Title: Nitration of Methyl Benzoate
Objective: To prepare methyl-3-nitrobenzoate from nitration of methyl benzoate
Introduction: Nitration is chemical process that introduces the nitro-group into an organic compound. There are many major industrial applications of nitration in the strict sense; the most important by volume are for the production of nitro-aromatic of nitro-aromatic compounds such as nitrobenzene. as nitrobenzene. They They are of wide importance as chemical as chemical intermediates and precursors. and precursors. In this experiment, the methyl-3-nitrobenzoate prepare from nitration of methyl benzoate with the reagents such as nitric acid and sulphuric acid.
The nitration of methyl benzoate is a typically electrophilic aromatic substitution. Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system is replaced by an electrophile. The position of the nitrogroups attach is determined by the other functional group that already attached to the ring. In this experiment, a nitro ( — NO2) group on a benzene ring which already has an ester group is 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.
HONO2 + H2SO4 H2ONO2
+
H2O + H2SO4 Overall reaction:
HO-NO2 + H-HSO4
+
H2ONO2 + HSO4
-
+
NO2 + H2O +
H3O + HSO4 +
-
NO2 + HSO4 + H2O
Apparatus and Materials: Beaker, Glass rod, Test tubes, Pasteur pipette, Buchner funnel, Thermometer, Melting point apparatus, Methyl benzoate, Concentrated sulphuric acid, Concentrated nitric acid, Methanol, Mixture of methanol:water(1:1), Ice
Experimental Procedure: 6mL of concentrated sulphuric acid was added into a 100 mL beaker. The beaker was cool in an ice bath (0°C) for 5-10 minutes. 2.8 mL of methyl benzoate was added to the cold sulphuric acid in the beaker, mix well and cool again to 0°C for about 5 minutes. 2 mL of concentrated sulphuric acid and 2 mL of concentrated nitric acid are added in a small test tube to prepare H2SO4/HNO3 mixture. The acid mixture was cooled in an ice bath (0°C). The H2SO4/HNO3 mixture was slowly added (drop-by-drop) to the H2SO4/methyl benzoate mixture (in the beaker) by using pasteur pipette. The mixture was swirled after each acid addition. The reaction beaker was kept in the ice bath during the addition. When the addition is complete (all the H2SO4/HNO3 mixture has been added), the mixture was allowed to warm to room temperature. The reaction mixture was standing for an additional 15 minutes to allow reaction to proceed to completion. The reaction mixture was poured onto about 10 g of crushed ice in a beaker. The product was isolated by vacuum filtration. The solid was rinsed with a tiny amount of ice-cold 50% methanol (mixture1:1 methanol-water). The crude product was recrystallised by using methanol as recrystallization solvent. The product yield and melting point were recorded.
Result:
Weight of product yield + weight of filter paper = 3.4304g Weight of filter paper = 0.5050g Weight of product yield = 2.9254 g Melting point = 80 - 83°C
Calculation: C6H5CO2CH3 + HNO3
C6H4CO2CH3 NO2 + H2O
Density of methyl benzoate = 1.08 g/mL Mass of methyl benzoate, C6H5CO2CH3 = 3.0240g Molar mass of C6H5CO2CH3 = 136.1487 g/mol No. of mole of C6H5CO2CH3 = 0.0222 mol
1 mole of C6H5CO2CH3 needed to produce 1 mole of C6H4CO2CH3 NO2 0.0222 mole of C6H5CO2CH3 needed to produce 0.0222 mole of C6H4CO2CH3 NO2 Molar mass of C6H4CO2CH3 NO2 = 181.1463 g/mol The theoretical yield of C6H4CO2CH3 NO2 = 0.0222 mol × 181.1463 g/mol = 4.0214 g
Percentage yield = =
= 72.75%
× 100%
Discussion: In this experiment, the nitration of methyl benzoate undergoes electrophilic aromatic substitutions. This reaction is involved the removal of hydrogen ion (proton) and replace +
by the nitronium ion, NO2 (electrophile) to become a substituent.
Figure 1: The mechanism of nitration of methyl benzoate
The sulphuric acid protonate the methyl benzoate to produce the resonance stabilised arenium ion intermediates. The electrophile is attack by the protonated intermediate at the meta-position of the aromatic ring. The reaction take place at the meta-position due to the ester group on the aromatic ring is meta-deactivator. The ortho and para positions are destabilised by the adjacent positive charge on the resonance structure. Lastly, the methyl-3-nitrobenzoate is formed as a major product due to the carboxyl and nitro groups towards the aromatic ring for both being powe rful electron withdrawing groups.
In the experiment, the methyl benzoate is being nitrated by using nitric acid, HNO3 and sulphuric acid, H2SO4. During the process, the reagent was added slowly because the reaction will be vigorous if added very fast. The temperature of the reaction also keep or maintained at very low to avoid the formation of dinitro product. An activation energy is needed for the substituted of nitro group on the aromatic ring. The increase of temperature will increasing the heat energy for activation where another nitro group will
be have a higher activation energy than the first substitution of nitro group to the ring. Thus, high temperature of this reaction increases the chance of forming of dintro product.
Figure 2: An example of dinitro product - Methyl 3,5-dinitrobenzoate.
The percentage yield of methyl 3-nitrobenzoate is 72.75% in which the weight of product obtained experimentally is 2.9254g while the theoretical yield is 4.0214g.
Figure 2. methyl m-nitrobenzoate (white solid)
The loss of product may due to the transfer of compounds e.g. the evaporation happened if liquid involved or the equilibrium system in the reaction is reached. The melting point of the methyl 3-nitrobenzoate that obtained in this experiment is 80 - 83°C. Although the value we got is close to the literature melting point of methyl m-nitrobenzoate with 78 80˚C
but that still have a slightly different value which may due to the product contain
the impurities or contaminator. There are several safety precaution should be taken in this experiment. Nitric acid and sulfuric acid are toxic and oxidizing. They can cause severe burns. Thus, safety goggles gloves, and lab coat should be wear. Next, the reaction of concentrated sulfuric acid between nitric acid is a highly exothermic reaction which the hot acid mixtures may bump and cause acid burns. So, make certain the acids are cold before mixing.
Conclusion: The methyl m-nitrobenzoate was prepared. The theoretical yield is 4.0212g while the actual yield is 2.9254 g so we get the percentage yield is 72.75%. The melting point of our product is 80-83°C. From the given physical constant we know that the literature melting point of methyl m-nitrobenzoate is 78-80˚C, so we can conclude that the product we get is methyl m-nitrobenzoate.
References: Nitration .2014. [online] Available at: http://en.wikipedia.org/wiki/Nitration [Accessed 5 July 2014]
Pahlavan/Cherif. Nitration of Aromatic Compounds: Preparation of methyl-mnitrobenzoate. [ pdf ] Available at: http://swc2.hccs.edu/pahlavan/2425L3.pdf. [Accessed 5 July 2014].
