Question

What are the parts of the dye molecules that affect the Rf of the spots as they move up the tlc plate? Please list several and explain why using Lewis structures.

Answer 1

Dyes are colored substances that has an affinity to the substrate to which it is being applied and imparts colour by absorbing some wavelengths of light more than others. Speaking chemically, theere are particular chemical moeties in dyes that are responsible for their strong coloration as well as chemical nature.

Regarding your question, parts of the dye molecules that affect the Rf of the spots as they move up the tlc plate are as follows:

1. The electronegative atoms present in a dye molecule, plays an important role in electron and charge transfer through the molecule (being an acceptor part), hence they, due to their high electronegative power provides resistance to the upward movement of the dye molecule up the tlc plate. (Lewis structure is shown below)

2. Conjugation through rings (bulky conjugated rings) provide higher emission, one of the vital feature of the dyes. (See examples of the two dyes given with buly phenyl rings helping in conjugation ). These bulky rings consequently add to the electropositivity of the molecule, hence helps in upward movement of the dye molecule up the tlc plate.