CHEM 242L University of Nevada Grignard Reaction Organic Chemistry Lab Report

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CHEM 242L

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Experiment #3-1: Grignard Reaction (micro-sacle approach) INTRODUCTION Organomagnesium halides were discovered by the French chemist Victor Grignard in 1900. Grignard received the Nobel Prize for his discovery in 1912, and organomagnesium halides are now called Grignard reagents in his honor. Grignard reagents have great use in organic synthesis. Grignard reagents are usually prepared by the reaction of an organic halide and magnesium metal (turnings) in an ether solvent: RX + Mg ArX + Mg Et2O Et2O RMgX Grignard reagents ArMgX Grignard reagents are very strong bases. They react with any compound has a hydrogen atom attached to an electronegative atom such as oxygen, nitrogen, or sulfur. We can understand how these reactions occur if we represent the Grignard reagent in the following ways:   R : MgX When we do this, we can see that the reactions of Grignard reagents with water and alcohols are nothing more than acid-base reactions; they lead to the formation of the weaker conjugate acid and weaker conjugate base. The Grignard reagent behaves as if it contained the anion of an alkane, as if it contained a carbanion:   R MgX Grignard reagent (stronger base) + H OH Water (stronger acid, pKa 15.7) R H Alkane (weaker acid, pKa 40-50) + OH Hydroxide ion (weaker base) Reivew organometallic compounds from the lecture textbook. 1 + Mg2+ + X REACTION SCHEME In this experiment, you will prepare a Grignard reagent (organomagnesium reagent). The reagent is phenylmagnesium bromide and will be converted to a tertiary alcohol. O Br + Mg ether MgBr O + Phenylmagnesium bromide Bromobenzene Methyl benzoate MgBr CH3OH MgBr2 MgCl2 + OH C HCl O C PROCEDURE (experiment video, https://youtu.be/x9aARD8ScPQ) • Formation of the Grignard Reagent: 1. Transfer about 10 mL of anhydrous diethyl ether from the stock bottle into a 25 mL Erlenmeyer flask, and you will use this flask during the course of this experiment. 2. Place 2.1 mmol of magnesium turnings, a spin vane, a couple of crystals of iodine, and 2 mL of diethyl ether in a 10 mL round-bottom flask. Keep this solution stirred gently. 3. Assemble the apparatus on a hot plate as shown, and seal off the open end of the Claisen head with a rubber septum. 4. Set the hot plate temperature properly to boil the solution gently. 5. Place 2.5 mmol of bromobenzene in a 5 mL conical vial and record the exact weight of the material transferred in your notebook. 6. Add 1 mL of diethyl ether to the vial containing bromobenzene. 7. Withdraw this bromobenzene/diethyl ether solution into a 3 mL syringe with a needle. 2 8. After inserting the syringe needle through the rubber septum, add the solution to the 10 mL round-bottom flask slowly (dropwise) in a period of 30 minutes. 9. Continue to reflux the solution until all magnesium turnings disappear. 10. Record any observation in your notebook. • Formation of the product – triphenylmethanol 1. Place 1.0 mmol of methyl benzoate into a 3 mL conical vial and record the exact weight of the materials transferred in your notebook. 2. Add 1 mL of diethyl ether to the vial containing methyl benzoate. 3. Withdraw this methyl benzoate/diethyl ether solution into a new 3 mL syringe with a needle. 4. Insert the syringe needle through the rubber septum and add the solution to the 10 mL flask containing Grignard reagent slowly over a 5 minute period. 5. Continue refluxing the solution for another 30 minutes. NOTE: If the solvent level decreases too low during the reflux, you should add more diethyl ether through the septum as before. 6. Record any observation in your notebook. 7. Cool down the reaction mixture to room temperature. 8. Add 3M HCl dropwise until no more bubbles form, and stir the solution on the hotplate without heating. NOTE: if your aqueous layer is not clear, consult with your instructor. 9. Take your 10 mL round-bottom flask to the fume hood, and add 20 drops of water slowly. 10. Carefully remove the bottom layer (aqueous layer) by using a pipette and discard. 11. Transfer the remaining organic layer and the spin vane to a clean 5 mL conical vial. • Isolation of the product 1. Add 1 mL of water to the 5 mL conical vial and stir vigorously for 1 minute. 2. Remove the aqueous layer. 3. Repeat the procedure #1 and #2. 4. Add 1 mL of saturated NaCl (aq) to the 5 mL conical vial. 5. Remove the aqueous layer. 6. Dry the organic layer with anhydrous sodium sulfate. 7. Transfer the organic layer to a 25 mL Erlenmeyer flask with a boiling chip. 8. Place the flask on a hot plate and evaporate about a half amount of the solvent (diethyl ether). 9. Add 20 drops of hexane to the Erlenmeyer flask until the solution reaches the saturation point for recrystallization. 10. Allow the solution to cool slowly to room temperature for crystallization. 11. Collect all crystals by vacuum filtration. 12. Determine the theoretical yield, the actual yield, and the percent yield. 13. Obtain your own IR and 1H NMR spectra of the final product. 3 Notes for the Template: 1. 