Chemistry

UNIT 4: BENZENE
Key unit competence:
To be able to relate the chemistry and uses of benzene to its nature and structure
Learning objectives
At the end of this unit , students will be able to:
• State the physical properties of benzene;
• Describe the uses of benzene;
• Outtline the preparations of benzene;
• Describe the chemical properties of benzene;
• State the conditions required for different reactions;
• Relate the conditions for the reactions of benzene to its chemical stability;
• Illutrate the mechanism of electrophilic substitutions on benzene.

Introductory Activity
From your prior studies in organic chemistry, it is known that carbon is tetravalent
while hydrogen is monovalent and compounds constituted by the two elements
are known as ‘hydrocarbons’. The structures and chemistry of the hydrocarbons
reflects to their uses as fuels and starting materials for many substances important
in life such as pharmaceutical drugs, solvents, packaging materials, clothes and
so on. In this activity you need to follow instructions given to explain how the
structure of a substance determines its chemical properties and uses.
1. Write down the molecular formulae for these five hydrocarbons
a. A molecule with 6 carbon atoms and 14 hydrogen atoms
b. A molecule with 6 carbon atoms and 12 hydrogen atoms
c. A molecule with 6 carbon atoms and 10 hydrogen atoms
d. A molecule of 6 carbon atoms with 8 hydrogen atoms
e. A molecule of 6 carbon atoms with 6 hydrogen atoms
2. From the molecules in 1) above, choose molecule(s) that fit(s) in the
description provided, and then draw its (their) structural formula (e).
a. Unsaturated hydrocarbon (s) that decolorize (s) bromine water and
alkaline potassium manganate (VII)

b. Saturated hydrocarbon (s)

c. Hydrocarbon (s) with empirical formula of CH
d. Unsaturated hydrocarbon (s) which do (es) not decolorize bromine
water and potassium manganate (VII).
e. Unsaturated hydrocarbon (s) that form(s) a white precipitate when
treated with ammoniacal silver nitrate and forms a reddish-brown
precipitate when treated with ammoniacal copper (I) chloride.
3. It is known that unsaturated hydrocarbons decolourise both bromine
water and alkaline potassium manganate (VII). Explain any assumption
you can suggest about the compound in question 2.d)

Some or all people are unique in their living attitudes and values. But being unique
does not mean to be isolated from others as people need each other in order to
complement and build a strong nation.

This is true for benzene. From the above activity question 3 you may have been stuck
while discussing on why this unsaturated compound has properties that are different
from other unsaturated hydrocarbons provided within the same activity. But this
does not mean that it is quite different from them. It will share some properties with
others but exhibit its identity or its unique properties from others.

In this unit, you will discover what makes benzene resistant towards some reactants
and its importance will be highlighted.

4.1. Structure of benzene
Activity 4.1
• Research in books or search engine about the structure of benzene.
• Read and make a summary on the historical development of benzene’s
structure.
Michael Faraday was the first to isolate benzene from coal. Benzene was found to
have the molecular formula of C₆H_6. However, its structural formula posed a problem
for many years.
For example, you can work out the structures of compounds whose molecular
formula is C₆H₆ and see how many you can find.

The structure of benzene must be only one, in which all the six hydrogen atoms
occupy equivalent positions. This was discovered by Friedrich August Kékulé Von
Stradonitz while daydreaming of a snake seizing its own tail. From this, he proposed a
ring structure of six carbon atoms with double bonds alternating with single bonds.

Furthermore, X-ray diffraction studies, first carried out by Kathleen Lonsdale, showed
that benzene is planar and all its C-C bonds are of the same length (0.139 nm which is
intermediate of C-C single bond and C=C double bond in alkenes) and bond angles
of the same size (120^o).

By comparing benzene with alkenes, the following points are noticed:

• Benzene fixes 3 moles of hydrogen, thus it has 3 double bonds,
• Benzene does not decolourise bromine water or acidified potassium
manganate (VII) and does not turn green the acidified potassium dichromate,
• Benzene does not react with water and hydracids under normal conditions.

From the above points it can be easily noticed that benzene is not quite an alkene,
due to its double bonds which do not occupy fixed positions. This change of positions
of the double bonds is referred to as ‘resonance’.

