Experiment # 5:

Preparation of Nitro-

Benzene

Date: 23-11-10

Muhammad Usman

(2008-chem-02-B)

1. Benzene:

Carcinogenic: Benzene exposure has serious health effects. The American Petroleum Institute

(API) stated in 1948 that "it is generally considered that the only absolutely

safe concentration for benzene is zero. The US Department of Health and

Human Services (DHHS) classifies benzene as a human carcinogen.

Highly flammable: It has a flash point of -11

oC. It can readily catch fire. So care should be taken

while handling it.

Toxic:

The short term breathing of high levels of benzene can result in death.

Properties of Benzene:

IUPAC Name Benzene

Other names Benzol

cyclohexa-1,3,5-triene

Appearance Colorless liquid

Structure

Molecular formula C6H6

Molar mass 78.11 g mol-1

Density 0.8765 g cm-3

Melting point 5.5 oC

Boiling point 80.1 oC

Solubility in water 1.8 g/L at 15 oC

Flash point -11 oC

2. Nitric acid:

Toxic:

Fumes from concentrated nitric acid are very damaging if inhaled

Corrosive:

Concentrated and moderately concentrated solutions are very corrosive

and can cause serious skin damage

Oxidant:

Contact with the eyes can cause serious long-term damage.

Properties of Nitric acid:

IUPAC Name Nitric acid

Other names

Oxoazinic acid

Aqua fortis

Salpetre acid

Spirit of nitre

Appearance Colorless liquid

Structure

Molecular formula HNO3

Molar mass 63.012 g mol-1

Density 1.5129 g cm-3

Melting point -42 oC

Boiling point 83 oC

Solubility in water Miscible

Acidity (Pka) -1.4

Flash point Non-flammable

Sulphuric acid:

Corrosive:

It is a strongly corrosive compound as it readily oxidizes anything.

Dangerous to Environment:

Although sulfuric acid is non-flammable, contact with metals in the event of a

spillage can lead to the liberation of hydrogen gas. The dispersal of acid aerosols

and gaseous sulfur dioxide is an additional hazard of fires involving sulfuric acid.

Very toxic:

Sulfuric acid is not considered toxic besides its obvious corrosive hazard, and the

main occupational risks are skin contact leading to burns and the inhalation of

aerosols. Exposure to aerosols at high concentrations leads to immediate and

severe irritation of the eyes, respiratory tract and mucous membranes.

Properties of sulfuric acid:

IUPAC name Sulfuric acid

Other names Oil of vitriol

Structure

Molecular formula H2SO4

Molar mass 98.06 g/gmol

Appearance

Density 1.84 g/cm3 (liquid)

Melting point 10 oC

Boiling point 337 oC

Solubility in water Miscible

Acidity (Pka) -3

Viscosity 26.7 cP (20 oC)

Flash point Non- flammable

Water:

It is a non-hazardous compound.

Environmentally safe:

Water is an essential part of life. Life cannot be sustained without water.

Food & eatables contain some amount of water in them. It is a non-

hazardous compound.

IUPAC name Water

Oxidane

Other names

Hydrogen oxide

Dihydrogen monoxide

Hydrogen monoxide

Hydroxylic acid

Hydrogen hydroxide

R-718

Oxygen dihydride

Oxygen hydride

Oxane

Structure

Molecular formula H2O

Molar mass 18.0152833 gmol-1

Appearance

Density

1000 kg m-3

(liquid)

917 kg m-3

(solid)

Melting point 0 oC

Boiling point 99.98 oC

Acidity (Pka) 15.74

Basicity (Pkb) 15.74

Viscosity 0.001 Pa.S (20 oC)

Preparation of Nitro-Benzene

Chemicals required:

1) Nitric acid 2) Benzene

3) Concentrated H2SO4 4) Cold Water

Chemical reaction:

Apparatus Required:

Bolt head flask Dropping funnel

Knee tube Water condenser

Adopter Conical flask beaker

Stirrer

Procedure:

1. Place 35 cm3 of concentrated nitric acid in a 500 cm

3 round bottomed flask, and add slowly 40

cm3 of concentrated sulfuric acid, keeping the mixture cool during the addition by immersing the

flask in cold water.

