Question

1) Calculate the Rf value of a spot that travels 5.7 cm, with a solvent front that travels 13 cm. 2) A student spots an unknown sample on a TLC plate and develops it in pentane solvent. Only one spot, for which the Rf value is 0.05, is observed. Is the unknown material a pure compound? What can be done to verify the purity of the sample using thin-layer chromatography? 3) You try to synthesize n-hexyl bromide as a product from a substitution reaction using from n-hexyl alcohol as the reagent. There is a risk that an alkene side product is produced. You run a TLC on a sample of your product (A) anda known mixture of the alkene side product and desired product and the alcohol reactant. Analyze the following TLC developed plate: Which spot is which substance in the standard and write 1 to 2 sentence explanation of what your results of your synthesis mean (i.e. did you make the desired product?) [Hint: The stationary phase is more polar than the mobile phase and this TLC has a polar stationary phase that particularly loves forming hydrogen bonding) The side product alkene Desired Product: n-hexyl bromide n-hexyl alcohol (Reactant) ОН TLC Plate:

Answer 1

1) Rf = distance traveled by (spot / solvent front) Given, distance traveled by spot = 5.7cm and that by solvent front = 13cm Rf = 5.7/13 = 0.44 2) Pentane is a non-polar solvent. And Rf value of a given spot is just 0.05. Hence this may not confirm the purity of the compund. There are may be more possible spots of impurities and to confirm that we need to increase the polarity of a mobile phase by adding such as ethyl acetate, or even further small portion of methanol. When the all the different mobile phases gives substantial Rf value with a single spot then we may say that the compound is pure. 3) As the stationary phase of TLC is polar (silica gel) it will bind more polar compounds strongly. The polarity order in the given three compunds is: Most polar Alcohol > alkyl halide > Alkene The stationary phase will have more strong interactions with alcohol than alkylhalide and least with alkene. Thus in TLC we should observe the Rf values as: alcohol the least Rf value < Alkyl halide < alkene highest Rf. Spotting A is the reaction mixure and B is the standards mixture. It is not possbile to measure the distance traveled by each spot hence may not be possible to calculate actual Rf value. However, I am using rough estimate: Distance traveled by spots:

Solvent front: 5cm 2-hexene 4cm n-hexylbromide: 2cm n-hexanol: 1cm Respective Rf values are: alkene = 4/5 = 0.8 alkyl bromide = 0.4 alcohol = 0.2 If we compare the Rf values of spots in the line A: we get spots corrosponding to hexanol and hexylbromide. There is no spot of alkene product in the reaction mixture. Thus reaction gave only the desired product and some unreacted reactant.