The average Rf value of unknown 25 with ethyl acetate as the reactions eluent

The average rf value of unknown 25 with ethyl acetate

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The average Rf value of unknown 25 with ethyl acetate as the reaction’s eluent is 0.842, whereas the average Rf value for the sample using hexanes as the eluent is 0.586. The difference between these two values is not excessive (less than 0.30), which indicates that the polarity of the unknown sample although isn’t strong, it isn’t too much less than the carbonyl functional group and is almost equal to the aromatic group, as the Rf values for biphenyl were close to the same values as the unknown. Therefore, based on these Rf values and the physical properties of the unknown sample (namely, the speed at which the compound evaporated and became increasingly concentrated), it can be determined that the unknown is an aromatic compound, containing the closed-ring structure as the functional group characteristic of this organic family. It should be noted that one source of error that occurred during this experiment is evident in TLC plate 5 (Figure 5.0), where on the TLC plate the samples from the unknown sample and the
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co-spot mixed together on the silica and formed 2 very large circles in the middle of the plate. Although the Rf values are seemingly unaffected, this could be due to the fact that aromatic compounds evaporate quickly into the open air, leaving what is left of the sample inside the beaker to become increasingly concentrated with passing time. This lack of accuracy could have affected the Rf values used to determine the functional group of the unknown sample, ultimately leading to its naming being incorrect. Questions: 1) It’s important to make co-spots last in order to prevent cross-contamination. Were the co-spot made first, it’s possible that some of the sample being analyzed would be transferred to the reference lane, confounding results. 2) Selecting a solvent system of appropriate polarity is critical in performing a successful TLC. If the solvent system is too polar (as was the case with ethyl acetate), both the sample and the reference compound will travel to the edge of the solvent front, rendering the TLC useless. One achieves similarly useless results with an insufficiently polar solvent, like hexanes. Neither of the compounds move far enough through the silica gel to be separated. 3a) 3b)
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3c) 3d) It’s better to use a spot of sample A than a spot of sample B for reference because, with B being the end product, the co-spot would overlap at the end. This would leave some ambiguity as to whether the reaction was truly complete. One would also lose their frame of reference for the Rf of compound A, as, when the reaction is complete, the reactant spot will have disappeared.
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4a) 4b)
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4c) 4d) It’s important in this case to use a co-spot because the reactant and product have very similar Rf values. One may not be able to judge how far apart they are just by “eyeballing” two different
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  • Spring '08
  • Chemical polarity, Benzophenone

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