The sp² hybridized orbitals of carbon is involved in sigma bond formation with other
two carbon atoms and one hydrogen atom to make a hexagonal ring. The remaining
unhybrid p-orbital is involved in side-ways overlapping with a neighbor carbon
atom to form a pi-bond. Since there is an equal probability of making the pi-bond
with either neighbor carbon atom, pi-electron remains delocalized over six carbon
atoms of the ring.

Checking up 4.1
Discuss and provide appropriate answers for the following questions:
1. a. Benzene has the molecular formula C₆H₆. Draw the Kekulé structure for
this showing all the atoms.
b. Draw the skeletal structure showing the way the Kekulé structure is normally
drawn.
2. How does the structure of benzene differ from the cyclohexane structure?

34.2. Physical properties, uses and toxicity of benzene
Activity 4.2
• Using the same resources (books or internet) as in activity 4.1, make a
research about the main points that should be talked about while discussing
the physical properties of any substance.
• Then, make a summary to be presented about properties, uses and toxicity
of benzene.

Benzene has the following physical properties:
• Benzene is a colourless volatile liquid with an aromatic (pleasant/sweet) smell.
• Benzene boils at 80.1 °C
• Benzene melts at 5.5°C.
• Like other aromatic hydrocarbons (arenes) benzene is insoluble in water.
• It is less dense than water (specific gravity or relative density is 0.88).. Describe the Structure and Bonding of Benzene

Benzene has many uses:
It has been used by chemists since 1800 because it is a good solvent for other organic
compounds. Benzene itself is an excellent solvent for certain elements, such as
sulphur, phosphorus, and iodine. It is found in crude oil. It is used to make plastics,
resins, synthetic fibers, rubber, lubricants, dyes, detergents, drugs and pesticides.

Benzene is highly toxic and is said to be carcinogenic.
A person exposed for long time to benzene (even at low levels), can develop anaemia
and leukaemia.

Benzene is formed in both natural and synthetic processes. Natural sources of
benzene include volcanoes and forest fires. It is a component of crude oil, petrol and
cigarette smoke.

Checking up 4.2
1. Benzene is flammable and carcinogenic. What do you understand by
the term “carcinogenic”?
2. What advice can you give to your friend who smokes?

4.3. Preparation of benzene
Activity 4.3
• Some of the reaction of all alkanes and alkynes discussed in senior five lead
to the formation of benzene. Use the following examples to describe how
each of the following conversions can be carried out
1. From CH CH to C₆H₆
2. C6H6 from n-hexane
3. Ethanol to C₆H₆
• To add other methods used to prepare benzene and to be able to describe
them, use the same sources (books/search engines) as in previous activities
to discuss about all the methods that can be used to obtain benzene.
• Take a note to share with others.
• Some of the reactions of alkanes and alkynes discussed in senior five lead to
the formation

All the raw materials provided in the activity above are from the topics covered
in senior five, so hopefully you performed them very well. The methods used for
preparing benzene are based on reduction reaction and decomposition reaction and
even addition reaction.

1. Industrial preparation (on large scale)
a. From petroleum oils: By catalytic reformation of petroleum products
By fractional distillation followed by reforming. Fraction of naphtha is
heated over Cr₂O₃ – Al₂O₃ at 500-550^oC and 15atm pressure (aromatisation).

When platinum is used at 15 atm pressure at 500oC, the process is called
‘platforming’.

b. By converting methylbenzene into benzene
Methylbenzene is much less commercially valuable than benzene. The
methyl group can be removed from the ring by a process known as
“demethylation”.
The methylbenzene is mixed with hydrogen at a temperature of between 550 and

650°C, and a pressure between 30 and 50 atmospheres, with a mixture of silicon

dioxide and aluminium oxide as catalyst.

c. From ethyne
When ethyne is heated in the presence of iron as catalyst or organo-Nickel, it undergoes
cyclization.

2. Laboratory preparation
a. From benzoic acid
In this method benzoic acid is heated with soda lime.

b. From benzenediazonium salt
In this method, the benzenediazonium salt formed by reacting phenylamine
with sodium nitrite and a mineral acid is treated with hyposphorous acid
(H₃PO₂) and water.

d. From cyclohexane
When cyclohexane is heated with Palladium or Platinum as catalyst and with
sulfur, it undergoes dehydrogenation forming benzene. When cyclohexane
is heated with sulphur, benzene is also produced.

