LAB 334 - NITRATION OF AROMATIC COMPOUNDS: PREPARATION OF METHYL-m-NITROBENZOATE LAB REPORT PDF

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NITRATION OF AROMATIC COMPOUNDS: PREPARATION OF METHYL-m-NITROBENZOATE LAB REPORT
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DEPARTMENT OF CHEMISTRY CHEM 334 SURNAME INITIAL ID NO. LAB GROUP DATE

MPHINYANE S. 201700690 THURS; 1000-1300HRS 13/02/2020

EXPERIMENT NO. 334.1

EXPERIMENT TITLE: NATRATION OF AROMATIC COMPOUNDS: PREPARATION OF METHYL-m-NITROBENZOATE.

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EXPERIMENTAL AIM The aim of the experiment is to determine and study electrophilic aromatic substitution reaction (EAS) particularly nitration of methyl benzoate go give methyl-m-nitrobenzoate; thus determine the weight, melting point and percentage yield of the pure methyl-m-nitrobenzoate. ABSTRACT

INTRODUCTION Aromatic substitution is electrophilic, due to high density in benzene ring. Benzene ring is one of components in most important natural products and other useful products. The species reacting with the aromatic ring is usually a positive ion or the end of a dipole. Nitration is one of the most important examples of electrophilic substitution. The electrophile in nitration is the nitronium ion which is generated from nitric acid by protonation and loss of water, using sulphuric acid as the dehydrating agent. Unlike nucleophilic substitutions, which proceed via several different mechanisms, electrophilic aromatic substitutions (EAS) generally occur via the same process. Because of the high electron density of the aromatic ring, during EAS reactions electrophiles are attracted to the ring's π system and protons serve as the leaving groups. Equation 1. During SN1 reactions, however, nucleophiles attack an aliphatic carbon and weak Lewis bases serve as leaving groups.

Ar - H arene

+

E+ electrophile

Ar - E + H+ substituted arene

Eq. 1

Generally, EAS reactions occur in three steps, Scheme I. During Step I, the electrophile is produced, Scheme I

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Usually, by the interaction of a compound containing the potential electrophile and a catalyst. During Step II, the aromatic π system donates an electron pair to the electrophile, forming a σ bond (an arenium cation) followed by deprotonation in step III in the present of a base ( HSO4-) affording the substituted 3

arene. EAS reactions are generally second-order processes, i.e., first order in electrophile and first order in arene. Thus, Step II. Is the rate-determining step (rds); rate = k2 [arene][electrophile].

Electrophilic Aromatic Substitution: Nitration of Methyl Benzoate

Benzene rings are components of many important natural products and other useful organic compounds. Therefore, the ability to put substituents on a benzene ring, at specific positions relative to each other, is a very important factor in synthesizing many organic compounds. The two main reaction types used for this are both substitutions: Electrophilic Aromatic Substitution (EAS) and Nucleophilic Aromatic Substitution (NAS). The benzene ring itself is electron-rich, which makes NAS difficult, unless there are a number of strongly electron-withdrawing substituents on the ring. EAS, on the other hand, is a very useful method for putting many different substituents on a benzene ring, even if there are other substituents already present. Electrophilic Aromatic Substitution chapter describes the factors involved in the regioselectivity for EAS reactions using benzene rings, which already have substituents on them. In this experiment you will put a nitro (—NO2) group on a benzene ring, which already has an ester group, attached to it (methyl benzoate). The actual electrophile in the reaction is the nitronium ion (NO2+), which is generated in situ ("in the reaction mixture" HNO3/H2SO4) using concentrated nitric acid and concentrated sulfuric acid.

Reaction: O

O C

OCH3

C

OCH3

HNO3 / H2SO4 NO2

Methyl Benzoate

Methyl m-nitrobenzoate

MW = 136.15 g/mol Density = 1.094 m/ml B.P. = 198-199 oC M.P. = -12 oC

MW = 181.14 g/mol B.P. = 279 oC M.P. = 78-80 oC

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Reaction mechanism The carbomethoxy group (-CO2CH3) directs the aromatic substitution reaction to the position that are meta to it. As a result the m-nitrobenzoate is the principal product from this reaction. Formation of dinitro products from this reaction is unlikely under the conditions in which you carry out your reaction. The reason for this is that both carbomethoxy groups as well as the nitro group (on the mono nitrated product) are deactivating groups making the second nitration less favorable. Concentrated sulfuric acid is the solvent for this reaction and is involved in the formation of nitronium ion (NO2+) from concentrated nitric acid. Water has a retarding effect on the nitration since it interferes with the nitric acid-sulfuric acid equilibrium (shown below) that generates the required nitronium ion (NO2+). H2ONO2+ + HSO4 -

HONO2 + H2SO4 H2ONO2+

NO2+ + H2O H3O+ + HSO4 -

H2O + H2SO4 Overall reaction:

NO2 +

HO-NO2 + H - HSO4

+

HSO4 -

+ H2O

Temperature also has an effect on the product distribution from this reaction. Higher the temperature, greater will be the amounts of dinitration products formed from this reaction.

