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

Nitration of An Aromatic Ring

m-Nitrobenzoic acid from Benzoic acid

Introductions

Electrophilic aromatic substitution (EAS) involves formation of new sigma bond from C=C in the arene nucleophile, removal of the proton by breaking the C-H sigma bond and reformation of C=C and restoration of aromaticity (Hunt 2016). EAS can be represented as follows:

[pic 1]

Furthermore, EAS mechanism involves series of events given below:

Stage 1. The reactive electrophile, E+, is formed from the reagents

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Stage 2. Arene C=C reacts slowly with the E+ to produce a resonance stabilised carbocation. The slow reaction is due to the resulting loss of aromaticity.

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Stage 3. To restore C=C and the aromatic system, there is a loss of H+ from the carbocation.

*at manual*

For the nitration of benzene, nitric acid is protonated by sulfuric acid, forming a nitronium ion and losing a water molecule. The nitronium ion (NO2+) functions as an electrophile and reacts with benzene to produce nitrobenzene and benzenesulfonic acid (Laali, Kenneth K., and Volkar J. Gettwert. “Electrophilic Nitration of Aromatics in Ionic Liquid Solvents.” The Journal of Organic Chemistry 66 (Dec. 2000): 35-40. American Chemical Society. )

[pic 2]

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The side reactions, which may accompany the formation of m-nitrobenzoic acid are given below:

1) Dinitration to form 3,5-dinitrobenzoic acid (Brewster Williams et al. 1955)

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Dinitration is not favored because benzoic acid is more reactive than m-nitrobenzoic acid.

2) Sulfonation to form m-carboxybenzenesulfonic acid (Carey 2006)

[pic 4]

Nitration is more favored than sulfonation because NO2 is more electrophilic than SO3.

Activated substituents donate electrons such as -OH, -NH2, -Me while deactivating substituents withdraw electrons such as -CF3, -NO2, -CN, -F, -Cl, -Br, -I (Carey 2006)

[pic 5]

VI. Discussion

        In the first part of the experiment, m-nitrobenzoic acid was synthesized and isolated from benzoic acid. Initially, 1.34 g of concentrated colorless nitric acid and 1 mL sulfuric acid were mixed and cooled in an ice bath.Then, 0.61g of white crystalline benzoic acid was added with 2.5 mL colorless concentrated sulfuric acid. Afterwards, the colorless nitric acid solution was added to the benzoic acid mixture of colorless liquid and white solids and took about 30 minutes, forming milky mixture with traces of yellow. The temperature was maintained between 0 degree to 10 degree Celsius by using an ice bath. The mixture was then poured into crushed ice and was stirred. The mixture was allowed to stand for 10 minutes at room temperature, settling the white cloudy solids and suspending  (???) the colorless liquid. The mixture was then decanted and was subjected to suction filtration. The white powdery precipitate was washed with pleanty of cold distilled water. It was then recrystallized by hot gravity filtration.

        Electrophilic aromatic substitution (EAS) was employed in the experiment. Aromatic hydrocarbons favors substitution instead of addition. It is because in addition, the nucleophile attacks the electrophile to form a cyclohexadiene derivative. Meanwhile in substitution, the nucleophile serves as a base and deprotonates an sp3 carbon to reform the double bond and restore the aromaticity (Carey 2006).

        Nitric acid and sulfuric acid were reacted first to generate nitronium ion, a strong electrophile needed for the reaction. Sulfuric acid, stronger acid, protonates nitric acid so the latter can dissociate to form the electrophile. Its effectivity as an electrophile is due to the positive charge of oxygen that can react with double bonds (Iverson 2011). Meanwhile, the sulfuric acid added to the benzoic acid protonates the latter and also  serves as a catalyst for the EAS reaction. Nitronium ion then reacts with protonated benzoic acid, forming a resonance-stabilized arenium carbocation intermediate. The 30-minute period of the dropwise addition is to reduce the side products brought by the reaction and for the conjugate base, hydrogen sulfate ion, then deprotonates the meta-intermediate to form the final product, m-nitrobenzoic acid by reforming the aromatic ring structure(Iverson 2011). The whole reaction is maintained by an ice bath because a cooled aromatic ester dissolved in sulfuric acid that is reacted with nitric acid is highly exothermic (Williamson & Masters XXXX).  

*INSERT REACTION MECHANISM*

        The experimental yield of the white powdery solids was calculated to be 23.46%. ItS melting point was determined to be 130 degree Celsius by the oil bath method. This is significantly lower compared to literature 141 degree Celsius. This may be due to combined recrystallized products of the experimenters. One of the products may be impure due to incomplete reaction of the nitric acid solution to the benzoic acid mixture.

*INSERT SHEMATIC ISOLATION*

        Benzoic acid is a substituted benzene. The electrophilic substitution of this kind is influenced by its substituent effects on reactivity and orientation.

         The substituents of benzene  are classified between activating and deactivating. It controls the reactivity of the benzene ring in a reaction by either increasing or decreasing the rate. This is further affected by the interaction of inductive effect and resonance effect. Inductive effect transmits electronegativity difference through the pi system. Meanwhile, conjugation of substituent function with the aromatic ring (resonance) allows flow of electron through the ring if the atom attached to the latter has lone pairs (Reusch 2013).