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• Dina L. Lacson

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Name: Dina L. Lacson

Date Performed: 09/20/16

BS Chemistry III

Date Submitted: 09/30/16 Experiment No. 4 SYNTHESIS OF NITROBENZENE FROM BENZENE

I.

DISUSSION Benzene (C6H6) is a colorless liquid. It is carcinogenic, highly flammable and toxic

and exposure to it may cause serious health effects. Nitrobenzene on the other hand, is the simplest aromatic nitro compound, having the molecular formula C6H5NO2 and is used in the manufacture of aniline, benzidine, and other organic chemicals. It has a colorless to pale yellow, oily, highly toxic liquid with the odor of bitter almonds undergoing nitration, halogenation, and sulfonation much more slowly than does benzene. Nitrobenzene is highly toxic and is readily absorbed through the skin so prolonged exposure may cause serious damage to the central nervous system, impair vision, cause liver or kidney damage, anemia and lung irritation. Inhalation and ingestion of this chemical compound can also have dangerous effects. It may be reduced to a variety of compounds, depending on the reaction conditions. Most nitrobenzene produced is reduced to aniline while smaller amounts are converted to azobenzene, hydrazobenzene, and phenylhydroxylamine. Nitrobenzene has been extensively used for making the shoe polishes as it dissolves dyes and penetrate the leather well. It is used as a scent for cheap soaps. It has also been used for making the floor polishes and also been used as an oxidizing agents in organic synthesis. The reaction of alkenes with nitric acid is not a generally useful reaction. Addition of nitric acid to the double bond is accompanied by more or less oxidation. However, benzene is quite stable to most oxidizing agents and its reaction with nitric acid is not a generally useful reaction. Addition of nitric acid to the double bond is accompanied by more or less oxidation. However, benzene is quite stable to most oxidizing agents, and its reaction with nitric acid is an important organic reaction.

O +

N

+

HNO3

H2SO4

O

-

+

H2O

The acid mixture was added in portions so that the temperature does not exceed 50⁰ C. Sulfuric acid is a catalyst, allowing nitration to occur more rapidly and at lower temperatures. After all of the acid has been added, the reaction mixture is cooled and the oily nitrobenzene separated, washed and distilled. Selection of the appropriate nitrating agent and the conditions of reaction is based upon factors such as reactivity of the compound to be nitrated, its solubility in the nitrating medium, and the ease of isolation and purification of the product. The nitro group is an important functional group in aromatic chemistry because it may be converted into many other functional groups. The nitration reaction thus provides a route to many substituted aromatic compounds. Many properties of the nitro group can be interpreted on the basis of a resonance hybrid of two Lewis structures:

In these structures, the O—N—O system is seen to have an allylic anion type of π system. In a mixture of nitric acid and sulfuric acids, an equilibrium is established in which many species are present. One of these species is the nitronium ion, NO 2+, which has been detected by spectroscopic methods. In the mixture of acids, it is produced by a process in which sulfuric acid functions as an acid and nitric acid functions as a base.

(1)

H2SO4

(2)

H2O ONO 2

+

2 H 2SO4

+ + +

HONO 2

H2O NO 2

+

+

H2SO4

H 3O

+

+

HONO 2

H 3O

+

+ +

HSO 4 NO 2

NO 2

+

-

+ + 2 HSO 4 HSO 4

The mechanism is similar to other sulfuric acid catalyzed dehydrations. Sulfuric acid protonates the hydroxyl group of nitric acid, allowing it to leave as water. The structure of nitronium ion is known from spectroscopic measurements. It is related to the isoelectric compound, carbon dioxide. The molecule is linear, and is a powerful electrophilic reagent. O

O N O

-

O

+

+

H

+

N

H2SO 4 O

O

-

+

H O

+

N

+

H2 O

O

H

It reacts directly with benzene to give a pentadienyl cation intermediate. The nitronium ion reacts with benzene to form a sigma complex. Loss of a proton from the sigma complex gives the nitrobenzene. O

H

+

N

4

O

-

+

N

+

N

O

benzene

H SO

O O

O

-

sigma complex

nitronium ion

-

+

H2SO 4

nitrobenzene

(resonance-delocalized)

It can be noted that the reaction occurs on nitrogen rather than oxygen. Reaction at oxygen gives a nitrite compound, R—O—NO. Nitrites are unstable under such strongly acidic conditions and decompose to products containing C—O bond. These oxidation products react further to give highly colored polymeric compounds. The formation of more or less tarry byproducts is usual side reaction in most aromatic nitration reactions. The final product produced after distillation was a pale yellow oil which have the characteristic odor of almonds. The boiling point of nitrobenzene is 210- 211 ⁰C. C6 H6

+

NO 2

+

+

HSO 4

-

C6H5NO 2

+

H2SO4

Due to several errors and some factors such as the temperature, a final product of nitrobenzene was not synthesized. The hot plate used for the experiment has not reached a temperature of 210⁰C, thus a nitrobenzene product was not obtained. The loss of yield might also because of incomplete reactions due to certain factors such as the temperature, or when

the reactants have not completely come into contact with each other, or when the experiment was done without waiting for the mixture to complete its reaction. During isolation and purification process a lot of product was most probably lost especially when washing the nitrobenzene with Na 2CO3 which was done 5 times before the nitrobenzene was neutral in the litmus test. Some of the sample was probably discarded with the Na2CO3. During distillation the mixture was not distilled to dryness, which might have also account for loss of product. There is also the possibility of loss due to nitrobenzene further nitrating to m-dinitrobenzene. If a residue remains in the distilling flask it is probably m-dinitrobenzene which is formed when the temperature of nitration is not carefully controlled. Water is added during the first part to prevent the mixture from nitrating further into m-dinitrobenzene.

II.

CONCLUSION This experiment generally aims to synthesize nitrobenzene from benzene.

Nitrobenzene (C6H5NO2) is the simplest aromatic nitro compound used in the manufacture of aniline, benzidine, and other organic chemicals. Benzene showed neither the typical reactivity nor the usual addition reaction of alkenes. Addition of nitric acid to the double bond is accompanied by more or less oxidation. However benzene is quite stable to most oxidizing agents and its reaction with nitric acid is an important organic reaction. The reaction used for this synthesis was therefore an electrophilic aromatic substitution reaction using an appropriate Lewis acid catalyst. The nitronium ion from concentrated nitric acid reacted with benzene forming nitrobenzene. The synthesis was carried out by adding benzene to a mixture of nitric and sulfuric acid and then the nitrobenzene was isolated and purified by separation from the acid, a series of washing and finally, distillation. There was no final product of nitrobenzene obtained in the synthesis due to failure in heating the sample in a hot plate with a temperature of 210 ⁰C, thus the percent yield of nitrobenzene was not able to be calculated.

III. 

REFERNENCES Adams, Johnson and Wilcox. 1966. Laboratory experiments in organic chemistry. 5th



Edition. The Macmillan Company. pp560. Clugston M. and Fleming R. (2000). Advanced Chemistry (1st ed.). Oxford: Oxford



Publishing. p. 108. Doyle, Michael P.; Mungal, William S., Experimental Organic Chemistry. New York:

John Wiley and Sons, Inc, 1980  Gerald Booth (2007). "Nitro Compounds, Aromatic". In: Ullmann's Encyclopedia of 

Industrial Chemistry. John Wiley & Sons: New York Mcmurry, S. (2008). Fundamentals of Organic Chemistry 6th Edition . Thompson



Corporation. Streitwieser and Heathcock. 1976. Introduction to organic chemistry. Macmillan Publishing Co., Inc. pp 1279.