Please help with these questions. Thanks! 1. Suppose you run TLC of reaction mix
ID: 810178 • Letter: P
Question
Please help with these questions. Thanks!
1. Suppose you run TLC of reaction mixture using dichloromethane/methanol (1:1) as an eluent. TLC shows one spot with Rf= 0.98. Does it mean that given mixture contains only one component? Why/why not? What specifically would you change in the conditions for running TLC to obtain more accurate information about contents of the mixture? 2. Suppose you need to separate compounds A and B. Values of Rf for A are 0.89 in dichloromethane and 0.60 in hexane. Values of Rf for compound B are 0.79 in dichloromethane and 0.35 in hexanes. Which solvent would be better for separation of A and B and why? 3. Another popular analgesic in naproxen (the ingredient of Aleve). (a) Look up and draw the structure of naproxen. (b) Suppose you carry out a TLC separation of phenacetin, acetanilide, and naproxen (refer to the experiment 2 and 3 for the structures of acetanilide and phenacetin), on silica gel. Rank in these three compounds from the one with the highest Rf to the one with lowest Rf? Arrange the following compounds in order of increasing Rf in TLC: naphthalene (A), benzoic acid (B), 2-hexanone (C), 1-aminohexane (D), 12-cyclohexanediol (E) (assume the same mobile and stationary phases for each compound).Explanation / Answer
1. No, the compound could contain two, three, or more components.
But in order for the Rf factor to be that high, the compound that is being analyzed is very nonpolar and is eluting up the plate very quickly. So in order to decrease the Rf factor so that you can see clear separation if the compound is composed of a mixture, you should change the solvent system and perform another TLC analysis. My suggestion is to change the solvent system by decreasing the polarity of the solvent system by either changing one of the solvents, specifically dichloromethane or by changing the solvent mixture ratio by increasing dichloromethane and decreasing methanol. That should allow the compound to interact with the TLC plate longer so that you can observe some type of separation during TLC analysis. Another suggestion is to change the TLC plate (i.e, Al2O3 instead of SiO2). Al2O3 plates are better for separation of nonpolar compounds, and is probably what I would do first.
2. Hexane is a suitable option for separating A and B since they both generate Rf with said solvent are separated enough to identify who is A and who is B. For the case with Hexane Rf of A is 0.60 and Rf is 0.35 for B , which gives 0.25 Unides difference Rf of separation between a compound and the other.