In the Fischer Esterification reaction shown below, identify which spot on the T
ID: 898691 • Letter: I
Question
In the Fischer Esterification reaction shown below, identify which spot on the TLC corresponds to the starting material and ester product, eluted in 30% ethyl acetate in hexanes (ligroin). Explain your reasoning for each selection.
How would the spots change in terms of Rf values (smaller or larger) if mobile phase was switched to a) 10 % ethyl acetate in hexanes b) 60 % ethyl acetate in hexanes. (think polarity)
A student who set up reaction above ended up with 80% yield of the ester product, and was also able to recollect 15% of starting carboxylic acid. What is the quickest way to separate the desired ester product from its carboxylic acid derivative? (Hint: you’ve done this in lab)
Explanation / Answer
Part A
1)If the compounds are not colored then they can be visualized using an ultraviolet lamp or a chemical stain.
2)There are a wide variety of chemical stains based on the functional groups present.
2nd part
Explanation
1) The choice of solvent system is crucial for good separation.
2)If the developing solvent has a high polarity, all of the spots will run to the top of the plate, and Rf will be zero.
3)With a very non-polar solvent, the spots will not move at all from their initial positions, and again Rf = 0.
4)The best separation is often achieved by using a mixture of a non-polar solvent with a polar solvent.
5)The polarity of the developing solvent is adjusted by changing the ratio of polar to non-polar solvents in the mixture.
6)The best developing solvent should give an Rf value of 0.3 to 0.7 for the desired compound and a Rf of at least 0.1 between the desired compound and any impurities.