Feb. 10, 2010
Title: The Friedel-Crafts Alkylation Reaction Synthesis of 1,4-di-t-butyl-2,5-dimethoxybenzene
with 1,4-dimethoxybenzene and t-butyl alcohol and sulfuric acid
A Friedel-Crafts reaction is a type of electrophilic aromatic substitution reaction that consists of the introduction of an alkyl group onto the benzene ring, otherwise known as alkylation. The first step of Friedel-Crafts alkylation requires the generation of an intermediate carbocation to act as the electrophile during the aromatic substitution process. This is typically done through a reaction of an alkyl halide with a Lewis acid, but this can also be done by the addition of\ H+ to the double bond of an alkene, or by treatment of an alcohol with concentrated sulfuric acid. We will use the treatment of an alcohol with concentrated sulfuric acid to generate the electrophilic species. During this experiment, we will prepare 1,4-di-t-butyl-2,5-dimethoxybenzene by the Friedel-Crafts reaction of 1,4-dimethoxybenzene with t-butyl alcohol and sulfuric acid. The electrophile for this reaction is the t-butyl carbocation which is generated by the treatment of two equivalents of t-butyl alcohol with concentrated sulfuric acid. The overall reaction is shown below:
Table of Reagents:
| mol. wt. (g/mole)| mp (˚C)| bp (˚C)| density (g/mL)| 1,4-Dimethoxybenzene| 138.17| 57| 212| |
t-Butyl alcohol| 74.12| | 83| 0.79|
Concentrated H2SO4| 98.08| | 327| 1.84|
1,4-Di-t-butyl-2,5-dimethoxybenzene| 250.37| 104-105| | | Methanol| 32.04| -97| 65| 0.79|
Safety and Waste Disposal:
1. Concentrated sulfuric acid and acetic acid are highly corrosive and can cause severe burns. Wear gloves and take great care not to spill these acids on your skin or clothing. Should a spill happen, wash the area at once with copious amounts of water. Clean up any spills on the benchtop or floor immediately by neutralizing the spilled acid with either sodium carbonate or sodium bicarbonate and washing the area with water. 2. Methanol and ethanol are flammable. No flames are permitted in the lab. During recrystallization, keep hot ethanol vapors away from the hot plates. 3. Methanol and ethanol wastes should be discarded in the labeled bottles provided. Aqueous acidic wastes can be poured down the drain with lots of water to dilute them.
We measured out 0.300 g of 1,4-dimethoxybenzene, 1.0 mL of glacial acetic acid, and 0.5 mL of t-butyl alcohol. We added these to a 10-mL round-bottom flask and placed them into an ice bath to cool. Next we measured out 1.0 mL of concentrated sulfuric acid and began adding it, dropwise, to the round-bottom flask while making sure to mix thoroughly after each drop. Once all the acid had been added, we stirred the mixture well with a glass stirring rod, and then removed the flask from the ice bath and allowed the mixture to warm to room temperature. We then let the mixture stand at room temperature for an additional 10 minutes to complete the reaction, and then cooled it again in the ice bath directly following. Next we began adding approximately 5 mL of cold water, dropwise, in order to precipitate the product. Next, we used a Pasteur pipet to carefully remove the solvents from the solid crystals. Then we washed the crystals again with an additional 5 mL of cold water and then removed the liquid with a Pasteur pipet again from the crystals/solid. Next, we washed the crystals with three portions of cold methanol (0.5-1.0 mL each portion) while keeping the round-bottom flask in the ice bath. To start recrystallization, we added the solid to approximately 1.5 mL of warmed ethanol and then proceeded to warm the mixture until dissolution was complete. We then cooled the solution to induce crystallization and then filtered the recrystallized solid by suction filtration on a Büchner...