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A Grignard Synthesis of Triphenylmethanol

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Chem‐106

Grignard Synthesis of Triphenylmethanol
Objective: The purpose of this experiment is to synthesize triphenylmethanol from benzophenone via Grignard reaction. The product will be isolated through extractions and purified by recrystallization. Reaction efficiency will be evaluated through percent yield, percent recovery, and the purity of the final product will be determined by IR, TLC, and mp determination.

Chemicals: bromobenzene, magnesium turnings, diethyl ether, benzophenone, biphenyl, triphenylmethanol, iodine,
6 M HCl, brine, anhydrous MgSO4 or Na2SO4, 10:90 EtOAc/hexanes.

Glassware and equipment: 100 mL RBF, air condenser, Claisen adaptor, 60 and 125 mL addition funnel, short stem glass funnel, two 50 mL Erlenmeyer flasks, 10 mL graduated cylinder, lab jack, crystallizing dish, magnetic stir bar.

Techniques: reflux, extraction, vacuum filtration, recrystallization, TLC, mp, IR spectroscopy. Introduction
In 1912 Victor Grignard received the Nobel prize in chemistry for his work on the reaction that bears his name, a carbon-carbon bond-forming reaction by which almost any alcohol may be formed from appropriate alkyl halides and carbonyl compounds. The Grignard reagent RMgBr is easily formed by redox reaction of an alkyl halide with magnesium metal in anhydrous diethyl ether solvent. R-Br + Mg → RMgBr The Grignard reagent can be viewed as an ionic species consisting of carbanion R-, with Mg2+ counterion and an additional Br- counterion. The carbanion R- is very reactive, and functions both as an extremely strong base and an extremely strong nucleophile.

O

Br

Mg anhydrous ether

MgBr O

H3O+
HO

Please review the mechanism of this reaction for your reference. Byproducts 1. Biphenyl, Ph-Ph, which is formed in competition with the Grignard reagent PhMgBr. Following initial electron transfer, the phenyl radical Ph• can either accept another electron leading to the desired carbanion, or combine with another phenyl radical to make biphenyl. This product is formed as a result of quick addition of bromobenzene to magnesium turnings.

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R-MgX MgBr + Biphenyl, major byproduct + R'-X Br R-R' + MgX2 (Wurtz coupling)

2. Benzene (Ph-H), resulting from protonation of the carbanion. The carbanion is very basic, so if there is any water in the solvent or in the glassware, or if moist air is allowed to enter the reaction mixture, some of the carbanion will be protonated. Formation of this product results from presence of any moisture in the reaction mixture: so, please, do not sneeze, cough, or spit on your reaction!!!
R-MgX R-MgX + + H2O R'-OH RH R'H + + MgXOH MgXOH

Reaction quenched

3. Activation of magnesium: Pure magnesium is an active metal, so active that any magnesium that has been exposed to air is inevitably coated with a film of magnesium oxide on its surface. This oxide film blocks the bromobenzene from actually contacting active magnesium, and thus prevents the requisite electron transfer. One way to expose active surface is to simply break several turnings before placing them into reaction flask. The other way is to break the turning inside the flask using your glass stirring rod: this procedure is not the safest as it often results in breaking the fragile glassware. The third way is addition of an activator such as a small iodine crystal. The iodine serves two functions. a. The first is as an indicator. The color will disappear when the magnesium is activated and is able to do redox chemistry with bromobenzene. b. The second is as an activator. Iodine is sometimes able to chemically “clean” the surface of the magnesium so that fresh, active magnesium is exposed so that it can do redox chemistry with bromobenzene. However, it doesn’t often work! 4. Unreacted starting material (Could be the Ph-Br, the Mg, and/or the ester).

