This experiment is about the synthesis of 3-nitrobenzaldehyde through nitration. The nitration of benzaldehyde is an example of an electrophilic aromatic substitution reaction, in which a proton of an aromatic ring is replaced by a nitro group. Many aromatic substitution reactions are known to occur when an aromatic substrate is allowed to react with a suitable electrophilic reagent, and many other groups besides nitro may be introduced into the ring. Although the reaction produced a low yield at the end, the yield is calculated from the reaction and limiting reagent. Keywords: electrophilic aromatic substitution, nitration, aldehyde, nitrating group.
Introduction
Electrophilic substitution happens in many of the reactions of compounds containing benzene rings - the arenes. Electrophilic Aromatic Substitution is a reaction in which the hydrogen atom of an aromatic ring is replaced as a result of an electrophilic attack on the aromatic ring (Attkins & Carey, 1990). There are steps to an electrophilic substation. First, attack of the electrophile on the aromatic ring, creating a resonance-stabilized carbocation called an arenium ion, which is an ion that is the result of an electrophilic attack on a benzene ring. And deprotonation of the arenium ion by a weak base to regain aromaticity. Nitration is defined as replacing a hydrogen with a nitro (NO2) group. Nitration requires the presence of sulfuric acid (H2SO4) as a catalyst. Mechanism of Nitration.
Mechanism:
The nitronium ion is the electrophile generated by the nitric acid/sulfuric acid mixture. The reaction is "sluggish" with nitric acid-sulfuric acid. Note that the reactions are reversible, and shifted to the desired products by manipulating concentrations (i.e., adding more reactant, removing a product, etc.) The electrophile is the "nitronium ion" or the "nitryl cation", NO2+. This is formed by reaction between the nitric acid and the sulphuric acid.
Nitroaromatic compounds are among the largest and most important groups of industrial chemicals in use today. These compounds are organic molecules that consist of at least one nitro group (-NO2) attached to an aromatic ring. The vast majority are synthetic, although several biologically produced nitroaromatic compounds have been identified. The strong electronegativity of the nitro group stems from the combined action of the two electron-deficient oxygen atoms bonded to the partially positive nitrogen atom. When attached to a benzene ring, the nitro group is able to delocalize π-electrons of the ring to satisfy its own charge deficiency (Bloch, 2012). This not only provides charge to the molecule but also imparts unique properties that make the nitro group an important functional group in chemical syntheses.
The nitro group is strongly deactivating toward electrophilic aromatic substitution of the benzene ring. Both the conjugation state and resonance properties of nitro groups attached to aromatic rings result in partially positive charges at ortho and para positions that act to repel electrophiles, and as a consequence, attacks are directed toward the open meta positions. There are certain limitations to the usefulness of nitration in aqueous sulphuric acid. Because of the behaviour of the rate profile for benzene, comparisons should strictly be made below 68% sulphuric acid. For deactivated compounds, this limitation does not exist and nitration in sulphuric acid is an excellent method for comparing the reactivities of such compounds.The objective of this experiment is to synthesize 3-nitrobenzaldehyde and calculate the percent yield of the product from the reaction.
Results and discussion
Benzene was treated with a mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature not exceeding 50°C. As temperature increased, there was a greater chance of getting more than one nitro group, -NO2, substituted onto the ring. Nitrobenzene was formed.
or:
The concentrated sulphuric acid was acting as a catalyst. The electrophilic substitution mechanism. Stage one.
Figure 1. Stage 1 of the mechanism of nitration. As the NO2+ ion approached the delocalised electrons in the benzene, those electrons were strongly attracted toward the positive charge. Two electrons from the delocalised system were used to form a new bond with the NO2+ ion. Because those two electrons aren't a part of the delocalised system any longer, the delocalisation was partly broken, and in the process the ring gained a positive charge. Stage two
Figure 2. Stage 2 of the nitration mechanism The second stage involved a hydrogensulphate ion, HSO4-, which was produced at the same time as the NO2+ ion. This removed a hydrogen from the ring to form sulphuric acid - the catalyst had therefore been regenerated. The electrons which originally joined the hydrogen to the ring were now used to re-establish the delocalised system. Table 1. Observations from the experiment proper
Observations:
89 mL conc. H2SO4.
Clear solution + 45 mL fuming HNO3.
Clear solution + 10.2 mL benzaldehyde Solution turns yellow if stirred continuously while adding benzaldehyde.
But solution will produce red orange fumes and increase heat. + ice.
White fluffy precipitate.
After vacuum filtration.
White gum-like precipitate + 125 mL diethyl ether.
Precipitate dissolves and solution turns into pale yellow color + 125 mL 5% NaHCO3.
Immiscible with solution. Golden yellow in color.
While the experiment was being executed, observations were taken. No visible change was seen when sulfuric acid and nitric acid were mixed together. The sulfuric acid must be cooled while nitric acid was added so that the temperature would not increase. When benzaldehyde was added, the solution turned yellow. Nitric compounds are often yellow in color and posses agreeable odor. White fluffy precipitate formed when iced was added to the mixture. And a gum like precipitate formed when it was vacuum filtered. It was then washed with diethyl ether and sodium bicarbonate which dissolved and turned the white precipitate into a pale yellow solution. Petroleum ether or diethyl ether was washed with sodium bicarbonate in order to minimize contamination. The solution transferred to the beaker was allowed to evaporate and a yellow solid was formed.
Calculations from the data gathered
Pre-weighed vial 12.2338 g
Vial with crude sample 15.3010 g
Weight of crude sample 3.0672 g
Theoretical yield 15.00 g % yield 19.85 %
The solid was broken down to pieces and transferred to a pre-weighed vial. The weight of the crude sample was then calculated. The theoretical yield was calculated from the reaction of the experiment.
Then, the percent yield was attained from the theoretical yield and the actual yield. The percent yield was 19.85%. One of the reasons for the small yield was that it was not immediately filtered upon the addition of ice. Nitroaromatic compounds are relatively rare in nature and have been introduced into the environment mainly by human activities. This important class of industrial chemicals is widely used in the synthesis of many diverse products, including dyes, polymers, pesticides, and explosives. Unfortunately, their extensive use has led to environmental contamination of soil and groundwater. The nitro group, which provides chemical and functional diversity in these molecules, also contributes to the recalcitrance of these compounds to biodegradation. The electron-withdrawing nature of the nitro group, in concert with the stability of the benzene ring, makes nitroaromatic compounds resistant to oxidative degradation.
Experimental methodology
Concentrated H2SO4 that amounted to 89 mL was filled on a 500 mL volumetric flask while it cooled with an ice bath. A 45 mL of fuming HNO3 was added carefully and stirred continuously. To this nitrating mixture, 10.2 mL benzaldehyde was added and further cooled so that the temperature was kept constant at 15 degrees Celsius. The solution was stirred continuously for an hour. The ice bath was removed and the reaction mixture was stored overnight at room temperature inside the fume hood. The reaction mixture was poured on a 1 L-beaker on 500g crunched ice. The yellow precipitate formed was sucked off over a Buechner funnel and washed with 200 mL of cold water. The moist crude product was dissolved in 125 mL tert-butyl methyl ether (diethyl ether) and shaken out with 125 mL of a 5% NaHCO3 solution. Then, the solution was transferred into a beaker and stored overnight inside the locker. After the solution had dried, it was transferred into a vial and weighed.
