Chem 241 Lab Manual - Lab 3 - Friedel-Crafts Acylation

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CHEMISTRY 241 Organic Chemistry Laboratory 1 Lab Manual Lab 3 – Friedel-Crafts Acylation Professor Lawrence Goldman Professor Tomi Sasaki Department of Chemistry University of Washington

Chemistry 241 Lab 3: Friedel-Crafts Acylation and Column Chromatography 2023-2024 2 L AB 3: F RIEDEL -C RAFTS A CYLATION AND C OLUMN C HROMATOGRAPHY Scenario: Your supervisor has asked you to prepare some pure diacetylferrocene to be used in a research project. The only related compound you have in stock is ferrocene, so you will need to convert the ferrocene into diacetylferrocene. Your first task will be to determine what reaction to use. In this case, the Friedel-Crafts acylation seems like a good choice. Your second task will be to separate your diacetylferrocene away from other compounds such as acetylferrocene or unreacted ferrocene. Your third task will be to see how the reaction conditions can be modified to favor diacetylferrocene as the major product (or alternatively, to favor acetylferrocene as the major product, in case your supervisor decided that will be the next research target). As part of these tasks, you will need to apply skills from the previous experiments, such as deciding what purification technique will be useful at separating ferrocene, acetylferrocene, and diacetylferrocene. And deciding what analytical technique(s) will be best at determining how well your reaction and purification worked. Theory: A question that arises in organic chemistry is how to design ideal reaction conditions. If not enough of one reactant is used, the reaction might not go to completion. If too much is used, material may be wasted. In either case, unwanted byproducts may be formed. Part of this experiment will be to determine the ideal amount of acetyl chloride to use in order to select for either acetylferrocene or diacetylferrocene as the major product. Friedel-Crafts acylation reactions are a type of electrophilic aromatic substitution reaction. In these reactions, an acyl chloride (R(C=O)Cl) is combined with a Lewis acid catalyst to generate an acylium cation (RCO + ).

Chemistry 241 Lab 3: Friedel-Crafts Acylation and Column Chromatography 2023-2024 3 The electrophilic acylium cation can then react with the aromatic compound in an electrophilic aromatic substitution reaction. The acylium ion performs an electrophilic attack on the aromatic ring to generate the arenium ion. Deprotonation of the arenium ion restores aromaticity and regenerates the Lewis acid catalyst. In our experiment, the acyl chloride will be acetyl chloride (R = CH 3 ) and the aromatic substrate will be ferrocene. Ferrocene is a neutral metal complex that contains an Fe 2+ ion “sandwiched” between two planar cyclopentadienyl anions. The cyclopentadienyl anion has six pi electrons delocalized (note the resonance structures shown below) throughout the ring, and is thus aromatic (recall Hückel’s 4n+2 rule). The related compound cyclopentadiene (C 5 H 6 ) is not fully conjugated, and is therefore non-aromatic. Since the cyclopentadienyl moiety contains a carbanion and carbanions are generally unstable, one might expect the C 5 H 5 – group to be quite reactive. However, on top of the ion being aromatic, the iron forming a complex with the cyclopentadienyl anion stabilizes it further and ferrocene is fairly stable. Other metals will also form a complex with ferrocene, including cobalt, nickel, chromium, zinc, manganese, molybdenum, osmium, rhodium, ruthenium, zirconium, vanadium, titanium, and uranium.

Chemistry 241 Lab 3: Friedel-Crafts Acylation and Column Chromatography 2023-2024 4 In your reaction, both mono- and diacetylation will occur. Neither recrystallization nor acid-base extraction will allow you to separate these products, but don’t lose hope! We will use two related techniques to analyze and separate the mixture of ferrocenes. Both of them separate compounds based on their polarity. Theory of Column Chromatography: Column chromatography is essentially TLC on a larger scale. We fill a burette with silica, load our mixture at the top, and let solvent flow from top to bottom. As with TLC, the more polar the compound, the slower it moves because it is interacting more strongly with the silica. Depending on their size, columns can be loaded with 10mg-100g of material, making them ideal for separating compounds that couldn’t be separated with simpler methods like extraction and distillation. Below is an example of a column separating red, orange, and yellow material. Just like the TLC, the compounds can be observed as they separate. Colorless compounds can also be separated with TLC/columns, but a different approach must be used to determine where the compounds are. Safety Information: In this experiment you will use acetyl chloride and aluminum chloride. These reagents are corrosive and release HCl upon reaction with water, including moisture in the air. Avoid contact with these materials; take appropriate precautions, including wearing gloves and all normal lab safety equipment (goggles, lab coat, etc.). If you do get some of these chemicals on your skin, immediately wash that area with lost of cold water. Keep these materials in the fume hood at all times. Alert your TA immediately about any chemical spills. Pre-lab Information: There is 1 pre-lab quiz for this experiment. Reading: PLKE Techniques 7.7 (drying tubes), 12.4-12.6 (micro-scale extraction), and 19 (column chromatography); Loudon 6th 16.4-16.5, 15.7D or McMurry 5.1-5.2 and 5.6.

