of mixture of substance . different compounds in sample mixture travel at different rate due to difference in their attraction to the stationary phase and because of difference in solubility in solvent. They will be spotted differently according to their Rf values. fractions spotted near thec solvent front have low polarity while starting material will have high polarity compartively, so it will be spotted near the base.
Rf of the starting material can be increased by increasing the
polarity of the solvent system. High polarity of the solvent
weakens the interaction of starting material with the silica.
Strong solvent push the material up the plate.