2. Do not turn in this first page (which is this page you are looking at now). Remove all text inside of the parenthesis as well as the example paragraphs. Your final write-up shouldn’t contain any parenthesis except for the ones you choose to add. There will be point deduction if you still have them in your final, submitted draft. NAME: (your name) CHEM 242L SECTION #: (your section #) UNLV 2020 Summer Session III GRIGNARD REACTION Purpose: (Specifically state the purpose of the experiment and evaluation methods in a few sentences.) Procedure: (Write a paragraph of the detailed experimental procedure including observation in 3rd person past tense. In this report, do not copy your notebook procedure. See the following example for your understanding.) Example: 316 mg of ferrocene (1.7 mmol) and 2 mL of acetic anhydride were placed in a 10 mL of roundbottom flask, to this mixture, 10 drops of 85% phosphoric acid was added slowly with stirring. A watercooling condenser was attached, and the mixture was heated for 20 minutes in a 90 ˚C water bath. After cooling down, 1.0 mL of ice-cold water was added dropwise to the reaction mixture. The diluted mixture was poured into a 50 mL beaker containing about 10 g of ice. The mixture was neutralized by adding approximately 5 g of solid sodium bicarbonate (about 0.25 g at a time) until the CO 2 stops bubbling off and pH = 5 – 6 with the pH paper testing. The crude solid product was collected by vacuum filtration and washed thoroughly with cold water. After determining the mobile phase by TLC, the crude product was purified by column chromatography using ethyl acetate/hexane (1:9, v/v). Data/Results: (Record all collected and calculated data from the experiment video. If possible, a tabular form is preferred.) Reagents Required # of mmol Magnesium 2.1 mmol Bromobenzene 2.5 mmol Methyl benzoate 1.0 mmol Required mass Actual mass used Actual # of mmol (mg) (mg) • The mass of the pre-weighed watch glass = • The mass of the watch glass + the product = • The actual yield of the product = • The limiting reagent = • The theoretical yield = (Calculation required, use the formula function of the MS word or insert your calculation as an image) • The percent yield = (Calculation required, use the formula function of the MS word or insert your calculation as an image) • IR Analysis: Identify all respective peaks and assign all functional groups from the provided IR spectrum (If necessary, insert additional rows in the table) Peaks Position Observed (cm-1) • Assignment (functional group) NMR analysis: Identify all chemical shifts and assign protons from the provided NMR spectrum. H (a) H (b) H (c) O H (d) Chemical Shifts (ppm) Assignment (proton groups) Conclusion: Summarize your results and draw conclusions from the experiment using complete sentences in a narrative form. Did you prepare the proper substances or perform the experiment properly? Always include your yield data (grams and percent) and any other pertinent data collected (melting point, TLC Rf values, etc.). If your yield was low, discuss any possible sources of error. Can you suggest any improvements in the procedure? If you were given an unknown, always include the unknown code with your analysis. Example: "Acetylsalicylic acid was synthesized from salicylic acid. The yield was 5.32 grams, which represents a 76% yield. The melting point of the acetylsalicylic acid was determined to be 131-134 °C which is slightly below the literature data of 135-136 °C, indicating the presence of impurities. The crystals were white and powdery in appearance, but did give a slight purple color on testing with ferric chloride, indicating the presence of some unreacted salicylic acid in the product. Post Lab Questions 1. Why is it very important to use moisture free glassware and a moisture free environment during the preparation of Grignard reagent? 2. Why is it important to add bromobenzene dropwise and slowly to the Magnesium/Ether mixture? 3. State two purposes of adding 3.0M HCl. 4. What is the definition of a good recrystallization solvent?
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NAME: (your name)
CHEM 242L
SECTION #: (your section #)
UNLV 2020 Summer Session III

GRIGNARD REACTION
Purpose:
The purpose of this laboratory session was to synthesize triphenylmethanol through the
Grignard reaction. The Grignard reagent was obtained from the reaction between
magnesium and bromobenzene in dry diethyl ether which was then reacted with methyl
benzoate to yield the desired product. The product was characterized by IR and NMR
spectroscopy.
Procedure:
Grignard reagent preparation:
10 mL of anhydrous diethyl ether was placed into a 25 mL Erlenmeyer flask. This flask
was used during the preparation of the reagent in further steps whenever the solvent was
required. Separately, in a 10 mL round-bottom flask, 51.4 of magnesium turnings, a spin
vane, a few crystals of iodine, and 2 mL of diethyl were added and gently stirred. A Claisen
head was connected to the flask and one of its open ends (the one directly above the
flask) was sealed off with a rubber septum, while in the other a clean water condenser
was attached to prevent the solvent evaporation. The apparatus was placed on a hot plate
and the water condenser was turned on. Then, 1 mL of diethyl ether and 410.2 mg
bromobenzene was added to a vial. The solution was transferred to the 10 mL round
bottom flask on the heating mantle with the help of a syringe. After 30 minutes passed the
Grignard reagent was prepared while observing the solution turned mil...


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