TLC PLQ

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Chemistry

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Feb 20, 2024

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CHEM 240 • Laboratory Hobart and William Smith Colleges Modules Post Lab Questions (Fill out this document and submit electronically) Please make sure your answers are in blue! HW1. A student ran the following reaction: Match the following scenario with the representative TLC (sm= starting material, co = cospot, rxn= reaction pot) Scenario Answer 1. The reaction did not run to completion (not all sm was consumed) C 2. The reaction ran to completion (all starting material was consumed). D 3. The sm spot was run with too polar an eluent. B 4. The sm spot was run with too nonpolar an eluent E 5. No reaction has occurred. A 6. Too much sm was loaded onto the adsorbent F The student could make the eluent more polar. The amount of Hexane used could be lowered in comparison to the ethyl acetate mixture CHEM 240 Laboratory Post-Lab Materials

CHEM 240 • Laboratory Hobart and William Smith Colleges Modules (EtOAc) which would make the eluent more polar. The ratio could be 1:2 Hex:EtOAc or any other ratio where Hexane was in a lesser amount. By increasing the polarity of the eluent, the compound will be moved further up the plate, creating more separation. This is because the more polar eluent will interrupt CHEM 240 Laboratory Post-Lab Materials

CHEM 240 • Laboratory Hobart and William Smith Colleges Modules the intermolecular forces interacting between the compound and the absorbent allowing the compound to move mor HW2. A student ran the following TLC using 1:1 Hex:EtOAc and observed the TLC below. a. How might the student alter the eluent ratio to effect a better separation? Explain your reasoning. b. The student could make the eluent more polar. The amount of Hexane used could be c. lowered in comparison to the ethyl acetate mixture (EtOAc) which would make the eluent CHEM 240 Laboratory Post-Lab Materials

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CHEM 240 • Laboratory Hobart and William Smith Colleges Modules d. more polar. The ratio could be 1:2 Hex:EtOAc or any other ratio where Hexane was in a e. lesser amount. By increasing the polarity of the eluent, the compound will be moved further f. up the plate, creating more separation. This is because the more polar eluent will interrupt g. the intermolecular forces interacting CHEM 240 Laboratory Post-Lab Materials

CHEM 240 • Laboratory Hobart and William Smith Colleges Modules between the compound and the absorbent allowing the h. compound to move mor i. The student could make the eluent more polar. The amount of Hexane used could be j. lowered in comparison to the ethyl acetate mixture (EtOAc) which would make the eluent k. more polar. The ratio could be 1:2 Hex:EtOAc CHEM 240 Laboratory Post-Lab Materials

CHEM 240 • Laboratory Hobart and William Smith Colleges Modules or any other ratio where Hexane was in a l. lesser amount. By increasing the polarity of the eluent, the compound will be moved further m. up the plate, creating more separation. This is because the more polar eluent will interrupt n. the intermolecular forces interacting between the compound CHEM 240 Laboratory Post-Lab Materials

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CHEM 240 • Laboratory Hobart and William Smith Colleges Modules and the absorbent allowing the o. compound to move mor The amount of Ethyl acetate used could be increased in comparison to the Hexanes which would make the eluent more polar. The ratio could be 1:2 Hex:EtOAc or any other ratio where Hexane is has lower amount compared to EtOAc. By increasing the polarity of the eluent, the compound will be moved further up the plate, creating higher separation. This is because the more polar eluent will interrupt the intermolecular forces interacting between the compound and the absorbent allowing the compound to move mor b. Generate in ChemDraw a TLC plate that predicts what the TLC would look like given your eluent change. The TLC tool is under chromatography tools drop-down sixth up on the right. To insert different shapes for the spots, the “orbital tools” are helpful. (7 th down on the right, most useful for this application are the dark shaded  -orbitals. These are directly below the s- orbitals, which are also useful in a pinch.). You may need to draw the orbitals OUTSIDE of the TLC plate, then grab the “Marquee” tool (top right box selection tool). You should now be able to reshape and move the spots to your heart’s content. You can also right-click or use the menu to change colors! CHEM 240 Laboratory Post-Lab Materials

CHEM 240 • Laboratory Hobart and William Smith Colleges Modules HW3. Look at the structures below of two common dyes, which you may recognize as “standards” that were available for comparison with the markers. (Note: these were most likely not the same dyes in the marker) Indicate the most polar area/functionality for each compound. (Copy or redraw the image below and paste into ChemDraw, then use the circle or oval tools in the “drawing” dropdown, 8 th on the right. Alternatively, the arrows are the 6 th dropdown on the right.) methyl red methylene blue HW4. Explain why the eluent system you used in the marker TLC was 3:2 acetone:ethanol and not hexanes:ethyl acetate like we used in the TLCs in HW2 [Hint use your knowledge from P11 and D2 and D3 and consider the structures of common dyes in HW3 ]. Both acetone and ethanol are very polar functional groups and the polarity gradient created in the 3:2 acetone: ethanol eluent is much higher than hexanes:ethyl acetate eluent. The structures illustrated above are also very polar. Methyl red has a carboxylic acid functional group and methylene blue has a permanent full charge making it very polar. Hence, a very polar eluent is needed to move the dyes up the plate due to the consequential strong intermolecular forces interacting between the dyes and the absorbent–polar eluents are better at interrupting those interactions and moving the compounds farther. CHEM 240 Laboratory Post-Lab Materials

CHEM 240 • Laboratory Hobart and William Smith Colleges Modules HW 5. List at least two concepts from this or any previous lecture courses that are foundational to understanding the thin layer chromatography lab: Polarity and solubility rules are crucial to understanding the thin layer chromatography lab. It is important to know how molecules will behave in certain solvents based on their polarity–where more polar compounds will not travel that far up the TLC plate. For example, low polarity compounds can be eluted with low polarity solvents, while higher polarity compounds require solvents of higher polarity. Moreover, the solubility of different compounds in the eluting solvent plays an important role in how fast they move up the TLC plate. Absorption is also crucial to understanding the TLC lab. Absorption is a process by which molecules interact with the molecules on the surface of a solid called the absorbent–the forces in that arise are from dipole-dipole and ion-dipole interactions, and H-bonds CHEM 240 Laboratory Post-Lab Materials

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