Safety Note Caution: Avoid contact with the acids used in this experiment and the reaction product. Prevent contact with the skin, eyes, and clothing; work in the hood. An acid spill is neutralized using solid sodium carbonate or bicarbonate. The reaction is highly exothermic. A vigorous reaction will occur if the acid mixture is added too rapidly to the methyl benzoate. Concentrated nitric acid and concentrated sulfuric acid are both strong oxidizers, and strongly corrosive--wear gloves while handling them, and avoid breathing their vapors. Methyl benzoate and methyl m-nitrobenzoate are irritants -- wear gloves while handling them. Methanol is a flammable liquid, and is toxic -- no flames will be allowed in lab, wear gloves while handling it, and avoid breathing its vapors.

Chemicals Methyl benzoate Sulfuric acid (conc.) Nitric acid (conc.) Ice Methanol

Materials 150 – mL beaker 400-mL beaker 125-mL flask Stirring rod Mel-temp Suction filtration funnel 5

EXPERIMENTAL PROCEDURE 1.5 ml. of methyl benzoate and 4.0 ml. of concentrated sulfuric acid (drop-wise) were mixed in a 125-ml. Erlenmeyer flask, and chilled it in an ice bath. Continued to cool the mixture in the ice bath to reduce the heat, which produced in the reaction. After complete addition of sulfuric acid, added 2.0 ml concentrated nitric acid (measured in 10- ml graduated cylinder) drop - wise used a small graduated plastic pipette and mixed by gentle swirling. Continued to cool the reaction mixture. Allowed the reaction mixture to stand at room temperature for five minutes. Floated the 125-ml flask in a 400-ml beaker hot water bath. Removed the flask occasionally and swirled the content carefully. After fifteen minutes heating poured the reaction mixture in 100-ml of ice water contained in a 150- ml beaker, with stirring. Isolated the product by vacuum filtration, then washed the product with 20 mL cold water. Dissolved the product using 10 mL methanol placed in a hot water bath; Isolated the product by vacuum filtration, then washed the product with 10 mL cold methanol.

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RESULTS AND ANALYSIS

Mass of Petri-dish + filter paper Mass of Petri-dish + filter paper + product Mass of product

23.94g 24.87g 0.93g

The chemical equation: C6H5CO2CH3 (aq) + H2NO3 (aq)

C6H4CO2CH3NO2 (s) +H2O (l)

Calculation of the number of moles of C6H4CO2CH3NO2 Molar Mass (C6H4CO2CH3NO2) = 181.15gmol-1 Mass (C6H4CO2CH3NO2) = 0.93g Thus, n = M/MM = (0.93g) / (181.15gmol-1) = 0.005133866 = 5.13 × 10-3 mol From the reaction equation 1 mol of C6H5CO2CH3 produced 1 mol of C6H4CO2CH3NO2 Thus, n (C6H5CO2CH3) = 5.13 × 10-3 mol Calculation of mass of C6H5CO2CH3 M = n × MM = (5.13 × 10-3 mol) × (136.15gmol-1) = 0.6984495 = 0.70g

DISCUSSION In this experiment, nitric acid and methyl benzoate were reacted to form methyl-m-nitrobenzoate. The nitric acid could have added to the ortho, para, or meta position. This forms three possible products; methyl-o-nitrobenzoate, methyl-p-nitrobenzoate and methyl-m-nitrobenzoate when the reagent methyl benzoate is in the benzene species. The functional group on the benzene ring is an electron withdrawing group. This functional group deactivates the ring, and it is a meta-director. Electron withdrawing groups are meta-directors because their secondary carbocation intermediates are the most stable, compared to the ortho and para positions. This indicates that the meta product will be the major product when a benzene ring is mono-substituted with an electron withdrawing group. Because of this, methyl-m-nitrobenzoate is the major product over the two possibilities. Nitric acid was used to form the electrophile, the nitronium ion. The sulfuric acid aided the formation of the nitronium ion by donating a proton to nitric acid. Methyl benzoate was the nucleophile the reaction. Ice chucks were used in this reaction to help the product crystallize out of the solution. While water and 7

methanol helped remove impurities from the product. Ice water baths were used throughout the course of this experiment to prevent over-nitration of methyl benzoate. Although the ring was deactivated, overnitration could have occurred if the mixture was heated. Nitration is an introduction of nitrogen dioxide into a chemical compound acid. In the process the methyl benzoate was nitrated to form a methyl m-nitro benzoate. The reagents were added very slow to avoid a vigorous reactions and the temperature was maintained low to avoid formation of dinitro product. In this experiment, electrophilic aromatic substitutions involved the replacement of a proton on an aromatic ring with an electrophile that becomes substituent. The solvent sulphuric acid protonates the methyl benzoate, creating the resonance stabilized arenium ion intermediate. The electron deficient nitronium ion reacts with the protonated intermediate meta position. The ester group is the meta deactivator and the reaction takes place at the meta position because the ortho and para positions are destabilized by adjacent positives charges on the resonance structure .The major product is the meta product due to carboxyl and nitro groups both being powerful electron withdrawing groups. The percentage yield calculated for the product was 18.7%. A few errors occurred throughout the experiment that could have affected the yield. When inserting the nitric acid mixture drop wise into the flask, a few drops were spilled. This could have caused fewer molecules of the nitronium ion to react, thus causing a lower percentage yield. When transferring the crystallize mixture into the suction filtration apparatus, some solid product stuck to the inside of the flask, as well as the suction filtration funnel. Both of these errors could have contributed to the percentage yield error. The melting point for the product was a range from 75-64. The melting point

nitrobenzoate is a range from 94.00°C to 96.00°C. The melting point of methyl mnitrobenzoate is a range from 78.0°C to 80.0°C. 2

