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Grignard Reagents and As (Addition, Nucleophil ic) Additions Magnesium metal spontaneously reduces organohalides via a succession transfers. This results in the Grignard 'insertion reaction, as proposed in the 1920s single electron Procedure t was only Part A: Preparation of the Glassyware and Magnesium successful in ether solvents, and failed utterly n hydrocarbons such as benzene. 1. The Grignard reagent camot form in the presence of water, and any water that it comes Mg (solid) into contact with immed iately destroys i. Thus, during this part of the experime nt, all glassware mus MgBr imes, Br diethyl ether ept 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. Today, i is well established that the bonding between Mg and C is highly ionic, with a more correct structure as RC "MeBt. The critical role of the ether is now better understood the one et as Lewis Bases and chelate with the magnesium cation, stablilizing pairs on the ether oxygen the Grignard as it forms. 3 Weigh out 0.030 mol of Mg turnings and place them in the 100-m round bottom flask, along with the stirhar Only do steps 4 and 5 if your round bottom flask was not heated and dried previous to the start of the experiment The true for m of Grignard species are polymeric aggregates, as opposed to the simple monomeric forms commonly drawn, and the r rich chemistry continues to be studied today. etalli As a class of gent, Grignards can be added across a carbonyl double bond Wih a Bunsen burner flame, flame dry the apparatus, starting fromthe botton and (C-O). The nucleophi lic, negative carbon of the organomet allic attacks the electrophilic (5") -instead t should be easy to seethe water erdo g your way to om the .. carbonyl carbon, breaking the -bond. The positive metal then associates with the akoxide vapor moving product, forming a salt. This is the As (Addition, Nucleophilic) me chanism. Allow the apparatus to cool to the touch. DO NOT PROCEED to the next part until it Sucked into the apparatus, and by 5 s cOled passi O "MgBr llain drut fast koxide salt Part B:Formation of the Grignard Reagent A CH2 R-CH MgBr Start the cooling water going through the condenser bromobenzene into 10 mL of anhydrous ether and add Dispense 0.03 to the cooked 00-ml. flask e solution Aqueous hydrolysis wth acid destroys the salt and forms the fully protonated final alcohol product Stir the mixture for a few minutes, watching for the tell-tale signs of Grignard formation. The reaction should become coudy, 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 reflux for 10 minutes. Part C: Addition of Ketone Dissolve 0.030 mol of benzophe none in 20 ml. of anhydrous ether. You may need to r is very volatile. The homogenous Adjust the stopcock on the addition funnel, so that the ketone solution is added dropwise Pre-Lab Exercises (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. Please answer the following que stions briefly. They are due at the START of the lab session Part D: Extraction and Isolation For your reference, keep a copy of your answers so you can consult the m during the lab. Prepare an acid solution in a 250-ml Erleme yer flask containing 15 ml. and 35 g of crushed ice. Once the reflux period is complete, allow the reaction mixture to cool slgay d 3M H:SO 1 the name of the Draw out the chemical reaction sche me for this laboratory. Wha product? Sir the mix thure wah o the coll acid sohution Underneath your equations, in a neat table, collect the following information for magne sium, bromobenzene, benzophenone and the product: your stir rod, or swirl vigorously, so that all the salts dissolve. Add the reserved 10 s used (if reagent): (ii) theoretic al yield (if product); and (iv) L of acid . otom flask 1o y meltine nairt iforeanich Once all the solid has dissolved add this so lution to the acid mixture. Write a flow chart for the experiment Once the hydrolysis of all the salts is complete, transfer the solution to a separatory of ether. wash the aqueous layer wit m Combine the two aher solutions Esaturated sodium Wash the ether layer with 25 mL, of water, twice with 25 ml. 4 bonate, and finally twice with two 25 mL portions of saturated sodium chloride (brine). Dry the ether solution with magnesium sulfate, gravity filter into a cean flask of appropriate size. Evaporate the ether on a roto-evaporator. 6. Add 10 mL of high boiling ligroin to the residue and trturate the solid for a minute and the collect the crude product by suction filtration. Record the mass of crude product, and calculate a crude yield. Keepa small amount of crude for TLC. Part E Recrystallization from methanol. lize the crude prod 2. Collect the purified material by suction filtration, allow it to dry, weigh it, and keep a ra melting point and TLC. Hand the rest in a labe lled vial. small sample Part F: Purity Analysis Spot the crude product, the recrystallized product, as well as the starting material on TLC. Use as eluent 20% ethyl acetate in hexane. Use a UV lamp to visualize the spots Determine the me king point of your recrystallized material.

Answer 1

Scheme MgBr Н+Н20 B Dry ether + Mg OMgBr Magnesium OH Grignard reagent bromobenzene Benzophenone Alkoxide salt triphenylmethanol Molar mass(g/mol) Theoretical yield МРВР °С moles mass(g) 24.3 157.02 182.22 0.729 5.182 5.467 Magnesium Bromobenzene benzophenone 0.030 156(BP) 48.5(MP) 0.033 0.030 triphenylmethanol (product) 162(MP) 260.33 0.030 7.810 7,810 Formulas: Mass molar mass Theoretical yied: The benzophenone is a limiting agent and hence it gives 0.030 moles of product. Theoretical yield 0.030mol x 260.33 g/mol 7.810 g moles

Add 0.729 g of Mg turnings to 100 mL RB and dry the reaction Add 5.182 g of bromobenzene solution in dry ether to the reaction RB. And stir the mixture for few minutes. Add aditional 10mL dry ether to the reaction mixture. Aftre 10 minutes add 5.467 g of benzophenone solution in dry ether via addition funnel with the rate of 1-2 drops per second. Upon complete addition reflux the reaction mixture for another 20 minutes and the cool to the room temperature Extraction: Pour the reaction mixture on 15mL 3M H2SO4 and cruched ice mixture and stire the reaction mixture with the rod Transfer the reaction mixture to separatory funnel and seprate the two layers. Bottom layer is water and upper layer is ether. Wash ether layer with water twice with water and finally with saturated NaCL solution (Brine solution) Aqueous layer Ether layer

Aqueous layer Ether layer Contains product Contains inorganic salts Dry the ether layer over filter MgSO4 and Evaporate the ether on roto-evaporator and recrystalize the product methanol. triphenylmethanol OH