Question

please answer the prelab excercise:) I tried doing it but I did not get very far

Experiment 7: Grignard Reaction. Introduction Discovered by French chemist Victor Grignard, the reaction that now bears his name is an example of the preparation of an organometallic reagent. In this lab, phenylmagnesium bromide is the Grignard of interest, and it will be added to an aldehyde to produce an alcohol. Lab Objectives 1. To study organometallic reagents in general. benzophenone. 2. To prepare phenylmagnesium bromide and add it to 3. To gain experience with anhydrous conditions. Grignard Reagents and AN (Addition, Nucleophilic) Additions Magnesium metal spontaneously reduces organohalides via a succession of single electron transfers. This results in the Grignard 'insertion' reaction, as proposed in the 1920s. It was only successful in ether solvents, and failed utterly in hydrocarbons such as benzene. Mg (solid) R- Br diethyl ether MgBr Today, it is well established that the bonding between Mg and C is highly ionic, with a more correct structure as RC MgBr. The critical role of the ether is now better understood: the lone pairs on the ether oxygen act as Lewis Bases and chelate with the magnesium cation, stablilizing the Grignard as it forms. W polymeric aggregates, as opposed to the simple The true form of Grignard species monomeric forms commonly drawn, and their rich chemistry continues to be studied today. are As a class of organometallic reagent, Grignards can be added across a carbonyl double bond (C-O). The nucleophilic, negative carbon of the organometallic attacks the electrophilic () carbonyl carbon, breaking the n-bond. The positive metal then associates with the alkoxide product, forming a salt. This is the As (Addition, Nucleophilic) mechanism. O "MgBr fast alkoxide salt CH2 R R-CH MgBr Aqueous hydrolysis with acid destroys the salt and forms the fully protonated final alcohol product Procedure Part A: Preparation of the Glassware and Magneslum 1. The Grignard reagent cannot form in the presence of water, and any water that it comes into contact with immediately destroys it. Thus, during this part of the experiment, all glassware must be kept DRY at all times. 2 Take a pre-packed drying tube and assemble the apparatus for reflux as shown in pre lab talk. Do not attach amber tubing to the condenser yet. 3 Weigh out 0.030 mol of Mg turnings and place them in the 100-ml. round bottom flask along with the stirbar. Only do steps 4 and 5 if your round bottom flask was dried previous to the start of the experiment not heated and With a Bunsen burner flame, flame dry the apparatus, starting from the botton and working your way to the top. Do not overdeo it. -instead it should be easy to see the wa vapor moving as it is removed from the system water Allow the apparatus to cool to the touch. DO NOT PROCEED to the next part until i has cooled enough. Note that as it cools, air will be sucked into the apparatus, and by passing through the drying tube, it will remain dry! 5 Part B: Formation of the Grignard Reagent Start the cooling water going through the condenser. 1 Dispense 0.033 mol of bromobenzene into 10 ml. of anhydrous ether and add the solution 2 to the cooled 100-ml flask. Str the mixture for a few minutes, watching for the tell-tale signs of Grignard formation. The reaction shoukd become cloudy, and start to reflux on its own. Once the Grignard reaction has begun. place 10 ml. anhydrous ether into the addition funnel, and add it to the reaction. Allow reaction to rellux for 10 minutes. Part C:Addition of Ketone Dissolve 0.030 mol of benzophenone in 20 ml. of anhydrous ether. You may need to warm the solution slightly. but be careful as ether is very volatile. The homogenous solution is then poured into the addition funnel. Adjust the stopcock on the additiion funnel, so that the ketone solution is added dropwise (I to 2 drops per second). The reaction should reflux gently. After all the benzophenone has been added heat the entire mixture at reflux for another 20 minutes. 2 Part D: Extraction and Isolation Prepare an acid solution in a 250-ml Erklenmeyer flask containing 15 mL. of 3M H,SO. and 35 g of crushed ice. Once the reflux period is complete, allow the reaction mixture to cool slightly. Pour about 10 ml. of the acid solution into a graduated cylinder for later use. Pour the cooled reaction mixture into the cold acid solution. Stir the mixture with 1. your stir rod, or swirl vigorously, so that all the salts dissolve. Add the reserved 10 ml. of acid to the 100-ml. round bottom flask to hydrolyze any remaining salts. You may need to add a bit of ether to dissolve some of the material. Once all the solid has dissolved add this solution to the acid mixture. 2. Once the hydrolysis of all the salts is complete, transfer the solution to a separatory funnel. Separate the two layers and wash the aqueous layer with 15 ml. of ether. Combine the two ether solutions. 3. Wash the ether layer with 25 mL. of water, twice with 25 ml. of saturated sodium bicarbonate, and finally twice with two 25 ml. portions of saturated sodium chloride (brine) 4. Dry the ether solution with magnesium sulfate, gravity filter into a clean flask of Evaporate the ether on a roto-evaporator. 5. appropriate size. Add 10 ml. of high boiling ligroin to the residue and triturate the solid for a minute and the collect the crude product by suction filtration. Record the mass of crude product, and cakulate a crude yield. Keep a small amount of crude for TLC 6. Part E Recrystallization Recrystallize the crude product from methanol 1. Collect the purified material by suction filtration, allow it to dry, weigh it, and keep a small sample for a melting pont and TLC. Hand the rest in a labelled vial. 2. Part F Purity Analysis Spot the crude product. the recrystallized product, as well as the starting matersal on TLC Use as clucnt 20% ethyl acetate in hexane Use a UV lanp to visualine the spots Determane the melting poit of your recrystallized malerial 2 Pre-Lab Exercises Please ansv swer the following questions briefly. Draw out the chemical reaction scheme for this laboratory. What is the name of the product? Underneath your equations, in a neat table, collect the following information for magnesium, bromobenzene, benzophenone and the product: (i) molar mass; (i) grams used (if reagent): (i) theoretical yield (if product); and (iv) melting point (if organic). Write a flow chart for the experiment.

Answer 1

Find out the attached image files for answers. First image is for answer to question a that is reaction scheme. Second image is for other two question.

bry Bromobenzene Fhang magnesium trami&a Anbydm HO Qhar Gan2ophanome Triphemy methoanol

Theretica! yield meling PAnt Coapm mohr mass a ued eea8 24 Magnasium 0.727m -30.8c Boomolaanzene 157 5-18 18.5°c Benzophenone 5.469 I82 Tiphag 160-162 c methans 260 alenkotiom 15 mBronobenaere peoducan 260 m iphe methano 260 157 5-18. 260x 5.10 157 8.58 m C Flaro chast d the experinent weigh and dr ing turningo Day&Cool dher Dissolve ben20 phanone m 20 mL ehex Dispense Boomoaenzene in ether in o taok Adlihion Addihon them im uk lomL emer Add acid-ce mixture eshr Caol hhe mixture Reblux the Contant hor ao min toanh eher with water Racryhiee with methanol the ether Arahyse the pun ty