Friedel Crafts Alkylation

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Friedel-Crafts Alkylation Reaction
Friedel-Crafts Alkylation Reaction
Preparation of 1, 4-Di-t-butyl-2, 5-dimethoxybenzene
Microscale Experiment
Leah Monroe
April 8, 2003
Organic Chemistry Lab II
Experiment performed on March 20 and 25, 2003
Lab Partners (NMR only): Shannon Land and Jamie Yeadon
Abstract:
In this experiment, 1,4-dimethoxybenzene reacted with t-butyl alcohol to form 1, 4-Di-t-butyl-2, 5 – dimethoxybenzene via a Friedel-Crafts Alkylation mechanism. A small amount of 1,4-dimethoxybenzene was dissolved in acetic acid and t-butyl alcohol, along with H2SO4. The reaction was then allowed to go to completion at room temperature, and the crystals were washed with water and methanol. The percent yield of the product was 57.48%. This was lower due to the fact that many of the crystals were stuck in the reaction tube and could not be removed for weighing. Also, when a Pasteur pipet was used to remove the solvent from the reaction tube, many crystals were lost. The observed melting point of the product was 96 - 98°C, which is considerably lower than the literature value of 104 – 105°C. This may be due to impurities in the product or the sample may still have been wet. An NMR spectrum of the sample indicated that the sample was 1, 4-Di-t-butyl-2, 5 – dimethoxybenzene and that the reaction was successful. Friedel-Crafts Alkylation Reaction

Introduction: The purpose of this experiment is to synthesize 1, 4-Di-t-butyl-2, 5 – dimethoxybenzene by reacting 1, 4 – dimethoxybenzene with tertiary-butyl alcohol in the presence of sulfuric acid as a Lewis acid catalyst. The reaction will occur via the Friedel-Crafts alkylation mechanism, and involves the attack of the aryl group at the electrophilic trimethylcarbocation. The resulting product will be recrystallized using methanol and characterized using IR spectroscopy and melting point analysis. Materials Used:

1 beaker, 400-mL sand bath
ice for ice bath Erlenmeyer flask, 25-mL
several Pasteur pipets, 1.0-mL 10 x 100 mm reaction tube
glass stirring rod rubber bulb
Substances
Formula
Weight, g/mol
Amount Used
Moles
Used
Mole
Ratio
Melting
Point
°C
Boiling
Point
°C
Density
g/mL
1, 4-dimethoxybenzene
138.16
0.120 g
8.69 x 10-4
1 to 1
58 - 60
N/A
1.053
Acetic acid
60
0.4 mL
N/A
N/A
N/A
118
1.049
t-butyl alcohol
74.12
200 ìL
0.00213
2 to 1
N/A
83
0.7856
Concentrated H2SO4
98.06
2.0 mL
N/A
N/A
N/A
327
1.84
Methanol
32.0
1.0 mL
N/A
N/A
N/A
64.7
0.7914
Water
18.0
2.5 mL
N/A
N/A
N/A
212
1.0
1, 4-Di-t-butyl-2, 5 – dimethoxybenzene
250.17
product
product
1 to 1
104 - 105
N/A
N/A
Reaction and its Mechanism:
The limiting reagent for this reaction is 1, 4 – dimethoxybenzene. See calculations section for explanation. Overall Reaction: C8H10O2 + 2C4H10O �� C16H26O2 + 2H2O 1,4-di-t-butyl-2,5-dimethoxybenzene H2SO4 1, 4-dimethoxybenzene OCH3 OCH3 + OH CH3 CH3 CH3 Tert-butyl alcohol OCH3 OCH3 H2O + Mechanism:

H H H O SO3H Sulfuric Acid OH CH3 CH3 CH3 O CH3 CH3 CH3 + + CH3 CH3 CH3 C Friedel-Crafts Alkylation Reaction
Mechanism (con't) + CH3 CH3 CH3 C OCH3 OCH3 + OCH3 OCH3 H + OCH3 OCH3 H + OCH3 OCH3 H -OSO3H OCH3 OCH3 + H OCH3 OCH3 + + CH3 CH3 CH3 C OCH3 OCH3 -OSO3H H OCH3 OCH3 + H OCH3 OCH3 OCH3 OCH3 H2SO4 + 1,4-di-t-butyl-2,5-dimethoxybenzene Procedure:

Dissolve 120 mg 1,4-dimethoxybenzene in a 10 x 100 mm reaction tube with 0.4 mL acetic acid. Add to it 0.2 mL t-butyl alcohol and heat gently until the mixture is in solution. Once it's in solution, cool the mixture in ice. Then, add 0.4 mL concentrated H2SO4 (18 M) to the tube dropwise with a Pasteur pipet. After each drop is added, mix the solution thoroughly with a glass stirring rod. After all the H2SO4 has been added, remove the reaction tube from the ice and allow it to warm to room temperature. Allow the tube to remain at 20-25°C for approximately 10 minutes in order to complete the reaction. Then cool the mixture in ice so that...
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