2. A standard sample of compound A was placed on a silica TLC plate. After eluti

Question

2. A standard sample of compound A was placed on a silica TLC plate. After elution in 30/70 ethyl acetate/hexane, the sample showed a single spot at 3.4 cm (relative to the initial point) after the solvent had risen to 7.1 cm (relative to the initial point). A sample which was thought to contain this compound was analyzed by TLC and showed two spots after elution in the same solvent system. The solvent height for this plate was 6.5 cm (relative to the initial point) and the spots were located at 3.6 cm and 5.2 cm (relative to the initial point). Can either of these spots possibly be compound A? Explain.

Explanation / Answer

Retardation factor (OR) Retention factor (Rf): Each spot has a retention factor (Rf) which is equal to the distance migrated over the total distance covered by the solvent.

The Rf formula is

Rf = distance traveled by sample / distance traveled by solvent

Standard sample Rf = 3.4 / 7.1 = 0.48

Compound Rf = 3.6 / 6.5 = 0.55 and again for another spot Rf = 5.2 / 6.5 = 0.8

Hence either of these spots does not represents standard sample A as these Rf values are different.

A specific compound will show same Rf value in the same solvent mixture with different TLC plate heights.

NOTE: only enantiomers cannot be seperated by TLC as they show same Rf value, this requires stereo-specific chemical method called as chiral resolution.

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