Checking up 4.3
Discuss and describe how you can obtain benzene starting with inorganic
reagents, showing necessary conditions at every step.

following stages.

can present about the stability of benzene.

hexagonal ring with three pi-bonds in an alternate manner.

replaced by an electrophile.

hexagonal ring of carbon atoms as shown below:

electrophilic substitution reactions.

structure).

The enthalpy of hydrogenation of cyclohexane is -120 kJ/mol.

expected heat of hydrogenation is 3 times i.e. 3 x (-120) kJ/mol = -360 kJ/mol.

resonance energy.

Note: Because of this extra stability, benzene:

•    Does not undergo reactions with halogens and halogen acids which are characteristic of alkene,

•    Does not react with water in the presence of H+ and does not react with acidified KMnO4

•    Cannot be represented by these structures because of its inertness

•    Under drastic conditions, it however reacts with Cl2 or Br2 in the presence of ultraviolet light/light or halogen carrier,

•    Reacts so fast with oxygen, by producing yellow luminous flame which is sooty.

Checking up 4.4

Refer to your results from the activity 4.4 to discuss and conclude on this:
Do your results support that real benzene is more or less stable than the Kekule structure? Explain your answer.

4.5. Reactions of Benzene

Activity 4.5 From the previous topics discussed in this unit, you have found that benzene has some uniqueness from aliphatic unsaturated compounds.
Use the same resources to find out
•    How benzene reacts and

•    Its reactions with different substances and their respective mechanisms

As seen in the previous discussions, since benzene contains carbon-carbon double bonds, it might be expected to undergo electrophilic addition reactions readily as it is the case for alkenes. This is not the case, however, and benzene does not decolourise bromine water. Neither does it readily undergo any other addition reactions.

The reason for this is that the delocalized system in benzene is stable, and addition reactions would break up this delocalization and lead to the formation of the products which are less stable than benzene itself. Benzene thus tends to undergo electrophilic substitution reactions rather than addition reactions.

4.5.1. Electrophilic aromatic substitution reactions
Aromatic compounds undergo substitution reactions with electrophiles in which one or more hydrogens of the benzene ring are substituted.

Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as “Electrophilic Aromatic Substitution”. The catalysts and co-reagents serve to generate the strong electrophilic species needed to perform the initial step of the substitution.

Many substitution reactions of benzene have been observed and the five most useful are listed below.

The specific electrophile in each type of reaction is listed in the right hand column.

All electrophilic substitution reactions of benzene follow the same mechanism. After the formation of the electrophile, a two-step mechanism has been proposed for these electrophilic substitution reactions.

In the first, slow or rate-determining step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate. In the second, fast step, a proton is removed from this intermediate, yielding a substituted benzene ring.i

Briefly, electrophilic aromatic substitution reaction is realised in 3 steps:

1. Electrophile formation

2. Attack of the ring by electrophiles

3. Deprotonation = loss of H+

1. Halogenation Benzene reacts with chlorine or bromine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a chlorine or bromine atom.

•    The reactions happen at room temperature.

•    The catalyst has to be a Lewis acid known as “halogen carrier”. The most commonly used catalysts are: aluminium (or iron) chloride, AlCl3/ FeCl3 or aluminium (or iron) bromide, AlBr3/ FeBr3 if you are reacting benzene with bromine.
   
Example: The reaction with chlorine ( Chlorination)
The reaction between benzene and chlorine in the presence of either aluminium chloride or iron gives chlorobenzene.

2. Friedel-craft-acylation Acylation involves substituting an acyl group, RCO-, into a benzene ring.
145Chemistry Senior Six Student Book
The most reactive substance containing an acyl group is an acyl chloride (also is known as an acid chloride). These have the general formula of RCOCl.

2. Friedel-craft-acylation Acylation involves substituting an acyl group, RCO-, into a benzene ring.
145Chemistry Senior Six Student Book
The most reactive substance containing an acyl group is an acyl chloride (also is known as an acid chloride). These have the general formula of RCOCl.

3. Friedel-Crafts Alkylation

This reaction involves substituting an alkyl group into a benzene ring. Hydrogen on the ring is replaced by a group like methyl or ethyl and so on.

a. Using haloalkanes

Benzene reacts with chloroalkanes in the presence of anhydrous AlCl3  or FeCl3  as a catalyst under reflux at 50oC to form alkylbenzenes

b. Using alkenes

Alkylbenzenes other than methylbenzene can be formed by reacting benzene with alkenes in the presence of HCl and AlCl3, under reflux at temperatures below 50oC. Mechanism:
Step 1: The alkene reacts with the HCl in the same way as in electrophilic addition reactions:

The carbocation behaves as the electrophile.
Step 2 and Step 3 proceed in the same way as in the alkylation reaction described above.
The overall reaction can be written as follows:

The more stable cation gives the major product, methylethylbenzene (or isopropylbenzene).

