Based on the figure above: a) give the relative polarity in decreasing order for

Question

Based on the figure above:

a) give the relative polarity in decreasing order for compounds 5-8.

b) which compound would you expect to have the greatest Rf and least Rf values if cyclohexane is used as an eluting solvent?

c) which compound would you expect to have the greatest Rf and least Rf values if ethanol is used as an eluting solvent?

d) Consider the initial elution of polar compund 6 with cyclohexane, and shortly afterward, you gradually add increasing amounts of ethanol to the elution solvent mixture consisting now of cyclohexane and ethanol. Briefly give an account of the movement of 6 on a TLC plate or a chromatography column before ethanol addition and after ethanol addition. Briefly explain.

20-2 20-23 3 H HO Hi Coasider compouads S-8 below in order to answer questions 7 8 20 Give the relative polarity in decreasing order for compounds 5-8 21 Walch compound (S-8) would you expect to have the greatest Re, and least Rf value if cyclohexane is ased as an eluting solvent. Briefly explain" you expect to ave the greatest Rp, and least Re value if 22 Waics compound (5-3) would you expect to have the greatest Rg, and least Re value if ethanol (CH3CE20H) is used as eluting sol venL ·Briefly explain- 23 Consider the inital ecution of polar compound 6 with cyclohexane, and shortly afterward, you gradually add Increasing amounts of ethanol (a polar solvent) to the elution solvent mixture consisting now of oycdobeane and ethanol. Briely give an account of the movement of 6 on a TLC plate or on a chironiatograpby columa betore ethanol addition and after ethanol addition. Briefly explain

Explanation / Answer

20. The relative polarity of compounds 5-8 in decreasing order is 6 >> 7 > 8 > 5. This order of polarity can be easily understood if we remember than –OH group is highly polar. Simply having more –OH groups attached to benzene will increase the polarity of the compound. Thus, compound 6 (1,2,4-benzenetriol) is the most polar compound. Between 7 and 8, compound 8 (o-methylphenol) has a methyl group ortho to the phenolic –OH which makes o-methylphenol less polar than phenol. Finally 5 (benzene) has no polar group and hence is the least polar.

21. Compound 6 (1,2,4-benzenetriol) will have the highest Rf value when cyclohexane (non-polar) is used as the eluting solvent. This can be easily explained. The TLC plate has silica (polar) as the adsorbent. More polar compounds will adsorb strongly to the polar stationary phase while less polar compounds (compound 5, benzene in this case) will adsorb to the stationary phase weakly. When a non-polar solvent like cyclohexane is used, benzene will flow more easily with cyclohexane (like dissolves like) and fall off easily. In other words, compound 5 (benzene) will have the least Rf value. Compound 6 being the most polar compound, will have low solubility in cyclohexane and will spend more time interacting with the stationary phase. It will travel up the TLC plate slowly and have the highest Rf value.

22. In this case, we need to remember that ethanol is a really polar solvent. Another important thing to note is that with polar solvents, the “like dissolves like” rule is not obeyed. Instead, we must remember that polar solvents dissolve more compounds starting with non-polar compounds, then medium polar compounds and finally highly polar compounds. Thus, when ethanol is used as the solvent, compound 5 (benzene, least polar among the given compounds) dissolves first and spends more time in the mobile phase. Compound 6 (1,2,4-benzenetriol, most polar) dissolves in ethanol least and spends less time in the mobile phase. Since the time that a compound spends in the mobile phase is directly proportional to its Rf value, thus, compound 5 (benzene) will have the highest Rf value and compound 6 (1,2,4-benzenetriol) will have the least Rf value when ethanol is used as the solvent.

23. In this case, compound 6 will bind strongly to the stationary phase and move up very slowly with cyclohexane. This will lead to increased retention of compound 6 on the TLC plate or the chromatographic column. However, as increasing amounts of ethanol are added to cyclohexane, the polarity of the solvent system increases. Polar solvents dissolve more compounds starting with non-polar compounds and highly polar compounds are dissolved last. Thus, as the solvent system becomes more polar, compound 6 will dissolve into the solvent system more slowly and “come off” the plate or column more easily.

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