Nitration Of Methyl Benzoate Lab Report

Nitration of Methyl Benzoate to form Methyl-m-nitrobenzoate via Aromatic Substitution

Linh Ngoc Thuy Nguyen
Seattle Central Community College
Professor: Dr. Esmaeel Naeemi
Date: February 21st, 2012
Abstract
In this experiment, methyl-m-nitrobenzoate, followed the electrophilic addition of aromatic ring, would be formed from the starting material methyl benzoate and nitric acid, under the catalysis of concentrated sulfuric acid. The reaction between nitric acid and sulfuric acid resulted in the formation of nitronium ion NO2+. It then acted as strong electrophile that nitrated the benzene ring. After that, vacuum filtration and recrystallization were methods used to obtain the final product with minimal impurities. The mass of product
collected was 3.835g, making a percent yield of 94.5%. The product was successfully made since nitro group can be observed at peak 1531.3 in the IR spectrum, doublet peak at 4.0 ppm in the proton NMR, and peak at 60 ppm in the C13 NMR. The purification of 95.2% can also be observed in the GCMS.
Introduction
Nitration is an electrophilic aromatic substitution reaction, where a nitro group is being added into the benzene ring, in return to the losing of a hydrogen.
First, nitronium ion is made by the reaction between nitric acid and sulfuric acid, with sulfuric acid acting as a protonated reagent. The nitronium ion is a strong electrophile that can react with benzene ring to form the arenium ion intermediate, despite the fact that it can temporarily lose its stability provided by the resonances. After that, it regains its stability by deprotonating the intermediate and yields the final product, in this case is methyl-m-nitrobenzoate. This experiment will be carried out under controlled temperature of 15oC or lower, since higher temperature will result in the second nitro group addition. In this experiment, student will observe the mechanism of electrophilic aromatic substitution reaction through the nitration of methyl
benzoate.
Experimental
Methyl benzoate (3.05g, 0.0224g) was added to a 100-mL beaker contained concentrated sulfuric acid (6mL) that had been cooled to 0oC .
The mixture was then cooled to 0oC or below using ice-salt bath before adding slowly 1-1 ratio concentrated sulfuric acid and concentrated nitric acid (4mL). The mixture was stirred continuously to maintain the reaction temperature below 15oC for a period of 15 minutes. The acid mixture then poured over crushed ice (25g), and waited for 5 minutes until the ice was completely melted. The product, after isolation by vacuum filtration, was washed with two portions of cold water (30mL) and ice-cold methanol (25mL). Recrystallization of product was carried out with minimum amount of solvent methanol, which resulted in the final product of methyl m-nitrobenzoate (3.84g, 0.0212mol), with the percent yield of
94.5%.
Results:
Percent Yield:
(0.02117mol Methyl-m-nitrobenzoate)/(0.02240mol Methyl Benzoate) x 100% = 94.50%
GCMS:
4.837% of product at retention time of 3.179 min was the solvent used for GCMS test
95.163% of product at retention time of 3.402 min was the meta product of methyl-m-nitrobenzoate.
Product melting point range:
70.3oC – 75.6oC
Proton NMR:
Multiplets at δ 8.37-8.47 ppm with the intergration value of δ 40.71 ppm
Carbon NMR:
Peaks between δ124.61 ppm and δ 148.24 ppm were for the 6 carbons that located on the benzene ring.
Peak at δ164.96 ppm was for the carbon in the carbonyl group.
Peak at δ 52.82 ppm was for the carbon next to the carbonyl group.
Peak at δ76.71-77.36 ppm was for the solvent using for NMR test.
IR:
Peak at 3092.51 indicated C-H aromatic stretch.
Peak at 2954.8 indicated C in CH3 stretching.
Peak 1734.1 indicated carbonyl-ester stretching.
Peak at 1531.3 indicated nitro group stretch. Discussion:
In this experiment, the electrophilic aromatic substitution reaction mechanism was investigated and used to produce methyl m-nitrobenzoate from methyl benzoate and nitric acid-sulfuric acid mixture. Nitro group was added to the aromatic ring in the meta position since the carbonyl and nitro group was the deactivating, electron withdrawing group. The methyl benzoate was first mixed with concentrated sulfuric acid and then adding together with mixture of 1-1 ratio concentrated sulfuric acid and concentrated nitric acid. The sulfuric acid protonated to the nitric acid in order to make nitronium ion since nitric acid is not a strong enough electrophile to react with benzene ring, coz the ring is highly stable due to the resonances and less likely to give up the stability. The nitronium ion with positive charge would be an ideally electrophile, under slightly elevated temperature, to react with the double bond in the methyl benzoate ring. The addition of nitric/sulfuric acid was carried out slowly and gently over the period of 15 minutes to minimize the possibility of forming by-product, which is the dinitrobenzoate. Also reaction was carried out under temperature of 15oC or lower since higher temperature would boost up the reaction to form dinitrobenzoate – one of the main possibilities of impurity in the final product. The conjugate base was then used to deprotonate the intermediate to reform the aromatic ring structure, yielded the final product of methyl-m-nitrobenzoate. The product was first washed by ice (cold water) in order to get rid of any unreacted nitric acid/sulfuric acid and ice-cold methanol to get rid of by-products. Vacuum filtration technique was used to obtain the solidified product, which then recrystallize by minimum amount of hot methanol for the purification purpose.
Such careful purification, many steps of washing and recrystallization were paid off as the final product had a purification of 95.163%, as observed in GCMS scan at the retention time of 3.40. At the retention time of 3.18, there was a peak of 4.837% indicated the solvent CDCl3 using for this GCMS test. This peak cannot be impurities of dinitrobenzoate product since my product had a melting point of 70.
The percent yield of the experiment was 26%, and calculations are shown at the end of the paragraph. The melting point of the final product was 76.6°C-78.9°C, which compared to the literature value (78°C) was close enough to suggest a successful addition of the nitro group.1 The IR spectra also shows the characteristic aromatic ring stretches (3092.51, 1500-700, and 670), and at 1525.84 the nitro group stretch which indicates that the experiment was successful in adding the nitro group.
Percent Yield Calculations:

As seen in Scheme 02, there are 3 resonance arenium carbocation intermediates, with the positive charge at either the ortho, para, or meta position. The major product is the meta product due to the carboxyl and nitro groups both being powerful electron withdrawing groups. The meta attack on the methyl benzoate is the only attack that doesn’t yield have a resonance structure that isn’t highly unstable, so the meta attack is the most resonance stabilized (compared to the attack at either the ortho or para positions). While the percent yield was a little low for the experiment, the melting point data and the IR spectra both support the notion that the nitro group was successfully added to the methyl benzoate. Additional reading on the subject allowed for the defense of the meta attack as opposed to the ortho or para attack of the nitro group, which was a successful learning opportunity to see “nature’s laziness”, or the desire to form the structure with the most stability and lowest energy of activation