Question

Why is thin layer chromatography used to determine the purity of a sample when you could use an IR spectrum? How does TLC confirm the purity?

Answer 1

An IR spectra cannot completely determine the purity of the compound.

for example, if the compound has disubstituted alcoholic groups then also in the IR, we will get single - O-H (3200-3400 cm-1) stretch due to alcohol. we could not identify whether product has one alcohol substituent or two alcohol substituents. hence, ambiguity prevails in case of IR spectra.

on the contrary, in case of TLC, one can detect the purity of the compound by seeing the distance travelled by the spot, that is Rf value.

disubstituted alcoholic product will have lower Rf value while monosubstituted product will have larger Rf value.

Thus, TLC really helps in identifying the purity of the compound.