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i need help with the postlab questions please

Grignard reagent THE GRIGNARD REAGENT-PREPARATION AND REACTION In this experiment, you will prepare a Grignard reagent, phenyl magnesium bromide, from bromobenzene and magnesium metal. The Grignard reagent will then react with methyl benzoate to form triphenylmethanol. Introduction Grignard reagents, such as organomagnesium halides, were discovered in 1910 by French chemist Victor Grignard. The Grignard reaction is one of the most general methods for carbon- carbon bond formation in all of organic chemistry. In the first stage of this procedure, an organic halide reacts with magnesium metal to form an organomagnesium compound, which is known as a Grignard reagent. It is important to recognize that this is yet another example of an organic redox reaction. More specifically, the carbon atom that was initially bonded to the halide is reduced by two electrons, and the magnesium is oxidized by two electrons By forming the Grignard reagent, we have effectively reversed the polarity of the bond to carbon. Since carbon has a higher electronegativity than the metal magnesium, the C-Mg sigma bond is highly polarized, placing a partial negative charge on carbon. As a result, Grignard reagents act as carbanion equivalents, and they are both strong nucleophiles and strong bases. This means that great care must be taken to exclude even relatively weak acids (such as water or alcohols) from the reaction mixture in order to avoid the formation of byproducts arising from proton transfer. Grignard reagents are almost always synthesized in ethereal solvents such as diethyl ether or tetrahydrofuran (THF) for two very important reasons: (1) ethers do not generally react with Grignard reagents (i.e. they are stable to strong bases and nucleophiles) and (2) the lone pairs on oxygen help to stabilize the partial positive charge on magnesium and facilitate formation of the Grignard reagent. Once formed, Grignard reagents can react with a neral wide variety of carbonyl-containing compounds to form new carbon-carbon bonds in a ge process known as nucleophilic addition. The C-O bond is highly polarized, making carbonyl compou ester, the ultimate product is an alcohol. nds electrophilic at carbon. If the Grignard reagent reacts with an aldehyde, ketone or Overall Reaction Br Me PartA Part B Pre-lab questions (15 points) Show the mechanism of the reaction of bromobenzene with magnesium metal in anhydrous diethyl ether. There are many inert solvents that could be used but diethyl ether was chosen because it can assist the preparation of a Grignard reagent. How so? 1. 2. 56 Grignard reagent 3. The reaction to form a Grignard (and use it) requires "anhydrous" conditions, List one thing that is done before this lab that ensures "anhydrous" conditions: 4. Why do we use anhydrous diethyl ether in Part A of this reaction? Why does the procedure call for dry glassware? Think about what would happen if water was present. Write equation. 5. How many equivalents of Grignard reagent are used in this reaction compared to the limiting reagent? 6. What is the fate of the methoxy group from the methyl benzoate in this reaction? 7. Draw the setup of a simple distillation apparatus. Include the names of all glassware. 8. What is the molecular weight and melting point of triphenylmethanol? Chemicals Week l: Mg turnings (1.2 g), anhydrous diethyl ether (-40 mL), bromobenzene (4.7 ml), 12- dibromoethane (1 drop), methyl benzoate (2.5 mL), 6M sulfuric acid (25 mL) Week 2: MTBE (-20 mL), 3M sulfuric acid (25 mL), 80% Ethanol in water (-20 mL) Procedure During week one you will generate the Grignard reagent (Part A) and react it with the ester (Part the alcohol by recrystallization. Week 1: Part A - Preparation of the Grignard reagent: phenylmagnesium bromide. B). During the second week, you will protonate the alkoxide ion, isolate the alcohol, and purify Br MgBr o Et,o phenylmagnesium bromide aka "phenyl Grignard" ne Make sure that all glassware is clean and free of water. Impurities and water will interfere with the reaction. All glassware (Claisen adapter, condenser, addition funnel, and RB flask) must be dried in an oven overnight. You can ensure the removal of water from the glassware by rinsing it with acetone and then hexane and allow the solvent to evaporate. Use anhydrous diethyl ether that was dried using molecular sieves for one week. A coat of magnesium oxide may be formed on the top of magnesium turnings that have been exposed to air. This oxide film will prevent the manesium from being in contact with the bromobenzene. See step three below on how to overcome this setback Grignard reagent 57 Apparatus Setup Drying Tubes -H2O out Addition unnel Claisen Adapter (100-mL round bottom) Procedure Add a dry stirbar and-1.2 g of magnesium turnings to a dry 100-mL RB flask and assemble the apparatus as shown in the figure above. Add 8 mL of anhydrous ether to the RB to make a suspension of magnesium in ether. To the dry addition funnel (stopcock closed!!) add 10 mL of anhydrous ether and 4.70 mL of bromobenzene and carefully swirl to homogenize. Ensure that water is running through the condenser and heat the flask with a heating mantle at a low setting (2 or 3). Slowly, add a portion of the bromobenzene/ether solution (-20-30 drops) from the addition funnel onto the magnesium turnings. A chalky appearance or color change (brown) is indication that the reaction has started. If the reaction has not started after 10 min, add an additional portion of the bromobenzene/ether solution. If the reaction still hasn't begun after an additional 10 minutes, notify your instructor and ask him/her to add 2 or 3 drops of 1,2-dibromoethane to the mixture. This compound reacts very readily