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Methyl Benzoate is electron rich and thus undergoes electrophilic aromatic substitution. Nitration of methyl benzoate will form a major product, methyl m-nitrobenzoate that was further purified by re-crystallization. The electrophile or the nitronium ion attached to the meta position in the benzene ring because of the carboxyl group being meta-deactivator that made the reaction took place in the meta position and the ortho and para positions are destabilized. The mass of re-crystallized methyl m-nitrobenzoate was determined to be 0.257 g compared to the mass of methyl benzoate, 0.360 g that resulted to …… % yield. The melting point of the methyl m-nitrobenzoate resulted to 74°C compared to its true melting point 78°C was computed to have 5.13% difference. Many side-products like different orientations in the benzene ring will be formed during chemical reactions. To see how much were formed and how pure was the product was, its TLC Profile was calculated that resulted into Rf = 0.79 wherein the result from iodine vapor indicating that the product synthesized was pure. The results of the experiment are therefore accurate and useful in familiarizing with reactions and mechanisms that occur during nitration.
Aromatic hydrocarbons are compounds with one or more benzene rings (3). Due to electron delocalization by its alternating double bonds, it is characterized by being more stable compared to alkenes (2). Benzene, being aromatic, is a cyclic compound, an unsaturated compound due the presence of pi bonds. In contrast to the double bonds causing the unsaturation of benzene, it is strong and unreactive because of its resonance structure where there is rearrangement of electron pair. They undergo EAS or Electrophilic Aromatic Substitution reaction due to the fact that they are electron rich. The nitration of methyl benzoate, C6H8O2, undergoes such type of reaction with concentrated nitric acid and sulfuric acid to yield methyl m-nitrobenzoate. From the methyl benzoate, methyl m-nitrobenzoate will be formed and will undergo purification by re-crystallizing with methanol. Thin Layer Chromatography profile and melting point of the product were used in the experiment to make sure that there are no by-products formed like compounds with o- and p- orientation (2).
MATERIALS AND METHODS
The mass of conical flask was determined and added with 20 drops of methyl benzoate. The mass was also determined as well as the mass of methyl benzoate itself. The methyl benzoate in the flask was placed in an ice bath while added by 2 ml of conc. H2SO4 while swirling. Then, the nitrating agent was prepared which is a mixture of 1 ml conc. HNO3 and 1 ml conc. H2SO4 chilled in an ice bath. The nitrating agent was then added to the methyl benzoate solution using a Pasteur pipet. The mixture was constantly stirred and still chilled in an ice bath. However, conc. H2SO4 must be added if cloudiness occurs to change the mixture into its clear appearance. Furthermore, the reaction mixture was added 10 g of crushed ice then stirred. As the ice melted, the solid product formed was separated using vacuum filtration where a filter paper was folded into 1/16 big enough to cover holes in the Buchner funnel. Then, the product was washed with cold distilled water, followed by 5% NaHCO3 and with cold distilled water until the filtrate became neutral.
The product was transferred in a watch glass and dried over a steam bath. Methanol was used to recrystallize or solidify the product and the percentage yield was calculated. The melting point of the recrystallized product was determined using the melting point apparatus. The TLC profile was determined using a TLC plate. The TLC plate was drawn with a line using a pencil 1 cm from the bottom and from the top then dipped in a beaker with ethyl acetate. The plate was dipped in ethyl acetate 3 times. When the solvent reached the other edge (the other line), the plate was removed from the beaker. Lastly, Iodine pellets were used to further determine the TLC profile.
RESULTS AND DISCUSSIONS
Methyl benzoate, C6H8O2, is an aromatic hydrocarbon, a methyl ester with a colorless appearance and used in perfumery. With Methyl benzoate, methyl m-nitrobenzoate will be synthesized and will further be purified by re-crystallization (2) with results shown in Table 1.
Table 1. Percentage Yield of methyl m-nitrobenzoate
Mass of conical flask
Mass of methyl benzoate and conical flask
Mass of methyl benzoate
Mass of re-crystallized methyl m-nitrobenzoate
The mass of methyl benzoate was determined by difference then cooled in an ice bath then added conc. H2SO4. The solution will still be colorless because methyl benzoate is soluble in H2SO4. Thus, it will form a homogenous mixture. On the contrary, it will form a yellow solution if the flask used is contaminated. The nitrating agent prepared in an ice bath because nitration is an exothermic that requires only 15°C and must not increase (1). HNO3 and H2SO4 should be added in the homogenous solution slowly while stirring to avoid side products formation like compounds with o- and p- orientation. Also, it will result to a low yield of the product desired. Likewise, fast addition will result to getting a high temperature that must be 15°C only. Addition of H2SO4 if cloudiness will occur is important since methyl benzoate is soluble in sulfuric acid and will produce a colorless solution.
