Nitration is the process of introducing a nitro functional

Question

Introduction:

Nitration is the process of introducing a nitro functional group to a benzene ring. The reaction occurs through a mechanism known as electrophilic aromatic substitution. In these reactions, the nucleophilic benzene ring attacks electrophilic species forming a resonance stabilized areniumcation that quickly eliminates a proton and forms a substituted benzene. In nitration, the nitronium ion is the electrophile and is formed from reacting nitric acid with sulfuric acid. Sulfuric acid is a stronger acid than nitric acid and thus pronates nitric acid. The pronated acid undergoes hydrolysis and forms a nitronium ion. The benzene attacks the nitronium ion and forms nitrobenzene cation. Sulfate ions act as a base and abstracthydrogen from the cation and forms nitrobenzene. When the benzene is already substituted prior to nitration, the substitution may promote or hinder the reaction. If the substitute has electron-withdrawing capabilities like carbonyl groups the reaction is hindered and prefers the metasubsitution. By withdrawing electrons from the benzene group, the benzene group is less nucleophilic. Electron donating groups  like alkyl groups make the benzene more nucleophilic and thus promote nitration. The substitution is in the ortho and para positions.

Procedure:

• Carefully measure out 3 ml of concentrated sulfuric acid and add to 50 ml Erlenmeyer flask
• Prepare ice bath with a metal pan holding ice
• Place flask in ice bath for 5'
• Then add 1.3 ml of methyl benzoate to flask while swirling
• Meanwhile, place a 50 ml beaker in the ice bath and add 1 ml of concentrated nitric acid to the beaker
• And then add 1 ml of sulfuric acid
• Cool the 50 ml beaker
• Add the cold nitrating mixture drop by drop to the 50 ml flask over 5-10' using a small Pasteur pipet
• Swirl flask intermittently
• After addition of the nitrating mixture, keep the flask on ice for an additional 10' while occasionally swirling
• Meanwhile place 10 g of ice in a 100 ml beaker
• After the 10', pour the reaction mixture into the ice, and stir with a glass rod or spatula until a precipitate forms
• Collect the precipitate using a buchner funnel
• After filtering, release suction and wash the precipitate with 3 ml of distilled water
• Reapply suction and filter again
• Repeat wash with water two more times
• Meanwhile cool a few ml of methanol in a beaker
• After washing the crystals with water, wash the crystals with 1-2 ml of ice cold methanol
• Transfer crude product to a 50 ml beaker
• Meanwhile, in a 50 ml Erlenmeyer flask, place 10 ml of methanol into flask and warm on a hot plate
• Recrystallize the crude product by adding the hot methanol to the beaker
• If solution has insoluble impurities, then filter through Buchner funnel
• Allow the solution to slowly cool
• Then collect the purified product using a Buchner funnel
• Transfer product to tarred vial and allow the crystals to dry
• Take melting point of product

Table of Chemicals:

Chemical	Molecular Weight (g/mol)	Melting Point (°C)	Density (g/ml)
sulfuric acid	98	-	-
Methyl benzoate*	136	-	1.087
Nitric acid	63	-	-
methanol	32	-	-
diethyl ether	74	-	-
toluene	92	-	-
2-propanol	60	-	-
Methyl m-nitrobenzoate	181.15	77.5-78.5	-

*Limiting Reagent

Mechanism:

Observations:

• 3.0 ml of sulfuric acid was placed to beaker
• 1.4 ml of methyl benzoate was then added
• The beaker was placed on ice
• In a separate beaker 1.0 ml of nitric acid was added to 1.0 ml of sulfuric acid
• This beaker was also put onto ice
• Drop by drop, the nitrous mix was added to the methyl benzoate beaker
• The reaction mixture was fuming a little
• A small amount of ice was added to the reaction mix
• A white precipitate formed
• The precipitate was vacuumed filtered
• The mass was .672 g

Results:

Recrystallization produced .672 g of methyl m-nitrobenzoate of product (33.2% yield).

Discussion:

In this experiment, methylbenzoate was nitrated to m-nitrobenzoate through the mechanism of electrophilic aromatic subsitution. .672 g of product was produced with a percent yield of 33.2%. Some possible sources of containments were substitution that occurred in the ortho and para positions. Meta substitution with deactivating groups is not 100% since the ortho and para positions still have resonance stabilization. Equilibrium could be a problem too, and the products could have reverted to the reactants.

Questions:

1. Starting with toluene, show all the steps that you would need to prepare the following two products.

2. Which of the following compounds would undergo nitration at the fastest rate?

3. Two students recrystallize crude methyl m-nitrobenzoate from ethanol instead of methanol. The product has amp of 45-47 °C and has a low yield. What product has the student isolated and what reactions have taken place with ethanol?

When the student was recrystallizing with ethanol, the ethanol must have done a transesterfication reaction with methyl benzoate group. In this process, the ethanol substitute for the methanol part of the ester. By having excess amount of ethanol (as used in recrystallization) the ethanol is promoted to substitute for the methoxy group. When recrystallizing with methanol, one avoids this reaction because the methanol is the same as the methoxy group on the ester. The product would be ethyl m-nitrobenzoate.