Nitration of Methyl Benzoate
PRELAB:
PRELAB EXERCISE: Draw the complete mechanism for the nitration of chlorobenzene.
Chlorine is an ortho-para director and deactivator of the benzene ring.
The nitration of methyl benzoate is a typical electrophilic aromatic substitution reaction.
The electrophile is the nitronium ion generated by the interaction of concentrated nitric
and sulfuric acids:
The nitronium ion then reacts with this protonated intermediate at the meta position,
where the electron density is highest, that is, where there is no positively charged
resonance form and yields the intermediate arenium ion which has the four resonance
forms shown here:
The arenium ion intermediate then transfers a proton to the basic bisulfate give methyl 3nitrobenzoate:
The ester group is a meta director and deactivator of the benzene ring. It is much easier to
nitrate a molecule such as anisole, where the methoxyl group is a w\ para director and an
activator of the benzene ring, as the following resonance structures indicate:
EXPERIMENTAL
IN THIS EXPERIMENT a cold solution of an aromatic ester that has been
dissolved in sulfuric acid is reacted with nitric acid. This highly exothermic reaction is
kept under control by cooling; then the mixture is poured onto ice. The solid product is
isolated by filtration and recrystallized from methanol, in which it is very soluble.
To 0.6 mL of concentrated sulfuric acid in a 10 X 100 mm reaction tube, add 0.30
g of methyl benzoate. Flick the tube or use magnetic stirring. Cool the mixture to 0°C;
add dropwise, using a Pasteur pipette, a mixture of 0.2 mL of concentrated sulfuric acid
and 0.2 mL of concentrated nitric acid. Keep the reaction mixture in ice. Using a stirring
rod, keep the reaction well mixed during the addition of the acids and do not allow the
temperature of the mixture to rise above about 15°C, as judged by touching the reaction
tube.
After all the nitric acid has been added, warm the mixture to room temperature
and, after 15 min, pour it onto 2.5 g of ice in a small beaker. Isolate the solid product by
suction nitration using a Hirsch funnel and a 25-mL filter flask. Wash the product well
with water and then with one 0.2-mL portion of ice-cold methanol. If the methanol is not
ice-cold, some product can be lost in this washing step. Save a small sample for a
melting-point determination and analysis by thin-layer chromatography (TLC) and
infrared (IR) spectroscopy.
The remainder is weighed and recrystallized from an equal weight of methanol in
a reaction tube. Alternatively, the sample can be dissolved in a slightly larger quantity of
methanol, and water is added dropwise to make the hot solution saturated with the
product. Slow cooling should produce large crystals with a melting point of 78°C. The
crude material can be obtained in about 80% yield with a melting point of 74°C-76°C. If
the yield is not as large as expected, concentrate the filtrate and collect a second crop of
product.
Cleaning Up.
Dilute the filtrate from the reaction with water, neutralize with sodium carbonate,
and flush down the drain. The methanol from the crystallization should be placed in the
organic solvents container.
2. Macroscale Nitration of Methyl Benzoate
In a 125-mL Erienmeyer flask, cool 12 mL of concentrated sulfuric acid to 0°C
and then add 6.1 g of methyl benzoate. Again, cool the mixture to 0°C-10°C. Now add
dropwise, using a Pasteur pipette, a cooled mixture of 4 mL of concentrated sulfuric acid
and 4 mL or concentrated nitric acid. During the addition of the acids, swirl the mixture
frequently (or use magnetic stirring) and maintain the temperature of the reaction mixture
in the range of 5°C-15°C.
When all of the nitric acid has been added, warm the mixture to room temperature
and, after 15 min, pour it on 50 g of cracked ice in a 250-mL beaker. Isolate the solid
product by suction filtration using a small Buchner funnel, wash well with water, and
then wash with two 10-mL portions of ice-cold methane!. A small sample is saved for a
melting-point determination. The remainder is weighed and crystallized from an equal
weight of methanol. The crude product should be obtained in about 80% yield with a
melting point of 74°C-76°C. The recrystallized product should have a melting point
of78°C. For your reference, the carbon and proton nuclear magnetic resonance (NMR)
spectra of the product are presented in Figures 28.1 and 28.2.
Cleaning Up, Dilute the filtrate from the reaction with water, neutralize with
sodium carbonate, and flush down the drain. The methanol from the crystallization
should be placed in the organic solvents container.
QUESTIONS
1. Hydrocarbons do not dissolve in concentrated sulfuric acid, but methyl benzoate-does.
Explain this difference and write an equation showing the ions that are produced.
2. What would you expect the structure of the dinitro ester to be? Consider the directing
effects of the ester and the first nitro group on the addition of the second nitro group.
3. Draw resonance structures to show in which position nitrobenzene will nitrate to form
dinitrobenzene.
4. Assign the peaks at 3101 cm-1, 1709 cm-1, and 1390 cm-1 in the IR spectrum of
methyl 3-nitrobenzoate (Fig. 28.3)