Experimental:
First class period, Step 1: Preparation of the Grignard Reagent
Glassware needed for the first reaction setup: please wash and dry during the previous class period and store in your drawer. a. 100-mL RBF b. “Claisen” adapter c. Air condenser d. 125mL separatory funnel with stopper e. Two 50mL Erlenmeyer flasks f. 10 mL graduated cylinder The reaction must be done in the hood!!! 1. Clamp the 100-mL round-bottomed flask equipped with a stir bar to a vertical rod using a two-finger clamp. 2. Weigh out about 0.5g of magnesium metal. (Record weight) 3. Break a couple of magnesium turnings, pour them into RBF, add 10 mL of diethyl ether and place Claisen head into the joint. 4. Add 125 mL sep funnel on one side and the air condenser to the other joint of the Claisen head. Clamp the glassware
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with keck clamps. Temporary stopper both outlets. 5. Pour 20 mL of ether into the separatory funnel and put stopper back on. 6. Measure out 2.1 mL of bromobenzene in a graduated cylinder. Record the volume as accurately as possible. Transfer it to a clean stoppered 50mL EF. Measure about 10mL anhydrous diethyl ether into the same cylinder, transfer again into the same EF. Swirl the flask and add about half of the mixture into the 100mL RBF with magnesium turnings. The rest of the mixture goes into the separatory funnel attached to the RBF. Rinse the graduated cylinder with about 7mL of anhydrous ether and add it to the mixture in the separatory funnel. 7. Initiate the reaction in the RBF by carefully shaking the flask. If the redox chemistry of the Grignard reaction initiates, the iodine color will go away, the solution will begin to get hot, there will be some bubbling, and solution may become slightly cloudy. 8. Slowly drain the bromobenzene/ether/iodine solution into the round-bottomed flask. Shake the RBF occasionally to speed up the reaction. Add additional 1-2mL anhydrous ether to the separatory funnel to rinse off the mixture, and drain it to the RBF. Stopper the separatory funnel. 9. The reaction should be so exothermic that it will be self-boiling for some time. If the rate of boiling subsides, apply a heating mantle (connected to the variac, not directly to the wall outlet) and apply heat to maintain a good rate of boiling. Do not exceed power setting of 20-25. The color of solution should turn brown. If it becomes heterogeneous and greenish-brown, your reaction mixture has absorbed moisture, and your yields will suffer tremendously. 10. Maintain boiling until most of magnesium metal has reacted. Since diethyl ether has a low boiling point, you might need to replenish the solvent to prevent the reaction mixture from drying. Add diethyl ether through air condenser using a clean dry pipette.

Grignard reaction just began: cloudy solution

Grignard reaction in progress

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Step 2: Reacting the Grignard Reagent with Benzophenone
1. After the hour of reflux is up, let the reaction cool down (an ice-water bath might help). 2. Mix 2.4g benzophenone with 10mL of ether in a clean 50mL EF and add the mixture to your separatory funnel. (Stopcock closed). 3. Remove the cold bath (if you have one on), then drain the ketone/ether solution into the RBF as rapidly as possible but make sure that the reaction doesn’t overheat too much. Try to shake the solution around as much as possible (hard to do when it’s clamped!) If things start to boil hard, reapply the cold bath. 4. If everything is added without excessive boiling, try to shake everything up, and give it five minutes to continue reacting. The reaction should take approximately 15 min. 5. If the reaction is still hot, cool it with the ice bath. 6. Add about 20 grams of ice and about 7-8mL 6M hydrochloric acid. The acid will react exothermically with both the anion and unreacted magnesium. The ice is there simply to absorb the heat. 7. Swirl well to promote hydrolysis and break the solid clumps. Use a spatula to break up the chunks. You should obtain two distinct layers: top organic layer with the product and bottom aqueous layer with MgBrOH and excess acid. Since the process is exothermic, some ether might be lost during the reaction, so add excess ether if necessary to maintain about 10mL of volume for the top layer. 8. Pour the mixture into your cleaned 125mL separatory funnel. (The magnesium doesn’t need to be totally dissolved) 9. Vent, swirl, and drain the bottom layer into a beaker. 10. Transfer the top layer into a different labeled Erlenmeyer flask, return the aqueous layer into the separatory funnel and re-extract it with additional 10 mL ether. 11. Discard the aqueous layer into aqueous waste container located in the waste hood. 12. Combine the organic layers, wash with brine, add about 1.0 g anhydrous sodium sulfate or other drying agent, stopper, parafilm, label with your name and leave it on the tray provided by your instructor.

