Electrophilic Aromatic Substitution: Meta-Selective Nitration of Methyl Benzoate with Sulfuric and Nitric Acid

This chemical reaction exemplifies Electrophilic Aromatic Substitution, employing a mixture of Sulfuric Acid and Nitric Acid to generate the Nitronium Ion (NO2+), the active electrophile attacking the electron-rich aromatic ring. The presence of electron-donating groups accelerates the reaction, often resulting in multiple nitro group attachments. However, in this laboratory exercise, the electron-withdrawing ester group (–CO2R) in methyl benzoate slows the electrophile attack, facilitating the isolation of a monosubstituted product.

Procedure:

1. Begin by adding 6 ml of concentrated sulfuric acid to a cooled 100 ml beaker in an ice bath for 5-10 minutes.
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2. Introduce 3.05 g of methyl benzoate into the cold sulfuric acid.
3. Allow the mixture to sit in the ice bath for an additional 5 minutes.
4. Prepare an H2SO4/HNO3 mixture by adding 2 ml of concentrated sulfuric acid to 2 ml of concentrated nitric acid; cool in an ice bath.
5. Slowly add the H2SO4/HNO3 mixture to the H2SO4/methylbenzoate mixture using a Pasteur pipet, swirling the mixture after each drop.
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6. Keep the reaction flask in the ice bath, maintaining a temperature range of 5-20.
7. Once the addition is complete, allow the entire mixture to warm to room temperature.
8. Let the reaction mixture stand for an additional 15 minutes for completion.
9. Pour the entire mixture onto approximately 25g of crushed ice.
10. Ensure all the ice is completely melted before proceeding with vacuum filtration.
11. Isolate the solid product through suction filtration using a Buchner funnel.
12. Wash the solid product with two 12 ml portions of cold water, followed by two 5 ml portions of ice-cold methanol.
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13. Recrystallize the crude product using methanol as the recrystallization solvent (equal weight).
14. Collect the purified product through vacuum filtration and wash with a small amount of cold methanol.
15. Dry, weigh the product, and determine its melting point to identify the isomer.

RESULTS

Table of weighing

Percentage yield = 25%

Melting point = 75-78

CALCULATION

C₈H₈O₂ + HNO₃ → C₈H₇NO₄ + H₂0

Mass of methyl benzoate = density x volume

= (1.094 g ml^-1)(2.8 ml)

= 3.0632 g

Mole of methyl benzoate =

= 0.0225 mol

Mass of nitric acid = (1.51 g ml^-1)(2 ml)

= 3.02 g

Mole of nitric acid =

= 0.0479 mol

methyl benzoate is the limiting reactant

1 mole methyl benzoate produced 1 mole methyl m-nitrobenzoate

Theoretical mass of methyl m-nitrobenzoate = (0.0225 mol)(181.14 g mol^-1)

= 4.0757 g

Actual mass of methyl m-nitrobenzoate = 3.1222 g

Percentage yield = x 100%

= x 100%

= 77%

In this experimental procedure, electrophilic aromatic substitutions were employed to replace a proton on an aromatic ring with an electrophile, converting it into a substituent. Sulfuric acid, acting as a solvent, protonated methyl benzoate, leading to the formation of a resonance-stabilized arenium ion intermediate. The electron-deficient nitronium ion then underwent a reaction with the protonated intermediate at the meta position. The ester group, functioning as a meta-deactivator, influenced the reaction to occur at the meta position due to the destabilization of the ortho and para positions by adjacent positive charges in the resonance structure. The dominant product was the meta product, attributed to the electron-withdrawing nature of both the carboxyl and nitro groups.

Upon re-crystallization, the product was weighed and found to be 3.1222 g, representing a 77% yield. The slight change in yield suggested possible impurities in the crude product or some product loss during re-crystallization. The measured melting point of the re-crystallized product (75°C to 78°C) closely aligned with the reported literature value of 78°C.

The lower yield might be attributed to a failure to maintain the temperature within the specified range (5-20°C) during the addition of the H2SO4/HNO3 mixture into the H2SO4/methylbenzoate mixture. Additionally, the high temperature could have led to the evaporation of some product.

In conclusion, the results indicated that methyl benzoate was the limiting reagent, and Nitric Acid was in excess. The theoretical yield of bromine was calculated to be 4.0757 g, resulting in a percent yield of 77%. After re-crystallization, the obtained product weighed 3.1222 g.

As for the question:

1. Methyl m-Nitrobenzoate is formed in this reaction rather than ortho/para isomers due to the ester group in methylbenzoate. The ester's electron-withdrawing nature makes nitrobenzene (NO2) preferentially occupy the meta position. Therefore, NO2 functions as a deactivating group, directing itself to the meta position.