Question

Explain why the presence of crystals of the sample at the starting point causes streaking of the TLC plate during development.

Answer 1

I think that's because the crystals are dissolving continuously as time progresses. This may also be due to improper drying of the TLC spot before developing. This problem generally happens when one is forced to use solvents like DMF or DMSO for spotting. Streakiness can also be a result of decomposition on the silica gel. You can check for this by doing a 2-D TLC. To try this, cut a square TLC plate and spot your stuff near one corner. Run the plate up one direction, then take it out, let the solvent evaporate from the plate and then turn it 90 degrees and run it up again. If your stuff is stable on TLC, then all of your spots should appear on the diagonal of the plate. If they are decomposing, then you will see off-diagonal spots.

The other thing that commonly causes streaks is some kind of isomerization of your compound. For example, with beta-ketoesters it is common to see some separation between the keto and enol tautomers on TLC. In some cases (where tautomerization is slow) you can completely separate these spots on TLC, but if the two isomerize on the plate, you often see a big streak.