Second Class Period: Purification and Characterization of Product connect to water aspirator
1. Decant the organic dried solution into pre-weighed 50mL dry RBF. Rotary-evaporate the solution to obtain crude brownish oily product. 2. Obtain the crude yield and record the percent yield. Save a couple of crystals in a labeled test tube (CP1) for TLC analysis. This product contains triphenylmethanol and biphenyl byproduct. This byproduct can be removed by simply dissolving your crude mixture in about 10 mL of warm petroleum ether (bp 30-60oC). After the mixture cools down to room temperature, transfer it on an ice bath for 5 min and then vacuum filter the solution. Wash the crystals with some ice-cold petroleum ether (use pipette to wash crystals). 3. Record the crude yield after the product is dried. Save a couple of crystals in a labeled test tube for TLC analysis (CP2), and save some of the mother liquor for the same purpose (BP). 4. Recrystallize the crude product from hot isopropyl alcohol. Record the pure yield and percent recovery from CP1. Save a couple of crystals in a labeled test tube for TLC analysis (PP).

solvent trap

Buchner funnel

filter flask

Fig. 3. Vacuum filtration apparatus
Make a TLC plate with six pencil marks for six tracks ready: 5. Run the TLC in 10:90 EtOAc/hexane mixture. Solutions of authentic materials will be provided (do not remove the spotters from these solutions to prevent contamination). 6. Mark down the results, calculate the Rf values, and answer the following questions:
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a = authentic biphenyl b = authentic benzophenone c = authentic triphenyl methanol d = crude product 1 (CP1) e = crude product 2 (CP2) f = purified product (PP) f = mother liquor (BP) eluting solvent: 10:90 mixture EtOAc: Hexanes

a

b

c

d e

f

Fig. 4. TLC analysis of product

Is biphenyl present in the crude mix? In the purified material? • Is methyl benzoate present in the crude mix? In the purified material? • Any other side products in the crude? • Did recrystallization purify the material at all? • Did crystallization get all of the product out of the solvent? 7. Take a melting point of your final product (literature value 162oC). 8. Take IR spectrum of the purified product (dissolve a couple of crystals in methylene chloride). 9. Transfer the purified product in a labeled vial, secure with rubber band and place in the tray provided for grading.

Guidelines for Formal Report: (20 pt)
Note: Attach all specra and label the important peaks (Draw the structures, label the functional groups or H, C, and match them with the speactra). Severe point deduction will follow if spectral data is missing. Abstract (2 pt) Target molecule and the reactions with mechanisms (2 pt) Tabulated results (6 pt): Product Yields (g/%) Color/Texture M.P. (oC) Purity (from TLC) Rf values

Crude 1

Crude 2

Purified

Discussion: Yields: Explain what could affect the yield on each step. Be specific and refer to your notebook. Melting points: Discuss the melting points in reference to the literature values and comment on the results via purity of the product. TLC: compare the TLC of the products and comment on the presence of the starting materials and any impurities observed. Tie this up with your yields and the meling points: your data should relate very closely. IR and NMR: when reporting IR and NMR of the products, please do not just list the chemical shifts and the wavenumbers on NMR and IR but rather comment on the changes from one compound to another in terms of reporter peaks. The NMR’s of the starting material and the product are presented below: Conclusion (2 pt): comment on any disadvantages or failures of this experiment and propose modifications/improvements for the synthetic procedure. Please be specific and note that general comments such as ‘use a different solvent, different glassware, etc’ are not valid to make your point!
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H NMR of benzophenone
O

8

7

6

5

4 PPM

3

2

1

0

13C

NMR of benzophenone
O

200

180

160

140

120

PPM

100

80

60

40

20

0

H NMR of triphenylmethanol

OH

8

7

6

5

4 PPM

3

2

1

0

13C

NMR of triphenylmethanol

OH

160

150

140

130

120

110

100

90

80 PPM

70

60

50

40

30

20

10

0

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