5. Nitration

Nitration happens when one (or more) of the hydrogen atoms on the benzene ring is replaced by a nitro group, -NO2. Benzene is treated with a 50:50 mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature not exceeding 50°C. The mixture is held at this temperature for about half an hour. Yellow oily nitrobenzene is formed.

C6H6 + HNO3 → C6H5NO2 + H2O

The concentrated sulphuric acid is acting as a catalyst and so is not written into the equations.
Mechanism:
Step 1: Nitric acid is a less strong acid than sulphuric acid, and acts as a base as the electrophile is formed.  

         H2SO4 + HNO3→  H2O + NO2+ + HSO4

Step 2: The NO2+ is the electrophile and attacks the delocalised ring, breaking it temporarily:

Step 3: The delocalised system then reforms itself by pulling in the electrons from the C-H bond. The H+ recombines with the HSO4- to form H2SO4.

The overall reaction is C6H6 + HNO3→ C6H5NO2 + H2O

The sulphuric acid behaves as a catalyst. The product is known as nitrobenzene.
4.5.2. Some addition reactions and combustion reaction

The benzene ring can undergo addition reaction under drastic conditions, breaking down its resonance

3. Combustion reaction

As other hydrocarbons, benzene burns in air forming carbon dioxide (or carbon monoxide in a limited supply of air) and water.

Checking up 4.5

Discuss and find out the answers for the following questions: Benzene can be nitrated to form nitrobenzene, C6H5NO2.
a. Draw the structural formula for benzene and give its empirical formula
b. State the reagents needed for the nitration of benzene
c. An electrophile is formed during the nitration of benzene

i. What is the formula of this electrophile?

ii. Write an equation for the production of the electrophile iii. Use curly arrows to show the mechanism for the nitration of benzene

C6H6(l) + 15/2 O2(g) → 6 CO2(g) + 3 H2O(l)  or  C6H6(l) + 9/2O2(g) → 6 CO(g) + 3 H2O(l)

Checking up 4.5 Discuss and find out the answers for the following questions: Benzene can be nitrated to form nitrobenzene, C6H5NO2.
a. Draw the structural formula for benzene and give its empirical formula
b. State the reagents needed for the nitration of benzene
c. An electrophile is formed during the nitration of benzene

i. What is the formula of this electrophile?

ii. Write an equation for the production of the electrophile

iii. Use curly arrows to show the mechanism for the nitration of benzene

4.6. Nomenclature and positional isomerism in derivatives of benzene
Activity 4.6 1. Name  the following molecules:

a. CH3CH2CH(CH3)CH3

b. ClCH2CH2CHOHCH3

c. CH3CH(C6H5)CH2CH2CH3

d. C6H5NO2

e. C6H4ClBr

2. Discuss about rules for naming aromatic compounds in this book or any other source (textbook or internet). Then, make a summary to be presented.

As you have seen from the previous lessons of this unit, benzene and its derivatives are referred to as aromatic compounds. The following diagram provides the structures of some aromatic compounds starting with benzene with one ring and then others with more than one ring and their respective names:

Some benzene derivatives have their traditional or popular names such as the following:

•    Di-substituted benzene derivatives with the prefixes “Ortho- or o-” for substituent groups on adjacent carbons (e.g, C1 and C2) in benzene ring. “Meta- or m-” for substituents separated by one carbon atom (e.g, C1 and C3). “Para- or p-” for substituent groups on carbons on opposite sides of ring (e.g, C1 and C4). The positions on the benzene ring are as follows:

Benzene derivatives consisting of two substituents attached to the ring could be distinguished among three positional isomers (ortho- , meta- and para- isomers).
These are named either by numbers or by using non numerical prefixes (ortho, meta and para).
Notice that there are 2 identical ortho positions (2, 6), and 2 identical meta positions (3,5).

Checking up 4.6

Discuss and provide appropriate answers to the following questions:

1. You are provided with C6H4Br2. Give three different structural formulae of isomers of C6H4Br2 and name them.

2. Provide all the structures and names of compounds having the same molecular formula as C6H5NO3.