Question

Referring back to question 5 above, in the TLC plate on the left the eluting mobile phase solvent was ethyl acetate, in the plate on the right it was 97% ethyl acetate and 3% methanol. Explain why the Rf values in the plate on the right are larger than those in the plate on the left. Your explanation must be in the terms of intermolecular forces, you are not allowed to only use the word "polarity". There are THREE dynamical processes that contribute to Rf (three processes that have rates associated with them). What are these THREE processes. The amount of material that is deposited on a TLC plate is very small and it would be impossible to weigh. But let's assume that you deposited 5 micrograms of acetaminophen on the plate. How many moles of acetaminophen would 5 micrograms be? Show your work including ALL UNITS.

Answer 1

6]

The Rf value is found to less when ethyl acetate alone is used as the mobile phase [ for the plate on the left side]. This means the compound has more inter molecular force of attraction with the stationary phase [which is silica or alumina plates which are highly polar].

[Intermolecular forces influences adsorption of molecules on the stationary phase are Hydrogen bonding, Dipole-Dipole interactions, Co-ordination and Vanderwaals interactions]

But when increased the polarity of the solvent by using a mixture of ethylacetate [97%] and methanol [3%]. The compound intermolecular force of attraction with the stationary phase got reduced and started to move faster in the mixture of these solvents and thereby shows a higher Rf value [ for the plate on the right side]

Rf value = Distance from baseline travelled by solute / Distance from baseline travelled by solvent front.

Thus when ethyl acetate alone is used the distance moved by solute is less and has less Rf value.

But when ethylacetate [97%] and methanol [3%] is used the solute moved more , and have higher Rf value.

Thus the mixture ethylacetate[97%] and methanol[3%] reduces the intermolecular force of attraction between the solute and the stationary phase and the solute got carried by this solvent mixture and thereby shows higher Rf value.

7]

• If the interaction between the compound and stationary phase is strong , it moves up slowly in the TLC plate
• Non polar compounds move up the plate speedly and have higher Rf value
• Polar substances travel very slowly and have less Rf value

8]

The molecular weight of acetaminophen is 151.2 g/ mol

Number of moles = weight/ molecular weight

Weight = 5 micrograms = 5x 10^-6 gm

Therefore no of moles = 5x 10^-6 gm / 151.2 = 3.3 x 10^-8 moles