Ostap Hlynskyy
CHM 3006
Nitrating Methyl Benzoate: Electrophilic Aromatic Substitution Substitution Abstract:
The purpose of this experiment was to synthesize methyl nitro benzoate from methyl benzoate, concentrated HNO 3, and concentrated H 2SO4 via an electrophilic aromatic substitution reaction. The HNO3 and H2SO4 were combined to form a nitrating solution, which was mixed with a mixture of methyl benzoate and H 2SO4. The product was then isolated and recrystallized using 95% ethanol. Following recrystallization, melting point was used to identify and characterize the ◦ product of the reaction. The melting point was determined to be 75 C, which indicates metaregiochemistry. The percent recovery of this reaction for the recrystallized product was 26.42%. Introduction:
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. In this experiment we will put a nitro ( — ( — NO 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 (NO 2 ), which is generated in the reaction mixture using concentrated nitric acid and concentrated sulfuric acid. Reaction scheme:
The electronic nature of the substituent also directs the position of electrophilic substitution. There are three different regioisomers for disubstituted aromatic rings: orth, meta, and para.
O
O C
OCH3
C
OCH3
HNO3 / H2SO4
NO2
Methyl Benzoate
MW = 136.15 g/mol Density = 1.0837 g/cm³ o
B.P. = 198-199 C o
M.P. = -12 C
Methyl m-nitrobenzoate
MW = 181.14 g/mol o
B.P. = 279 C o
M.P. = 78-80 C
Results and Discussion:
Procedure Part 1 – Place equal volumes of ice and tap water into a 400-mL beaker so that the beaker is approximately 75% full. Then, prepare chilled by pouring approximately 60 mL distilled water into a 125-mL Erlenmeyer flask. Place the flask into the ice-water bath. Part 2 – Label two 15 x 125-mm test tubes ―nitric acid‖ and ―sulfuric acid‖, respectively. Transfer 1 mL concentrated nitric acid into the tube labeled ―nitric acid.‖ Then, transfer 1 mL concentrated sulfuric acid into the tube labeled ―sulfuric acid‖. Chill both the test tubes with the acids in them in the ice-water bath for 15 minutes. Then, use a Pasteur pipet to very slowly add the cold sulfuric acid drop by drop into the cold nitric acid. Swirl the reaction mixture after every three drops are added. After adding all the sulfuric acid to the nitric acid, allow the nitrating solution to stand in the ice-water bath for 10 minutes. Part 3 – Place approximately 1 mL methyl benzoate into a 50-mL Erlenmeyer flask and add 2.0 mL concentrated sulfuric acid. Clamp the flask containing the mixture to a support stand. Lower the flask into the ice-water bath for 5 minutes, being sure to prevent the bath water from entering the reaction flask. After the 5 minutes are up, use a Pasteur pipet to slowly add the nitrating solution in the test tube to the Erlenmeyer flask containing the methyl benzoate and sulfuric acid. After all the nitrating solution has been added, allow the mixture to stand in the ice-water bath for 30-45 minutes. Make sure you swirl the flask every 5 minutes. After the 30-45 minutes are up, add 20 mL chilled water to a 100-mL beaker. Slowly and carefully add the cold reaction mixture, with stirring, to the chilled water. Then, allow the chilled solution to stand 5-10 minutes to complete crystal formation. Part 4 – Isolating, Purifying, and Characterizing the Product Filter the reaction mixture via vacuum filtration. Wash the crystals with approximately 20 m L of chilled water in order to remove any residual acid. Then, allow your product to air dry in the filter funnel for 10 minutes. Then, put your product crystals into a 25-mL Erlenmeyer flask and add 10 mL of 95% ethanol. Swirl the flask to mix. Heat the flask in a steam bath until the crystals are in solution and the solution boils. Then remove the flask from the sand bath and allow it to cool to room temperature. Add ice-cold ethanol to the flask and swirl to loosen the crystals. Then filter the crystals using vacuum filtration. Part 5 – Determine the melting point of recrystallized methyl nitrobenzoate. Data & Calculations Mass of crude methyl nitrobenzoate: 0.895g Mass of recrystallized methyl nitrobenzoate: 0.381g Percent yield: 26.42%
◦
Melting point of recrystallized product: 75 C Literature Value: 78 - 80°C
Theoretical Yield 3
Mass of methyl benzoate used = (1 mL methyl benzoate)×(1.0837 g/cm³)×(1 cm /mL) = 1.0837g Theoretic yield = [(1.0837g methyl benzoate)×(1mol methyl benzoate)×(1mol methyl nitrobenzoate)×(181.13g methyl nitrobenzoate)]/[(136.15g methyl benzoate)×(1mol methyl benzoate)×(1 mol methyl nitrobenzoate)] = 1.442 g methyl nitrobenzoate Percent Yield For recrystallized product = (mass of recrystallized methyl nitrobenzoate)/(theoretical yield) × 100% = (0.381g methyl nitrobenzoate)/(1.442g methyl nitrobenzoate) × 100% = 26.42% Crystallization losses are one source of error. Nitrations are energetic reactions under harsh conditions. Consequently, by products can readily form which decrease the overall yield of the reaction. In this lab procedure one source of error is that not all of crude product was used in the recrystallization, because of transfer losses. Another source of error could have been that some impurities could have crystalized out of solution during recrystallization slightly lowering the melting point. Conclusion:
The electrophilic aromatic substitution reaction between methyl benzoate and a nitrating solution of sulfuric and nitric acids was successful and yielded methyl m-nitrobenzoate. The percent yield of the recrystallized product was 26.42%. The melting point of the recrystallized product was 75°C. This was then compared with a data table given in the experiment in order to determine the regiochemistry of the obtained product. The observed melting point was close to the literature value of 78 - 80°C for the meta-substituted product. Therefore, analysis of the melting point determined that the product was methyl meta-nitrobenzoate, as could be predicted. Since the – NO2 group t is meta-director.
Questions:
1. (a) The percent yield of my product was 26.42%. (b) The melting point of my product was 75°C. (c) The observed melting point of 75°C was most closely related to the literature melting point of 78 – 80°C for a meta substitututed product. Therefore, my product has a meta- regiochemistry. The meta-product forms because the ortho- and para- products both have very unstable resonance forms with 2 positive charges next to each other. The two positive charges right next to one another cause the resonance forms to be unstable, so ortho- and para- substituted products do not form in this reaction. The meta- position is electron-rich, and thus reacts best with the – NO2 group to form methyl m-nitrobenzoate. (d) The structure of my product is:
2. The resonance forms for the arenium ion formed during this reaction are as follows:
3. The first nitration proceeds much faster than the second two because the first – NO2 group to be added is added to the isolated toluene. The methyl group on the benzene ring is an activating group. It causes the benzene ring to be more electron rich and thus, have a high reactivity. – CH3 is an ortho-para activator. So, the first – NO2 group adds to the toluene in the para-position in
order to avoid steric crowding from the methyl group. The toluene has now been converted to para- nitrotoluene. The – NO2 group that is now on the benzene ring is a deactivating group, and causes the benzene ring to be not nearly as reactive as it was.
