Question: You and another student were each given an unknown compound. Both samples contained colorless material. You each used the same brand of commercially prepared TLC plate and developed the plates using the same solvent. Each of you obtained a single spot of Rf = 0.75. Were the two samples necessarily the same substances? How could you prove unambiguously that they were identical using TLC.

Question

My answer: To determine whether the two compounds are identical, they should both be spotted side by side on the same plate and the plate developed. If both compounds travel the same distance, they are probably identical. If the spot positins are not the same, the compounds are definitely not identical.

Multiple developments with different solvents may be necessary if there is a question that you may be dealing with more than one substance.

Does this look okay?

Sheryl

I think you almost contradict yourself. You start by saying what you would do and if the unknown travels the same distance the two compounds must be the same, then switch gears in the second paragraph and talk about the possibility of them being different. As a teacher, I would write something on the student's paper, "Well, which is it, yes or no?" I also think the teacher PROBABLY

(I hit the wrong key)....PROBABLY will tell you that it has already been established that the distance is the same because the Rf values are the same. There is nothing wrong with putting the spots on the same plate and developing both unknowns simultaneously but I wouldn't write that as a cure all for the problem because the Rf values have been measured and I'm sure the teacher will think that is good enough.

Actually what is the relationship between enantiomers ,identical compounds & constitutional isomers