Science / Preparation Of P-Nitroaniline
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Autor: anton 29 November 2010
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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. Ethanolâ€™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).
â€¢ 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.
â€¢ The addition of sulfuric acid and nitric acid produced a slightly cloudy solution.
â€¢ Once the solution was cooled in an ice bath it became clear and colourless.
â€¢ During the nitration process, the mixture went from a medium brown to a dark brown solution.
â€¢ 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.
â€¢ At the end of refluxing, the yellow precipitate dissolved to give a clear amber solution.
â€¢ Filtrate obtained from vacuum filtration was clear amber solution. The residue was very fine light brown powder.
â€¢ 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.
â€¢ 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 dissolved the solid and produced a clear solution.
â€¢ The mixture turned darker as it was placed on the steaming bath.
â€¢ Water is added and it acted as a nucleating agent. The solution became cloudy with solid particles forming in the centre of the flask.
â€¢ After the final vacuum suction, the end product appeared as tiny fibers shaped as needles that were shiny light yellow brown.
Table 1: Observed Results for p-Nitroaniline end product
Weight of watch glass 28.21g
Weight of watch glass + products 29.95g
Theoretical Yield 1.839g
Actual Yield 1.74g
% Yield Actual yield x 100% = 1.74 g x 100% = 94.62%
Theoretical yield 1.839g
Appearance of final product â€¢ Shiny, light-brown/dark yellow color
â€¢ Precipitate took the form of tiny fibers that look liked needles
Experimentation Boiling point 146 Ñ”C - 150 Ñ”C
The balanced equation:
PhNHCOCH3 + HNO3 + H2SO4  p-Nitroaniline
Table 2: Physical Properties of Compounds Used in Experiment
PhNHCOCH3 HNO3 H2SO4 p-Nitroaniline
Mol Wt (g/mol) 135.16 --- --- 138.12
Concentration --- 16M 18M ---
Amount Used 1.8g 1.6ml 7.0ml ---
Moles 0.01332 mol 0.0256 mol 0.126 mol 0.01332 mol
Limiting reagent In excess In excess Theoretical yield: 1.839g
Calculations for the moles of each reagent used:
PhNHCOCH3 (acetanilide): 1.8 g x 1 mol = 0.01332 mol
HNO3 (nitric acid): 16.0 mol x 0.0016 L = 0.0256 mol
H2SO4 (sulphuric acid): 18.0 mol x 0.007 L = 0.126 mol
Theoretical yield of p-Nitroaniline:
Since PhNHCOCH3 is the limiting reagent:
Mol p-Nitroaniline = mol PhNHCOCH3
Theoretical yield (mass in grams): = moles PhNHCOCH3 x 138.12 g
= 0.01332 mol x 138.12 g
= 1.839 g
Experimental yield of p-Nitroaniline:
Amount of p-Nitroaniline by mass obtained in lab: 1.74g
% yield: Actual yield x 100% = 1.74 g x 100% = 94.62%
Theoretical yield 1.839 g
% difference from theoretical = |1.74 g â€“ 1.839 g| x 100% = 5.38%
The final yield for the preparation of p-Nitroaniline experiment was 1.74g, giving a percentage yield of 94.62%. The theoretical yield obtained through calculations had a value of 1.839g, which shows that the experimental product produced had an acceptable yield. The final product obtained had a shiny, light brown to dark yellow colour. The precipitate took form of tiny fibers that looked like needles. To judge the purity of the product, melting points of the experimental product can be compared to that of the actual product. The experimental melting point obtained was a range of 146 - 150 Ñ”C. Compared to the theoretical melting point of p-Nitroaniline at 149 - 151 Ñ”C, thus the experimental product should have been of great purity (3). However, the final product was not completely shaped as needles which indicate there must have been some impurities present.
The error in experimental purity of the product could be due to side reactions. The substituent NHCOCH3 is an activator that directs an addition to the ortho / para position. Due to steric hinderance at the ortho position, the major product is the addition in the para position. However, a small amount of the ortho product is present that may have caused the experimental product to not be so pure.
A double nitration reaction could have also occurred resulting in the presence of dinitro compounds as a by-product.
Furthermore, human errors likely occurred that could cause the final product to have impurities. For example, product may have been lost during the transferring process. Vacuum filtration, which requires transferring of filtrate or precipitate, thus increasing the possibility of precipitates getting stuck on the side of the suction funnel or being directly filtrated though the filter paper into the filtrate. Time constraints in the laboratory would not allow a full recrystallization even with the use of ethanol and water to facilitate the dissolving and crystallization of the organic products.
To improve yield, one could allow more time for the product to recrystallize, which would ensure that the crystal precipitate obtained in the end is as pure as possible. Thorough washing of the glassware during the transferring of reactants and decreasing the amount of precipitate left behind would also be ways to improve yield. Extreme caution needs to be demonstrated to ensure no water gets into the nitration system, and precise amount of concentrated sulfuric acid is used to synthesize the nitronium ion. The absence of water ensures that the nitronium ion is not oxidized, so that is can act as an electrophile.
Para-Nitroaniline is commonly used as an intermediate in the production of dyes, such as Para Red, the first azo dye. In addition to dyes, it is also used as an intermediate in antioxidants, pharmaceuticals, gasoline, and as a corrosion inhibitor (4). Extreme caution should be taken with this product as it is toxic via inhalation, ingestion and absorption (4).
In summary, the experimental results were acceptable. The experimental yield was 1.74g, which was close to the theoretical yield of 1.839g. These values give a percentage yield of 94.62%, which is a very good value. The product is not of extreme purity due to some of the by-products that were created. The experimental melting point was measured to be 146 - 150Ñ”C, which is also close to the theoretical melting point of 149 - 151Ñ”C (3).
The synthesis of the nitronium ion is crucial in this experiment because it acts as the electrophile and carries out the nitration process. Nitration is the chemical addition of a â€“NO2 group on an organic compound. Therefore, nitric acid must be present, along with a concentrated acid such as sulfuric acid to ensure the nitronium ion will be created. These conditions all affect the nitration, and as a result affect the synthesis of the p-Nitroaniline.
1. Schore, Neil E.; Vollhardt, K. Peter C. Organic Chemistry: Structure and Function, 3rd edition; W.H. Freeman and Company: 2007.
3. David R Lide, ed, CRC Handbook of Chemistry and Physics; Internet Version 2007-8, (88th Edition), http://www.hbcpnetbase.com accessed January 30, 2008).
4. Aoyama, Y; Caira, M.R.; Desiraju, G.R. and Glusker, J.P. Design of Organic Solids; Springer: 1998.
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