Grignard reaction triphenylmethanol lab report

THE GRIGNARD REACTION: SYNTHESIS OF TRIPHENYLMETHANOL.

OBJECTIVES: To synthesize a Grignard reagent by reacting bromobenzene with Mg. To synthesize a tertiary alcohol by the reacting the Grignard reagent with and a ketone, specifically benzophenone.

Background

Grignard reaction

Grignard reagent are organometallic compounds synthesized by reacting Mg with an alkyl halide using ether (or THF) as solvent. They are extremely strong bases and they have to be protected from moisture and oxygen.

R-MgX + H2O R-H + HO-MgX

The synthesis of Grignard reagents is a radical reaction between the metal (Mg) and a transient alkyl radical. Grignard reagents are widely utilized for the synthesis of many compounds as they are a source of R:- groups. In particular, they can be used for the synthesis of alcohols.

O

C HH

O

C HR1

O

C R2R1

OH

C R2

R1

Nu

OH

C H

R1

Nu

OH

C H

H

Nu

A ketone or aldehyde reacts with the Grignard reagent to form an alcohol. The mechanism below is typical for the nucleophilic attack of an anion (such as a Grignard reagent) or another Lewis base on a carbonyl compound.

R-X + Mg RMgX

a Grignard reagent

Step 1

Step 2

O

C R2R1

O

C R2

R1

MgX

R

OH

C R2

R1

R

H

H

O H

R- Mg+X +

tetrahedral intermediate

+ +

Experimental

In today’s experiment you will first synthesize a Grignard reagent by reacting bromobenzene with Mg. In the second step of the synthesis, a tertiary alcohol is formed by the reaction of the Grignard reaction and a ketone, benzophenone.

The GTA will provide you with test tubes for the experiment. The tubes have been dried overnight and stored in a desiccator with fresh drying agent to keep them free of moisture.

1) Fill a crystallizing dish about ½ full with water. Set it on a hotplate until the temperature is about 45˚C (hotplate setting will read about 120˚).

2) While water bath is heating, add 0.05 g of magnesium turnings (about 2-3 chips) to the reaction tube. Using your 1 mL syringe, add 0.25 mL of bromobenzene to a vial. Using a dry syringe, inject 0.50 mL of anhydrous diethyl ether to the magnesium in the test tube, and 0.7 mL anhydrous ethyl ether to the bromobenzene in the vial.

3) Using your 1 mL syringe, inject about 0.1 mL of the bromobenzene-ether mixture into the reaction tube and mix the contents. Clamp the test tube so that the bottom (about ½ inch) is in the water bath. The ether will start to boil, but the vapors shouldn't get higher than about halfway up the test tube. Add one small chip of iodine, and the reaction should start..it will also turn brown. (If you raise the test tube out of the bath and still see bubbles on the Mg, then it has started). About every 5min, add another 0.1 mL of the bromobenzene/ether solution, keeping the vial capped between additions. Slow addition is important to avoid by-product formation.

4) Once all of the mixture has been added, allow the mixture to react for about 5-10 more minutes. Total time will be about 40-50 min and most of the magnesium should be consumed.

Synthesis of Triphenylmethanol

5) Weigh 0.364 g of benzophenone into a fresh vial, and dissolve in 1.0 mL of anhydrous diethyl ether.

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Mix the contents thoroughly (one partner should do this while the Grignard reagent is forming).

6) AFTER your Grignard reagent synthesis is complete, pull the benzophenone solution back into a syringe and add to the Grignard reagent at such a rate as to maintain a gentle reflux. To ensure complete mixing, swirl the tube vigorously.

7) The addition of benzophenone will result in a red color. Allow the reaction to proceed for 20-30 min with occasional mixing.

After the reaction is complete, cool the tube in ice and slowly add 2 mL of 3 M hydrochloric acid in order to consume excess magnesium. It will react vigorously. Make sure the solution is well mixed. The product should form as a white solid. Add enough diethyl ether (about 3 mL) to dissolve all of the precipitate and transfer the ether layer (top) to a separatory funnel. Add about 1 mL of ether to the HCl layer, mix and then transfer this ether layer to the sep funnel as well.

8) Add an equal volume of saturated sodium chloride solution to the ether layer and mix in the separatory funnel.

9) Drain off the aqueous layer (bottom), and then collect the ether layer in a small, dry erlenmeyer flask and dry the ether by adding a few pellets of anhydrous calcium chloride. Allow the solution to dry for 1-2 minutes.

10) Using a Pasteur pipette transfer the ether solution (keep your pipet vertical to avoid losing the solution!) to a 19/22 joint round-bottom flask. Wash the calcium chloride pellets with ether and add the rinse to the combined ether solution.

11) Evaporate the ether on the rotavap. A white solid (the product) may be seen. Any biphenyl present will appear as a liquid, so the product may not be visible.

12) Set the flask on ice for about 10 min. and add 0.5 mL of petroleum ether to remove the biphenyl -it will dissolve in the pet ether but your product should settle out. The product should form white crystals. Cool the mixture in an ice bath for 5 minutes and carefully remove the solvent/biphenyl mixture using a Pasteur pipette, leaving the solid product in the flask.

13) Add about 1-2 mL of isopropanol to the flask, and transfer mixture to a clean test tube (you can rinse with a little bit of isopropanol to transfer all of the solid). Add a boiling stick and rest on the hot plate (with swirling) to dissolve crystals. Cool to

room temperature slowly and then place the test tube in an ice bath until crystals have formed.

14) Isolate the crystals by vacuum filtration on the Hirsch funnel and allow them to dry. Determine the melting point and weight of the final product. Calculate the percent yield.

15) Do TLC using silica plates and 1:5 ethyl acetate/hexane. Visualize with UV light (or I2 if UV does not detect strong enough spots). Report the Rf values. Determine the number of products and identify them.

16) Take an IR spectrum of the product, identify the alcohol stretch.

Real life example of Grignard reagents

Porphyrin derivatives are used in biomedical applications as well as in catalysis and material science. One of the requirement for these applications is that porphyrins should display absorption bands in the 600–800 nm region. For the functionalization reactions, different approaches have been utilized over the years. One particulary effective is the use of Grignard reagents.

Post-lab Assignment

Write the mini-lab report following the instructions on D2L under “example of lab report”

References

L. D.; Costa, J. I.T.: Costa and A. C. Tomé; Molecules, 2016, 21(3), 320

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