Preparation of P-Nitroaniline

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Preparation of P-Nitroaniline

By | Feb. 2008
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Introduction
An electrophile is a reagent attracted to electrons and accepts an electron pair in order to bond to a nucleophile. Electrophiles will attack benzene and result in hydrogen substitution. However, this is not thermodynamically favoured because a sp3 hybridized carbon is generated, which disrupts the cyclic conjugation. In order to regenerate the aromatic ring, a proton is lost at the sp3 hybridized carbon. Thus, p-Nitroaniline can be prepared by means of electrophilic aromatic substitution.

To begin, nitric acid needs to be activated as it has little electrophilic power. Thus, concentrated sulfuric acid is added to protonate the nitric acid. Dehydration produces the nitronium ion, which is a strong electrophile and has most of its positive charge on the nitrogen atom. The nitronium ion, acting as the electrophile in the nitration reaction, will attack the benzene ring and produces the nitrobenzene ring. Resonance-stabilized arenium ion is created when the nucleophilic carbon pi bond on the acetanilide reacts with the nitronium ion. The aromaticity of the compound is lost in this important step.

Water, acting as the Lewis base, removes a proton from the carbon bonded to the nitro group, an aromatic benzyl structure is created again: nitroacetanilide; thus ending the nitration reaction (1). Next, an acid-catalyzed hydrolysis reaction occurs to remove the acyl group. The nitroacetanilide receives a proton from the sulfuric acid and the double bonded oxygen becomes positively charged. A sp3 hybridized carbon compound is formed once the water attacks the carbon bearing the oxygen. Numerous proton transfers occur once the mixture is placed under heat. Dissociation occurs of the sp3 hybridized carbon compound and HO2CCH3 and the substituted benzene structure are formed. Next, ammonia hydroxide is added to basify the mixture. The final product is p-Nitroaniline.

In order to purify the product, recrystallization is performed after hydrolysis. The...

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