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The melting point of the product obtained most closely matched the melting point for methyl m-nitrobenzoate. The melting point obtained is slightly higher and broadened. This is an indication of impurities present. These impurities could have been unreacted reagents, water, or methanol. 9

Finally, proton NMR analysis was done on the final product, shown in figure 5. The most shielded peak, peak A, was a singlet; located at 3.9 ppm. The second most shielded peak, peak B, was a triplet, located at 7.7 ppm. The fact that this peak is a triplet indicates that it is beside two neighboring hydrogens. These two neighboring 10

hydrogens are about equal in equivalency, and show up as peak C on the spectrum. This peak is located at about 8.5 ppm. The final peak, peak D, is the most de-shielded peak. It is a singlet located at 8.9 ppm. Because it is the most de-shielded, this indicates that it is between the two groups on the ring. Peak D and peak B are both indicators that 11

the major product obtained in this reaction was methyl m-nitrobenzoate. nitrobenzoate is a range from 94.00°C to 96.00°C. The melting point of methyl mnitrobenzoate is a range from 78.0°C to 80.0°C. 2

The melting point of the product obtained most closely matched the melting point 12

for methyl m-nitrobenzoate. The melting point obtained is slightly higher and broadened. This is an indication of impurities present. These impurities could have been unreacted reagents, water, or methanol. Finally, proton NMR analysis was done on the final product, shown in figure 5. 13

The most shielded peak, peak A, was a singlet; located at 3.9 ppm. The second most shielded peak, peak B, was a triplet, located at 7.7 ppm. The fact that this peak is a triplet indicates that it is beside two neighboring hydrogens. These two neighboring hydrogens are about equal in equivalency, and show up as peak C on the spectrum. 14

This peak is located at about 8.5 ppm. The final peak, peak D, is the most de-shielded peak. It is a singlet located at 8.9 ppm. Because it is the most de-shielded, this indicates that it is between the two groups on the ring. Peak D and peak B are both indicators that the major product obtained in this reaction was methyl m-nitrobenzo 15

In this experiment, nitric acid and methyl benzoate were reacted to form methyl nitrobenzoate. The nitric acid could have added to the ortho, para, or meta position. This forms three possible products: methyl onitrobenzoate, methyl pnitrobenzoate, and methyl m-nitrobenzoate. The reagent methyl benzoate is the benzene species. The 16

functional group on this benzene ring is an electron withdrawing group. This functional group deactivates the ring, and it is a meta-director. Electron withdrawing groups are meta directors because their secondary carbocation intermediates are the most stable, compared to the ortho and para positions. This indicates that the meta product will be 17

the major product when a benzene ring is monosubstituted with an electron withdrawing group. 3

Because of this, methyl mnitrobenzoate is the major product over methyl o-nitrobenzoate and methyl p-nitrobenzoate In this experiment, nitric acid and methyl benzoate were reacted to form methyl nitrobenzoate. The nitric acid could have added to 18

the ortho, para, or meta position. This forms three possible products: methyl onitrobenzoate, methyl pnitrobenzoate, and methyl m-nitrobenzoate. The reagent methyl benzoate is the benzene species. The functional group on this benzene ring is an electron withdrawing group. This functional group deactivates the ring, and it is a meta-director. 19

Electron withdrawing groups are meta directors because their secondary carbocation intermediates are the most stable, compared to the ortho and para positions. This indicates that the meta product will be the major product when a benzene ring is monosubstituted with an electron withdrawing group. 3

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Because of this, methyl mnitrobenzoate is the major product over methyl o-nitrobenzoate and methyl p-nitrobenzoate

CONCLUSION The methyl m-nitrobenzoate was prepared. The theoretical yield is 3.9852 g while the actual yield is 2.6996 g so we get the percentage yield is 67.74%. The melting point of our product is 75˚C - 78˚C and 76˚C - 78˚C. From the given physical constant we know that the literature melting point of methyl mnitrobenzoate is 78 - 80˚C, so we can conclude that the product we get is methyl m-nitrobenzoate.

QUESTION 1) Why is methyl m-nitrobenzene formed in this reaction instead of the ortho or para isomers? What is the melting point of the ortho and para isomers? Methyl m-Nitrobenzoate is formed in this reaction rather that ortho/para isomers because of the ester group of your starting product of methyl benzoate. The functional group of ester is an electron withdrawing group causing nitrobenzene (NO2) to become in the meta position. Thus NO2 is a deactivating group causing itself to be a meta director. More than the literature value.

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