[vikram]

I am trying to monitor the esterification of a non UV active amino acid.I used thionyl chloride and Methanol as reagents.

As amino group is unprotected, Ninhydrin should give positive result for both reactant and products. However, I believe there should be R_f difference between reactant and product.When I performed TLC and stained with ninhydrin, the reactant was strong brown and one spot at very high R_f value showed pale brown coloration.Now can I think that pale brown spot as my target ester?

I am doubtful because reactant and product both supposed to show strong coloration as amino group is not affected.On the other hand, will just methylating the acid result in high R_f change? Because I feel amino group is also polar, it would not allow spot to raise till high R_f end.

[phth]

one must use the correct solvent system there are lots of variations. A good review on some published ways: https://www.researchgate.net/publication/254338234_Thin-layer_chromatography_of_amino_acids_A_review

[Babcock_Hall]

It is unclear to me whether or not you ran a standard of the underivatized amino acid.  I would certainly expect a significant difference in R_f values assuming that you have a reasonable solvent system.

[kriggy]

Yeah just spot the starting material on the TLC plate so you know where it elutes. Im not sure what is your mobile phase but I would assume the ester having vastly different polarity from the parrent amino acid.

You can also use different stain, I use bromocresol green for that. When used, acidic compounds (I think pKa <5) show as yellow spot on blue background. Helped me a lot when doing troublesome amide formations.

Also, I have reasonable experience with cerium molybdate, which often gives different colors for each of my spots

[wildfyr]

Phosphomolybdic is a good general purpose stain.

[Babcock_Hall]

Bromcresol green has worked well in our hands for free carboxylic acids.

[rolnor]

In my experience, the pale brown is your product ester, I have made this reaction many times.