Grignard Reaction With A Ketone: Triphenylmethanol

Megan Wierzbowski
Organic II Laboratory
Wed. 11-1:50

Title: Grignard Reaction with a Ketone: Triphenylmethanol Part 1

Objective: The goal of this experiment is to learn to make Grignard reagents. The reactions of the Grignard reagents with ketones form tertiary alcohols. These reagents are highly air- and moisture-sensitive materials. We will observe the formation of the Grignard reagents, which magnesium metal is transformed into organometallic salts.

Experimental Procedure: Our experiment was as followed as in our text “Microscale Organic Laboratory” by Mayo from pages 275-283. The following modifications were made:
We performed the experiment through and including Isolation of Product.
We followed the regular phenylmagnesium bromide preparation
procedure, not the alternate procedure.
In the isolation of product part, we did not transfer the dried ether solution to a Craig tube and instead transferred the dried ether solution into a tared, screw-cap test tube.
The crude product was left to dry for the next week.

Reaction Scheme:

Data and Results:

Data and Results Table:

Bromobenzene
Diethyl Ether
Magnesium
Iodine
Benzophenone
Molecular Formula
BrC6H5
C4H10O
Mg
I2
C13H10O
Amount
76µL
1.3mL
0.020g
1 crystal
0.107g
Molar mass (g/mol)
157.02
74.12
24.3
253.8
182.21
Density (g/cm3)
1.495
0.7134
1.738
4.933
1.11
MP (°C)
−30.8
−116.3
650
113.7
48.5
BP (°C)
156
34.6
1091
184.3
305.4

Observations:
Iodine was a brown/red color
After adding the bromobenzene-ether solution, it slowly turned into a gray color than a clear color. But since I added too much iodine, my solution actually turned back into a brown color.
When adding the benzophenone, it was pink for a second, then turned cloudy/brownish color.
The solution then started to turn bubbly after adding the benzophenone
When extracting the ether solution, the color was yellow on top, the ether part, and clear on the bottom. I extracted the top portion that was yellow and collected it into another vial.
After adding the cold water, the ether solution was the top portion and was yellow, and the bottom was clear (the water portion)
When hydrolyzing the magnesium alkoxide salt, we added 3M HCl and continues to add the HCl until the litmus paper turned red (indicating that the aqueous phase was acidic)
After adding the anhydrous granular sodium sulfate, there were clumps inside of the Erlenmeyer flask.
Calculations:

1. Theoretical Yield of triphenylmethanol

Questions:

1. Write a mechanism for the Grignard reaction of benzophenone with phenylmagnesium bromide. Be as complete as possible and show electron flow for all steps.

2. Explain the special role of ethereal solvents like diethyl ether in the formation of Grignard reagents.

3. Triphenylmethanol can be prepared from phenylmagnesium bromide via several different Grignard reactions. Identify two other electrophiles (aside from benzophenone) that will react with PhMgBr to produce triphenylmethanol. Include a balanced reaction scheme for each of your answers.

4. a) Under the same exact conditions of Experiment 16, reaction of 4-bromophenol, instead of bromobenzene, would NOT give the analogous 3° alcohol product. Why? (HINT: why were special precautions taken to ensure anhydrous conditions when performing the Grignard
reaction?)

b) 4-Bromophenol would be converted to some product under these reaction conditions. Draw the product.

Works Cited:
1. Mayo, D. W.; Pike, R. M.; Forbes, D. C. Microscale Organic Laboratory with Multistep and Multiscale Syntheses, 5th ed.; John Wiley & Sons, Inc., 2011; pp 368-373.
2. Modifications for Girgnard Reaction with a Ketone: Triphenylmethanol Part 1 , Blackboard document.
3. http://www.chemicalbook.com