Nitration of an Aromatic Ring - M-Nitrobenzoic Acid from Benzoic Acid

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Furthermore, resonance structures are classified as electron donating groups (EDG) and electron withdrawing groups (EWG) which has lone pairs on atoms adjacent to the pi system and pi bonds to electronegative atoms adjacent to the pi system respectively. EDG activatesthe ring through a resonance donating effect which increases the electron density ring and directs it at ortho- and para- positions, it is more nucleophilic. Meanwhile, EWG promote deactivation through a resonance withdrawing effect which decreases the electron density on the ring and directs at meta- position. However, halogens are exception as they do deactivate rings but they direct ortho/para. EWG are less nucleophilic (Carey 2006 ?).

The electrophilicity of nitronium ion and sulfur trioxide can be compared by their structures. Both are electrophiles for nitration and sulfonation of benzene respectively but the nitronium ion is the more powerful one. NO2+ has an unpaired electron on its nitrogen atom, giving itself a positive charge (Kulkarni 2015). On the other hand, SO3 creates a partial negative charge due to the eletronegativity of the oxygen atom and a partial positive charge due to the sulfur atom (Malik 2014). Furthermore, nitronium ion is an example of EWG hence the stronger electrophile.

The final product can be confirmed as meta-substituted based on its melting point. By employing the oil bath method, the product melts at 130 degree Celsius. However, it is significantly lower than the theoretical melting point due to experimental errors. Hypothetically, the desired product, m-nitrobenzoic acid should have 141 degree Celsius sharp melting point. This will confirm the identity of a meta-substituted product because p-nitrobenzoic acid sublimes while o-nitrobenzoic acid melts at 147 degree Celsius.

The starting material and final product were subsequently subjected to chemical test for comparison namely bromine test, Baeyer’s test and reaction with ferrous hydroxide.

In both reaction with Br2 in Ch2Cl2 without light and Baeyer’s test, benzoic acid and m-nitrobenzoic acid returned similar negative results. It is because the substitution of a hydrogen involving EAS requires a catalyst and done under more vigorous conditions if there is no present acticating group in the ring, hence the lack of decolorization of bromine. Benzene, the main structure of both samples, is also resonance stabilized thus its negative result to Baeyer’s test. Ordinary pi bonds react with bromine and Baeyer’s tests due to its unsaturation. However, aromatic pi bonds are unable to decolorize bromine and KmnO4 because of its stability of its pi electrons rotating continuously (Gemellero 2015).

Meanwhile, ferrous hydroxide test was conducted to differentiate the final product fromthe starting material. It is a test for the presence of nitro groups. A positive test is the formation of the red-brown precipiate (ferric hydroxide) that persists for 30 seconds; the persistence of the positive result depends on the solubility of the nitro compound, while a negative test is signalled by a greenish precipitate (Doe 2014). In the experiment, benzoic acid and m-nitrobenzoic acid were treated with 1mL FeSO4 and alcoholic KOH. A greenish precipitate was formed in the benzoic acid mixture while a rusty brown precipitate was produced in the m-nitrobenzoic acid mixture. This signified that the final product contained a nitro group which was one of the objectives of the exercise.

Air must be excluded in the test with ferrous hydroxide (which was not done in the experiment). If the solution remained undeoxygenated, the presence of oxygen in an organic compound will oxidize ferrous iron to ferric, varying the color of the precipitate to green rust ( Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8).

*INSERT REACTION FOR THIS TEST*

Trinitrotoluene (TNT) is an explosive used mainly by ammunition manufacturers. It is made by adding three NO2 groups (from the reaction of nitric acid and sulfuric acid) to toluene. Toluene is nitrated initially, producing mononitrotoluene. The product is then renitrated to form dinitrotoluene. Finally, dinitrotoluene is nitrated with a mixture of nitric acid and oleum. TNT is then precipitated and granulated to obtain a crude product (Davis 1941).

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VII. Summary and Conclusion

Electrophilic aromatic substitution was employed in the experiment. It is constituted of three steps: a) generation of nitronium ion from the reaction of nitric acid and sulfuric acid, b) reaction of benzoic acid and nitronium ion to produce resonance-stabilized carbocation and c) deprotonation of the carbocation to restore the double bond and aromaticity of the compound. M-nitrobenzoic acid was synthesized and isolated from benzoic acid through EAS, producing white powdery precipitate.

The calculated experimental yield was 23.46% and the melting point was 130 degree Celsius. The low melting point was due to the experimental errors.

Substituted benzene accounts its substituent effects on reactivity and orientation. Reactivity is affected because of the interaction of the inductive and resonance effect. Orientation is affected by electron donating groups and electron withdrawing groups, the former directing to para- and meta- positions while the latter to meta- position. EWG is less nucleophilic than EDG.

The identities of the starting material and final product were confirmed by the bromine test, Baeyer’s test and Fe(OH)2 test. Benzoic acid and m-nitrobenzoic acid returned negative results in bromine and Baeyer’s tests because of the stability of the aromatic ring. Meanwhile, m-nitrobenzoic acid produced a positive result (rusty brown precipitate) in Fe(OH)2 test due to the presence of nitro group while benzoic acid formed a greenish precipitate due to lack of nitro group.