Please help me answer the problems on this page Give the relative polarity in de

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Please help me answer the problems on this page

Give the relative polarity in decreasing order for compounds 5-8. Which compound (5-8) would you expect to have the greatest R_f, and least R_f value if cyclohexane is used as an eluting solvent. "Briefly explain" Which compound (5-8) would you expect to have the gretest R_f, and least R_f value if ethanol (CH_3CH_2OH) is used as an eluting solvent. "Briefly explain" Consider the initial elution of polar compound 6 with cyclohexane and shortly afterward, you gradually add increasing amounts of ethanol (a polar solvent) to the eution solvent mixture consisting now of cyclohexane and ethanol. Briefly give an account of the movement of 6 on a TLC plate or on chromatography column before ethanol addition and after ethanol addition. "Briefly explain" Indicate which compounds below (9-13) can be visualized by using ultraviolet light (Le, UV) during TLC analysis. Indicate your choice by stating Yes or No. A 100 meters chromatography column (e.g., A) was found to contain 20 theoretical plates. What would be the HETP of the column? (Show calculations). Answer: What would be the HETP of a chromatography column (b) that is 100 meters in length and contains 10 theoretical plates? (Show calculations). Answer: Which column would give you the best separation, a or b? Explain "briefly" why this is so. Answer a or b:

Explanation / Answer

20. Relative polarity of compounds : -Oh group induces polarity to the structure. Higher number of -Oh in the molecule would make it more polar.

Decreasing polarity order : 6 > 8 > 7 > 5

21. Cyclohexane is less a non-polar solvent so compound which is least polar would dissolve (partition) preferentially in it and move fastest on the plate. Most polar compound would not dissolve (partition) in this solvent. Remember like-dissolves-like principle.

Thus, 5 will have higest Rf and 6 would have lowest Rf.

22. EtOH is a polar solvent. Addition of EtOH as eluting solvent would enable most polar compound to come out first and thus move fastest on the plate. Least polar compound would not partition (dissolve) in it.

Thus, 6 would have highest Rf and 5 would have lowest Rf.

23. Separation of compound 6 on column first by passing a non-polar solvent cyclohexane would not make the compound move much. Adding EtOH a polar solvent would enable dissolution or partitionaing of 6 in this solvent mixture and thus 6 would now move faster and come out of column.

Compounds that would be UV actve are,

24. Compound 9 : Yes                  25. Compound 10 : No                  26. Compound 11 : Yes

27. Compound 12 : Yes                  28. Compound 13 : Yes

29. HETP = L/N = 100 m/20 = 5 (a)

30. HETP = L/N = 100 m/10 = 10 (b)

31. As the HETP decreases the separation efficiency of a column goes up. Thus, of the two a would give better separation when compared to b.

Anser : a

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