Chemistry 241 Lab 3: Friedel-Crafts Acylation and Column Chromatography 2023-2024 5 Reminder on Notebook Pages: This is the first experiment where you are running a reaction. Therefore your lab notebook should include a proper reaction table (see pages ##-## for more details). Some, but not all, of the information you should include in the reaction table are amounts you are using, theoretical yields, and relevant physical properties. For this experiment, since you don’t know exactly how much acetyl chloride you will use, write your initial reaction table assuming 2 equivalents of acetyl chloride. Calculate your theoretical yields of acetylferrocene and diacetylferrocene assuming that the reaction produces 100% of each. Materials Chemical Name Safety Assessment Notes FW (g/mol) Equiv Amount mmol Density (g/mL) 4-Pentyn-1ol Irritant 84.12 1.0 1.4 mL 15.0 0.904 Pivaloyl chloride Lachrymator 120.58 1.2 2.22 mL 18.0 0.979 Pyridine Flammable, Affects CNS, Teratogen 79.1 10.0 12.13 µL 150.0 0.98 Dimethylaminopyridine Toxic 122.17 0.1 180 mg 1.5 (Product) Pent-4-yn-1-yl pivalate Toxic 168.23 1.0 2.52 g 15.0 (Product) Pyridinium chloride Corrosive, Irritant 115.56 1.0 1.73 g 15.0 Notebook Assignment: During this lab you will perform an extraction. In your notebook, draw a flow chart that includes all the pertinent details for the extraction (solvents, products, amounts, impurities, drying agents, etc.). You should be specific about which compounds are in each layer at each point in the extraction. If there are multiple extractions, include this information for each. This is NOT a flow chart for the entire procedure, just the extraction. D AY 1 - F RIEDEL -C RAFTS A CYLATION P ROCEDURE : Friedel-Crafts Reaction: You will work individually for this experiment. Half of you will stir your reactions, while half will not. There will also be 4 different volumes of acetyl chloride to use. So in total, there will be 8 variants of the procedure. Your TA will assign you to 1 of them. Our goal is to see which procedures are best for forming acetylferrocene or diacetylferrocene.

Chemistry 241 Lab 3: Friedel-Crafts Acylation and Column Chromatography 2023-2024 6 Add 95-105 mg of ferrocene and 145-155 mg of aluminum chloride into a snap-cap vial, cap it, and mix thoroughly. Add the solid mixture to a 50-mL round-bottom flask and attach a drying tube filled with Drierite and cotton to it. Transfer the appropriate amount of acetyl chloride assigned by your TA (5, 10, 15, or 20 drops) into an Eppendorf tube via pipette and cap it. Assume that each drop of acetyl chloride is 0.01 mL. Remove the drying tube from the RBF and add all of the acetyl chloride dropwise (ensuring that each drop makes contact with the solid mixture), then quickly reattach the drying tube. Reaction procedure 1 (half of you will be assigned this by your TA): Add a stir bar to your flask and stir the reaction for 30 minutes. Reaction procedure 2 (half of you will be assigned this by your TA): Allow this mixture to sit at room temperature for 30 minutes. At the conclusion of the reaction time, add a stir bar to the flask. Workup (for everyone): Add 3-4 mL of diethyl ether to the RBF and stir. Next, add 2-3 mL of DI water, then ~3 mL of 6M NaOH. Allow this mixture to stir until most of the solid residue at the bottom of the RBF has dissolved. Then, pipette the solution into a glass centrifuge tube, mix the layers well, and remove the ether (top) layer. Wash the aqueous layer twice more with 3 mL portions of diethyl ether and combine the ether layers. Dry the combined ether layers over sodium sulfate, decant the liquid off into a beaker, and wash the sodium sulfate with another 2-3 mL of diethyl ether. Analyze the dried ether extracts via TLC (using a mobile phase of 80% hexanes-20% acetone). Evaporate the solvent via a low flow of air. Safety note: You should dispense the acetyl chloride into an Eppendorf tube and then cap it in order to prevent it from hydrolyzing and to prevent anyone from inhaling vapors. TLC Analysis: The small aliquot from your ether extracts that you saved earlier (see above) will be used to prepare TLC plates. Prepare a TLC plate spotted with your reaction mixture aliquot and with mixed standards of pure ferrocene, acetylferrocene, and diacetylferrocene in diethyl ether. Your undeveloped plate should look like the one below: Make sure that your baseline is above the solvent. Develop your plate using 80% hexanes / 20% acetone as your solvent mixture. Calculate the R f values for all of the spots on your plate and determine which products were formed in your reaction. If you are having trouble seeing your spots, put your plate under the UV lamp. This is the stopping point for Day 1. Make sure to save your product for your use in Day 2.

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