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This 0 page Study Guide was uploaded by Gabriela Montalvo on Friday December 4, 2015. The Study Guide belongs to chm at University of Miami taught by Lab TA in Fall 2015. Since its upload, it has received 66 views.
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Date Created: 12/04/15
Gabriela Montalvo C10596017 CHM 205 Atul Parab October 13 2015 Experiment 33 Thin Layer Chromatography Post Lab Questions 1 pHydroxyacetophenone is very polar compared to 0Hydroxyacetophenone Because PS polar compounds have stronger dipoledipole interactions they require a higher output of kinetic energy temperature in order to break melt The large difference in the melting points between the two has to do with the difference in steric hindrance For example p hydroxyacetaophenone has a larger space provided for hydrogen bonding of hydroxide But for ohydroxyacetaophenone the double bond on the oxygen doesn t allow for easy hydrogen bonding to the hydroxide A Rf of 091 suggests that the polarity of the solvent used is much too high Ideally an Rf of about 03 is desired Switch to a solvent of lower polarity such as dichloromethane If the Rf is still too great employ a lowpolarity cosolvent such as hexane ie use hexane dichloromethane solvent pairThe solutes did not retard enough so a stronger solvent such as petroleum ether should be used B Again the solvent must be too polar and therefore you must pick a solvent that is less polar Also the solute was retarded too much a weaker solvent is needed such as ethyl acetate C Because the solvent covered the spots on the TLC plate baseline the solute had dissolved into the solvent and you will not get anything that separates on the plate For this you have to redo your plate and make sure that you spot high enough so that the solvent doesn39t react the baseline and dissolve the solute Also the solute was retarded too much to the point that it did not travel at all Less solvent should be used to solve this problem The order from bottom to top should be otoluic acid uorenol naphthalene The compounds with the larger Rf values are 4decanone xylene and cyclohelptane With regard to increasing polarity heptane lt toluene lt dichloromethane lt npropanol lt ethyl acetate lt ethanol The three important issues to keep in mind while spotting are not to put too much of the solute keep the time of contact very small and keep the solutes in their own lanes MCAT Practice Questions 7 A Spot with the smallest RF 8 C Double 9 A H20CH3CH220HBF3 II Purpose The purpose of this experimentation is to use the technique of chromatography in order to separate a mixture or a compound Such separation aids in the identification quantification and purification of the unknown substance This procedure of chromatography has its bases on the theory that there are different degrees of mobility for different compounds on the adsorbent layer Such movement is dependent on the chemical bond interaction strength the respective compound has with the adsorbent Chromatography can be split into two phases the mobile phase and the stationary phase The former is the point in which the mixture of compounds is dissolved into a liquid whereas the latter phase is when the solution is transported along an immobile non soluble adsorbent The compound used for the adsorbent in this thin layer chromatography experiment is silica gel The characteristic which makes silica gel an ideal adsorbent for the stationary phase is that it is very porous allowing it to have a large surface area for interaction with other molecules 111 Part A Solvent Front 01 cm 59 cm Spotting Line 10 cm from the bottom of the TLC ohydroxyacetaophenone distance 24 cm from the spotting line phydroxyacetaophenone distance 17 cm from the spotting line Rf phydroxyacetaophenone i01 029 Rf ohydroxyacetaophenone i01 041 Part B Unknown 4 Solvent Front 01 cm 52 cm Unknown distance 37 cm Rf for Unknown 4 01 071 Rf for Acetylsalicylic acid i01 071 Identification of 4 Bayer s Aspirin IV Results The Rf for phydroxyacetaophenone found in Part A of the experiment is 029 whereas the Rf found for ohydroxyacetaophenone is 041 With comparison of distance on the plates to acetopmenaphin ibuprofen acetylsalicylic acid and caffeine the unknown 4 is identified as Bayer s Aspirin Therefore it only contains high levels of acetylsalicylic acid V Discussion During the procedure there were many chances of error due to human and machine error In Part A we were trying to get a feel for how thin layer chromatography TLC works and see which substance 0Hydroxyacetophenone or pHydroxyacetophenone would go farther on the chromatography plate based on their differences in structure Based on our observations we can say that 0 Hydroxyacetopenone traveled farther than pHydroxyacetopenone based on the R F values received Even though these two compounds have the same functional group they still have some differences that could have caused this difference in R F value to happen For one 0Hydroxyacetopenone has its functional groups on the 1 and 2 carbons of the ring Because these groups are so close they interact with each other forming a benzene ring which makes the compound a little more stable Because 0Hydroxyacetopenone is more stable it makes it less polar and that is why it interacts more with the solvent and moves up the plate farther p Hydroxyacetophenone has a higher polarity then 0Hydroxyacetopenone one that more closely related to that of the plate and therefore it does not react as much to the solvent and doesn39t move as far on the plate In Part B we had an unknown compound and we had to see which compound it was based on the comparison to four known substances For Part B during placement of the plates my finger touched the plate This led to oil of my finger to imbed onto the gel causing it to spot under the UV light Such an error can cause confusion on which spot was for which compound leading to inaccurate calculated Rf values VI Conclusion The overall experiment was successful due to the unknown 4 being accurately identified as Bayer s Aspirin The spotting of the plates in Part B showed that the unknown was closet to acetylsalicylic acid in distance and in Rf value Whilst acetaminophen ibuprofen and caffeine have very low Rf values acetylsalicylic acid is the only compound that showed a large amount of mobility Such allowed the conclusion to be made that