Grignard Synthesis of Triphenylmethanol from Benzophenone

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Grignard Synthesis of Triphenylmethanol from Benzophenone

Purpose: The purpose of this experiment was to synthesize the tertiary alcohol triphenylmethanol from a Grignard reagent, phenyl magnesium bromide. The Grignard reagent was synthesized from bromobenzene and magnesium and then reacted with benzophenone to produce triphenylmethanol. It was important that water be excluded from the reaction, in order to prevent the formation of benzene. The reaction of phenyl magnesium bromide and benzophenone was quenched with sulfuric acid, and an extraction was performed in order to separate the organic phase containing the triphenylmethanol from the aqueous phase. The triphenlmethanol was then isolated and purified by crystallization and vacuum filtration.

Reaction Scheme:

Mechanism: The nucleophilic phenyl magnesium bromide attacks the electrophilic carbonyl of benzophenone to form a magnesium alkoxide. This is followed by hydrolysis by an acid to produce the alcohol, triphenylmethanol.

Procedure: The procedure followed was as is described in "Laboratory Manual for Organic Chemistry 2311," Fifth Edition, Jane E. Wissinger, Thomas Custom Publishing, Mason, Ohio, 2006. pp. 34-37. Modifications of the procedure included adding an additional 1.25 mL of bromobenzene to flask containing the magnesium before the start of the Grignard reaction, because of a broken stopcock which let all of the original 2.25 mL of bromobenzene into the flask initially. Then, this additional bromobenzene was slowly added to the flask dropwise in an attempt to start the reaction. However, because the reaction did not begin, the entire procedure up until this point was repeated, this time with an intact stopcock and just the 2.25 mL of bromobenzene as specified in the lab manual. A crystal of iodine was added in order to start the reaction. During the crystallization, as the solution was boiling and after four or five full pipets of petroleum ether were added, no cloudiness was observed. Therefore, instead of adding more drops of petroleum ether, the solution was taken off the hot plate and allowed to cool.

Reagent/Product Table (bold= measured, italics = calculated):

Bromobenzene, C6H5Br

Magnesium,
Mg
Benzophenone, (C6H5)2CO
10% H2SO4
Triphenylmethanol. (C6H5)3COH

1Mol Wt.

157 g/mol
24.3 g/mol
182.21 g/mol
98.08g/mol
260.3 g/mol

Grams

x
0.460 g
3.68 g
X
1.023 g

Moles

0.0214 mol
0.0198 mol
0.0190 mol
0.939 mol
0.00390 mol

mL

2.25 mL
x
x
50.1 mL
x

1Density
g/mL
1.49 g/mL
1.74 g/cm3
1.11 g/cm3
1.84 g/cm3
1.99 g/cm3

1bpt/mpt, ◦ C

130 ◦C /-31◦C
1090◦C /650 ◦C
305 ◦C /47-51 ◦C
337 ◦C /10 ◦C
360 ◦C /164-165◦C

1Solubility in water

insoluble
Insoluble
insoluble
soluble
insoluble

1Hazards

Irritant, environmental hazard

flammable
flammable
Corrosive, toxic
Eye and skin irritant
1 www.sigma-aldrich.com

Results/Observations:
Data for Triphenylmethanol

Theoretical Yield

Actual Yield
% Yield
Mpt range
Appearance

4.86 grams

1.023 grams
21 %

162.7-164.4 ◦C
Solid white, tiny crystals, no noticeable odor

Calculations:

Limiting reagent:

2.25 mL C6H5Br x (1.49 g C6H5Br/1 mL C6H5Br) x (1 mol C6H5Br/157 g C6H5Br) x (1 mol (C6H5)3COH/1 mol C6H5Br) = 0.0214 mol (C6H5)3COH

3.48 g (C6H5)2CO x (1 mol (C6H5)3COH/182.2 g (C6H5)3COH) x (1 mol (C6H5)3COH/1 mol (C6H5)2CO) = 0.0190 mol (C6H5)3COH

0.480 g Mg x (1 mol Mg/24.3 g Mg) x (1 mol (C6H5)3COH/1 mol Mg) = 0.0199 mol (C6H5)3COH

The limiting reagent is benzophenone.

Theoretical yield:

0.0190 mol (C6H5)3COH x 260.3 g (C6H5)3COH = 4.86 grams (C6H5)3COH

Percent yield:
1.023 grams (C6H5)3COH ÷ 4.86 grams (C6H5)3COH x 100 = 21 % yield

Observations: When flame drying the apparatus that would be used for the synthesis of the...
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