10. A colorless unknown substance obtained from a column chromatographic separat
ID: 1028474 • Letter: 1
Question
10. A colorless unknown substance obtained from a column chromatographic separation is spotted on a TLC plate and developed in a correct solvent. The spots do not appear when visualized with a UV lamp or an iodine chamber. What colorimetric test could you perform on the TLC plate to visualize the spots if the unknown substance was one of the following. Name the colorimetric test/chemical reaction and describe the color change you would observe if your prediction was correct.
a. an alkene
b. a primary alcohol
c. a phenol
Explanation / Answer
Among these three ,(a) an alkene is not shown any spot in iodine chamber or in UV lamp. For this, KMnO4 chamber is used to detect the spot. This KMnO4 solution is basic (NaOH is used with KMnO4). Alkene reacts with KMnO4 and forms diol and MnO2(dark brown).
Reaction is CH2=CH2 + 2MnO4- + 2OH- = CH2(OH)-CH2(OH) + 2MnO4 2-. Then further reaction occurs. It is 3 CH2=CH2 +2MnO4- +4H2O = 3 CH2(OH)-CH2(OH) + 2MnO2(dark brown) + 2 OH-.
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