2. Place a thermometer in this nitrating mixture, and then add very slowly 29 cm3 of benzene. This

should be added about 3 cm3 at a time, and the contents of the flask mixed after each addition.

The temperature of the mixture must not be allowed to rise above 50oC, and should be kept under

control if necessary by immersing the flask in cold water.

3. When all the benzene has been added, fit a reflux water condenser to the flask, and place

the latter in a water bath, which is then maintained at 60oC for 45 minutes. During this

period the flask should be withdrawn from the bath from time to time and vigorously

shaken in order to break up the nitrobenzene layer which would otherwise float on the

dense acid layer below.

4. After completion of heating pour the contents of the flask into a large excess of cold water (about

300 cm3) in which the nitrobenzene sinks to the bottom. Stir the mixture as vigorously as

possible. Decant off as much as possible of the supernatant aqueous layer, and transfer the

residual liquid to a separating funnel. Run off and retain the lower nitrobenzene layer, and discard

the upper aqueous layer. Return the nitrobenzene to the funnel, and shake it vigorously with an

equal volume of cold water. Allow the nitrobenzene to separate and run it off as before. Return it

to the funnel, and wash with an equal volume of dilute sodium carbonate solution, releasing the

pressure as necessary. Repeat the washing with fresh sodium carbonate solution until there is no

evolution of carbon dioxide.

5. Separate the nitrobenzene from the sodium carbonate solution a completely as possible

(see below), transfer it to a small flask, and add some anhydrous granular calcium

chloride. Shake until the liquid is completely clear.

6. Filter the nitrobenzene through a fluted paper directly into a 60cm3 distilling flask fitted with an

air condenser. Distil the nitrobenzene carefully, collecting the fraction that boils between 207oC

and 211oC.

Uses of Nitro-Benzene:

Aniline production:

Approximately 95% Nitro-benzene is used in aniline production.

Specialized applications:

1. As a precursor to Rubber chemicals, Pesticides, Dyes, Explosives, Pharmaceuticals

2. Shoe and floor polishes, leather dressings, paint solvents, and other materials to mask

unpleasant odors.

3. Used as an inexpensive perfume for soaps

Industrial preparation of Nitro-Benzene:

Industrially Nitro-Benzene is prepared from following procedure.

Feed tanks:

Raw materials are stored in feed tanks from where they can be supplied individually.

H2SO4 supply:

Hot Sulfuric acid at 90oC in run from the heat insulated storage tank (B).

A1-A

4 units:

These are the battery of the nitrators. Here reactants are mixed together and nitration reaction takes place.

Nitric acid:

Under vigorous agitation, sufficient 63% HNO3 is added to the nitrators to produce a mixed acid

containing 4 % HNO3.

Benzene supply:

Sufficient benzene is then delivered from its storage scale tank to react with all the nitric acid in the

nitrators.

Time required for reaction......................................................................10 minutes

Upon Completion of reaction, agitation is stopped & charge permitted to settle. While the separation of

the nitrobenzene and spent acid proceed, another nitration is started, thus providing a continuity of

process.

Neutralizer:

The crude nitrobenzene is drawn off through the side outlet (C) on the nitrators & sent to the neutralizer.

Spent acid recovery:

Acid heater:

The spent acid has

HNO3 content.............................................0%

Nitrobenzene..............................................small quantities

From outlets D1-D

4 spent acid is drawn off. Now it is directed towards the acid Heater (E). It is heated by

the low pressure steam to maintain the sensible heat of the spent acid (72% H2SO4).

Vapor separator:

The evaporator, operating under a vacuum of 29 in. , effectively removes the water of nitration by virtue

of sensible heat of the spent acid and returns the sulfuric acid to the system at its original 75% strength.

The small quantities of nitrobenzene present in water in the condenser is separated & added to the crude

charge.

References:

Unit processes in organic synthesis by P.H. GROGGINS

Fifth edition, page 112-113