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 solvent used is an ethanol-water mixture. Ethanol's strong dissolving abilities ensure that unwanted organic mixtures may be dissolved and water can reduce the solubility of the product to encourage recrystallization. Water is crucial to the nitration process. The nitronium ion is caused to oxidize, thus inhibiting its ability to act as the electrophile in the reactions following. This is due to the presence of water. Therefore, it is essential no water gets into the system at this point.
The mechanism for this reaction is given on the following pages.
Materials and Method
The procedure for this experiment is outlined in the Chem. 267L manual, under experiment #2, with no changes (2).
The addition of sulfuric acid to dissolve acetanilide produced a light brown solution with brown solid flakes. The cooling of this mixture produced no real change, with only a few more of the flakes dissolved. The addition of sulfuric acid and nitric acid produced a slightly cloudy solution. Once the solution was cooled in an ice bath it became clear and colourless. During the nitration process, the mixture went from a medium brown to a dark brown solution. As 25mL of ice water was added and stirred, the mixture turned yellow and a heavy yellow precipitate formed. Heat was given off; thus an exothermic reaction. At the end of refluxing, the yellow precipitate dissolved to give a clear amber solution. Filtrate obtained from vacuum filtration was clear amber solution. The residue was very fine light brown powder. As concentrated ammonia hydroxide was added to basify, the solution became yellow-orange and cloudy. As well, yellow-orange flakes formed on the surface. Heat was given off and smoke was seen rising from the beaker. The pH of the solution was 8 and the pH paper was dark green. After the second filtration, the filtrate was a clear orange solution and the residue was a bright yellow-mustard crystalline powder. Ethanol was added and this...