The mixture of HNO3 and H2SO4 produces nitronium ion that will attach to the benzene ring (5). Methyl Benzoate is characterized for being electron rich and thus capable of reacting to electrophiles and undergoes EAS or Electrophilic Aromatic Substitution with a mechanism via carbocation [C+] intermediate shown in Figure 1.
Figure 1. Complete Mechanism of Nitration via Carbocation Intermediate
It follows 3 steps that started from the generation of electrophile which is the nitronium ion, NO2. Then, the formation of carbocation intermediate where nitronium ion will be attacked by the nucleophile will follow. The carbocation in the second step is capable of resonating since aromatic compounds are resonance structure wherein the electron pair can move around the molecule. The resonance of the benzene ring makes them stable and favorable to the reaction since the more stabilize the benzene ring is, the more reaction. The last step is the dissociation of H+ and the reformation of aromatic ring where the electrophile will substitute in one of the hydrogens on the benzene ring. This mechanism resulted to an overall reaction shown in Figure 2.
Figure 1. Nitration of methyl benzoate
The solvent of the nitration which is the sulfuric acid will protonate the reagent, methyl benzoate, which will create stabilized carbocation intermediate. The electrophile or the electron poor nitronium ion, will react to the protonated intermediate in the meta position. The carboxyl group belongs to the e- withdrawing group that deactivates the aromatic ring. Since electron withdrawing group has a meta orientation and a deactivator, the reaction will take place in the meta position. Likewise, the ortho and para positions are destabilized (5). The major product now is the methyl m-nitrobenzene which has nitro and carboxyl group being both electron withdrawing groups oriented at the meta position.
After adding the nitrating agent, the reaction mixture was added 10 g of crushed ice until it solidified and filtrated by vacuum filtration to get a dryer product. It was then washed by cold distilled water and 5% NaHCO to make excess acid neutralize that made the product green-colored. The product was dried over a steam bath and re-crystallized with methanol for purification. By washing the product with methanol, by-products or impurities like substitution on different places on the aromatic ring (ortho and para positions), methyl-2-nitrobenzoate or maybe methy-3-nitrobenzoate formed during the previous reactions should purified to get the preferred product. However, the methanol is preferred cold to control loss of desired product. The re-crystallized methyl m-nitrobenzoate weighed 0.257 and had …… % yield. The melting point of the re-crystallized methyl m-nitrobenzoate was determined to be 74°C that is close to its true melting point being 78°C. The % difference was determined to be 5.13% as shown in Table 2.
Table 2. Melting point of re-crystallized methyl m-nitrobenzoate
Melting point of re-crystallized methyl m-nitrobenzoate
True Melting point of methyl m-nitrobenzoate
To see how much products were formed and to check its purity, the TLC profile or Thin Layer Chromatography should be done (6). Each trial as the TLC plate was dipped in the solvent (Ethyl acetate) should have only 1 spot that is equivalent to 1 compound. If 3 spots were formed in the TLC plate on 3 trials when the solvent moves on the top of the plate by capillary action, it simply means that 3 compounds were present in the product. Thus, making it considered as not pure. The spots will be clealy visualized when the plate is placed in an iodine vapor (4) here it sublimes from solid to liquid stain. Also, other compounds are capable of adsorbing iodine and become visible. On the contrary, the re-crystallized m-nitrobenzene formed had been considered as a pure compound as illustrated in Figure 3 and had an Rf value of 0.79.
Rf = dspot / dsolvent front
Rf = 3.75 cm / 4.6 cmC:\Documents and Settings\nicolle\My Documents\College Files\2nd Year - 2nd Sem\ORGCHEMLAB\EXPT4 NITRATION OF METHYL BENZOATE\TLC Plate.jpg
Rf = 0.79
dsolvent front = 4.7 cm
dspot = 3.7 cm
Figure 3. TLC Profile
The desired product, methyl m-benzoate, purified by re-crystallization was successfully synthesized from methyl benzoate by avoiding the formation of other side-products. However, if phenyl benzoate was used, it will also follow EAS or Electrophilic Aromatic Substitution