with magnesium to form ethylene gas and MgBr2; this removes the unreactive oxide layer on the magnesium, leaving it free to react with the bromobenzene. Once the reaction has started, add an extra 8-mL of anhydrous diethyl ether to the reaction mixture through the condenser. This serves to dilute the reaction mixture and to minimize the coupling reaction. The rest of the bromobenzene/ether solution should now be added dropwise to the reaction mixture at a rate that is just fast enough to maintain a gentle reflux. If it is added too fast, the reaction may get out of control, and the yield will be reduced owing to increased coupling. If the reaction becomes too vigorous turn off the heating mantle 58 Grignard reagent and reduce the rate of addition. This total addition should take about 15 min. If the spontaneous boiling of the mixture becomes too slow, increase the rate of addition slightly and/or turn up the heating mantle At the end of the reaction the solution will normally have a tan to brown color, and most of the magnesium will have disappeared, although residual bits of metal usually remain. No characterization is required for Part A. Once prepared, the Grignard reagent must be immediately reacted with the ester. Do not stop here but continue to part B Week 1: Part B- The reaction of phenylmagnesium bromide with methyl benzoate to give triphenylmethanol. Dissolve 2.50 mL of methyl benzoate in 8 mL of anhydrous diethyl ether and place this solution in the addition funnel (stopcock closed). Heat the RB flask gently (mantle setting 1-2), and then begin the slow, dropwise addition of the solution of methyl benzoate to the phenylmagnesium bromide solution that is in your RB. This reaction is exothermic; control the rate of reaction by adjustment of the addition rate and/or removing the heating mantle, if necessary White solids may form during the reaction; this is a sign that the reaction is proceeding normally. After the addition is complete and the exothermic reaction has subsided, heat the reaction mixture at gentle reflux for 20 min. Near the end of this time, prepare a mixture of HSO (25 mL, 6M) and 20 g of ice in a 125-mL. Erlenmeyer flask. After allowing the RB flask to cool to room temperature, pour the reaction mixture into the Erlenmeyer flask containing the H SOvice mix and swirl. Perform several rinses with MTBE (-5 mL) to transfer your entire product into the E. flask and rinse the walls of the RB and E. flasks. Once the bubbling has stopped, stopper the Erlenmeyer and store in the location given by your instructor. Week 2 - Liquid-liquid extraction, recrystallization, and characterization. Add enough MTBE to dissolve all the solids and transfer the entire mixture to a 125-mL separatory funnel. Shake the funnel vigorously but carefully, venting often to relieve pressure. Remove the aqueous layer and wash the remaining organic layer first with HSO (25 mL, 3M) and then with deionized water (20 mL). Discard the aqueous layer. Collect the organic layer and dry over anhydrous magnesium sulfate. Grignard reagent 59 Gravity filter into a 100-mL RB flask, set up a distillation apparatus and distill away most of the solvent. Remove the heating mantle and recrystallize from a mixture of ethanol and water (80/20) (reread Mohrig book about mixed solvent recrystallizations if you are rusty on recrystallizations). Recrystallize on a hot plate and in an Erlenmeyer flask Isolate the product by vacuum filtration, using a cold solution of ethanol and water 80/20). Allow the solids to dry and record the isolated mass. Often by letting the product vacuum filter for 5-10 minutes, the product will be dry enough to start characterizing. Obtain and record the following information. 13. Crude product description 14. Pure product description 15. Pure product weigh/percent recovery 16. Overall percent yield 17. Melting point 18. IR 19. NMR Post-lab REQUIRED CALCULATIONS Remember to do the following calculations in your laboratory notebook: 1. Calculate the moles of Mg used. 2. Calculate the moles of bromobenzene used. 3. Determine which component, Mg or Bromobenzene, is the limiting reagent. 4. Calculate the theoretical yield of benzoic acid. 5. Calculate the percent yield of triphenylmethanol. Post-lab Questions Q1. Think specifically about the various processes involved. Briefly summarize your reaction yield results by commenting on the following: (4 points each) te a co selfis oomed and will a. Think about the reaction you set up. List one reason the reaction itself is doomed and will not produce 100% yield. Now consider the work-up process you did (that part that occurs once you started adding aqueous acid to isolate the product). List one reason for a decreased yield based on your work-up process that will prevent you from attaining 100% yield (the more obvious vacuum filtration washes and transfer difficulties do not count). b. Q2. MP Discussion: Cite your MP and discuss the purity of your triphenylmethanol. (2 points) Q3. Lab report should include analysis of the spectra produced by drawing the structure on the NMR or IR and labeling which peaks go with which functional groups or protons. (5 points) In this reaction, we isolated a tertiary alcohol and not the intermediate ketone. This is due to the reactivity of the ketone compared to the starting ester. Explain why it is not possible to isolate the ketone from these reaction conditions, even if we had used less Grignard reagent. (5 points) Q5. Grignard reagents can react with a number of different electrophiles to give a wide variety of products. Suggest mechanisms for each of the following: ( 10 points) 60 Grignard reagent MgBr followed by HCI workup OH MgBr O followed by HCI workup MgBr followed by HCI workup MgBr

Answer 1

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