Bayer s Aspirin was the unknown compound therefore containing 325 mg of acetylsalicylic acid per tablet Such high amounts of mobility show small amounts of interaction with the adsorbent According to literature the lack of interaction between the two signifies a low polarity characteristic of the unknown compound Gabriela Montalvo C10596017 CHM 205 Atul Parab October 13 2015 Experiment 3b Column Chromatography I Post lab questions 1 Ferrocene eluted from the column first because it is the less polar molecule and doesn39t react as much with the absorbent It was eluted off with the addition of petroleum ether which is not a very polar solvent We had to change the solvent from petroleum ether to MtBE in order to move the more polar component of our mixture Acetylferrocene which was eluted only after the addition of the polar solvent Ferrocene moved through from the column first and traveled a farther distance up the TLC plate because it was less polar compared to the Acetylferrocene Therefore it did not react as much with the absorbent as much as Acetylferrocene did and was able to move through the absorbent easier after being washed with petroleum ether It is important to limit the heat in the sand bath because if the temperature is too high then it will melt our collected fractions Especially when it came to the Acetylferrocene which has a melting point around 810C if the sand bath was too hot then there is a potential problem of melting the Acetylferrocene on the spot a You can find two different solvents ones that are more closely related to that of the two different halogenated alkenes That way one alkene will elute when the first less polar solvent is added and then the other will elute in the more polar solvent is added b In this case the absorbent is too polar and reacts with the components of the mixture more than the solvent did Therefore you can either find a more polar solvent to elute the mixture or find a less polar absorbent The elution order of the three compounds is naphthalene first then 0toluic acid and then uorenol If there were an unknown mixture that contained three compounds first I would start with a solvent that was not very polar This would cause elution of the first compound that is not as polar as the other two compounds Then I would add a solvent that is slightly more polar so that the second compound could be eluted off the column next Then lastly I would add a highly polar solvent so that the rest of the compound would be eluted as the most polar All of this must be done so that you can visually see the different parts of the mixture being separated A new fraction must be started every time a specific volume elutes Thus the fractions are separated based on volume instead of by color MCAT Review Ouesti 8 C pNitrophenol 11 Data After separating the FerroceneAcetylferrocene mixture through column chromatography we were able to measure the amount of each component there was in our initial sample and the melting point of these components Weight of FerroceneAcetylferrocene 0105 g Weight of Alumina 0108 g YellowOrange Band Ferrocene Weight of Erlenmeyer ask with boiling 14835 g chip Weight of Erlenmeyer ask with boiling 14874 g chip and compound Weight of Compound 0039 g Melting Point Measured 1632 C 1664 C Melting Point Expected 17 25 C Calculated R f 070 Percent Recovery 3714 RedOrange Band Acetylferrocene Weight of Erlenmeyer ask with boiling 14556 g chip Weight of Erlenmeyer ask with boiling 14588 g chip and compound Weight of Compound 0032 g Melting Point Measured 801 C 833 C Melting Point Expected 8183 C Calculated R f 019 Percent Recovery 3048 TLC Analysis Solvent Front Line 47 cm YellowOrange Band Ferrocene 33 cm RedOrange Band Acetylferrocene 09 cm III ResultsDiscussion During column chromatography we were able to see the different compounds of our FerroceneAcetylferrocene mixture separate at different times based on polarity The Ferrocene was separated down the column when using the less polar solvent petroleum ether and then the acetylferrocene moved through the column when using the more polar solvent MtBE This shows that Ferrocene is the less polar of the two and it reacts less with the alumina absorbent Acetylferrocene is the more polar component and reacts more with alumina absorbent taking a more polar solvent to move it down the column We can also see this in the TLC plate attached Ferrocene moved through the absorbent using the petroleum ether solvent and has the larger R f value of 070 Acetylferrocene moved through the absorbent using the MtBE solvent and yielded a smaller R f value of 019 Then we took our separated Ferrocene and Acetylferrocene and evaporated the liquid from the sample From this we could calculate the recovery of each compound from our starting mixture The Ferrocene component had a higher percent recovery of 3714 The Acetylferrocene on the other hand had a lower percent recovery of 3048 This could have been due to a mistake made during the elution process when we tried to speed up the drip rate and accidentally allowed some of the redorange substance to elute into Test Tube 3 Some other factors that could have been a cause of a lower percent recovery might have been due to a mixed separation due to column chromatography or by overheating the compound during the evaporation process The last part of the analysis that was conducted on our compounds was the melting point analysis Both the melting point for our Ferrocene and Acetylferrocene were in the range of the expected melting point There could have been errors in this part of our experiment due to the impurities in the sample as it mixed with other compounds from the chromatography process When observing our crystals that were made we can see that Ferrocene had a much more pronounced crystal which could account for its high melting point Then acetylferrocene was more of a powder and that could be a reason why it had a lower melting point IV Conclusion In conclusion Ferrocene was a less polar molecule that moved through the column faster forming a yelloworange band with the addition of petroleum ether and had a higher observed melting point of 1632 C 1664 C Acetylferrocene was a more polar molecule that moved through the column chromatography slower forming a red orange band with the addition of MtBE and showed a lower observed melting point of 801 C 833 C
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