BENZENE
Week: Date:
Period: Duration: 1 HR
20 MIN. Average
age of learners: 16YEARS
Subject: CHEMISTRY Class:
SS TWO
Topic: BENZENE
Sub topic: Reference materials:
(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA
(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO
(3) INTERNET
Instructional
materials:
Entry behavior:
The students have been taught terminologies of electrolysis
Behavioural objective: At the end of the lesson the students should be able to:
i. explain aromatic compound and their characteristics
ii. draw the structure of benzene
iii. state the physical properties of benzene
iv. state the similarities with an alkanes
v. mention the uses of benzene
CONTENT
BENZENE
Aromatic
compounds are benzene and compound that resemble benzene in chemical behavior.
They have the same molecular formula CnH2n-6 where n is
equal or greater than 6. Benzene is the parent substance of the aromatic It is a hydrocarbon obtained from the
distillation of coal tar. It is a member of a large family of organic compounds
called aromatic compounds.
The
term aromatic was initially used because these compounds possess certain
characteristic odor (or aroma), but now, it is used to describe compounds which
possess certain common features.
Some
of these features are:
1.
They are cyclic (i.e., ring) compounds.
2.
Their molecules are planar.
3.
They are unsaturated, and contain delocalized electron clouds above and below
the ring.
4.
They obey Huckel’s rule - i.e., there are (4n+2) electrons involved in the
delocalization, where n is the number of rings available.
Therefore,
for benzene (n =1) - there are 6 delocalized electrons in the ring.
5.
They exhibit resonance.
Structure
of Benzene
Benzene
has a molecular formula of C6H6 which indicates it
to be unsaturated.
In
1865, a German chemist, August Kekulé, proposed that the structure of benzene
is a ring called cyclohexatriene. It has a ring structure of six carbon atoms,
each carbon atoms with an attached hydrogen atom.
The
bonding of benzene is considered as a delocalized electron cloud spread out
over the whole ring of benzene.
Physical
Properties of benzene
1.
Benzene is a liquid with a sweet smell (or aroma).
2. It
burns in air with a sooty flame - due to high proportion of unburnt carbon.
3. It
is insoluble in water, but dissolves organic compounds.
4. It
has a boiling point of 80oC and freezes at oC.
5.
its vapour is toxic and carcinogenic.
6. it
is highly inflammable.
Chemical properties of benzene
1. Combustion: Benzene burns in air with a smoky
(luminous) flame. This is due to its high carbon content. (combustion is
incomplete).
2. Substitution
reaction: As a
saturated hydrocarbon benzene undergoes substitution reactions with ethane,
chlorine, bromine, concentrated tetraoxosulphate (vi) and trioxonitrate (v)
acids.
(a) With ethane: under the influence
of heat, pressure, and anhydrous aluminium (iii) chloride catalyst, benzene
reacts with ethene to form ethylbenzene.
C6H6
+ CH2=CH2 à C6H5C2H5
(b) With halogens: In the presence of
catalyst type called “halogen carrier” e.g iron, iron (iii) or aluminium (iii)
chloride, benzene reacts with chlorine or bromine to form halogenobenzene.
C6H6
+ Cl2 à
C6H5Cl
+ HCl
(c ) Nitration: If benzene is
treated with a nitrating mixture, that is mixture of concentrated solutions
of HNO3 and H2SO4
at room temperature, a nitrobenzene is rapidly formed with the evolution of
heat.
d) Sulphonation: when benzene is
reflux with concentrated tetraoxosulphate (vi) acid for several hours, one
hydrogen atom is substituted for by hydrogen trioxosulphate ( iv) group (HSO3-).
The reaction is faster with “Oleum”.
3.
Addition
reaction ( As an unsaturated hydrocarbon)
I.
With
hydrogen: At a temperature of 180oC using finely divided nickel
catalyst. The benzene nucleus is destroyed and cyclohexane is formed.
C6H6
+ 3H2 à
cyclohexane
II.
With
chlorine: Benzene in the presence of direct sunlight or ultraviolet light,
combines slowly with chlorine at room temperature to form
1,2,3,4,5,6-hexachlorocyclorocyclohexane ( benzene hexa chloride)
Uses of benzene
(i).
Benzene is used as a fuel.
(ii).
it is used as organic solvent for fat and oil
(ii).
Phenylethene (styrene) - polymerised into polystyrene (a plastic product).
(iii).
Phenol - polymerised into compounds used in dyes and as disinfectant.
(iv).
Cyclohexane - polymerised to g ive nylon - used in carpet.
PRESENTATION
Step I: The teacher explains aromatic hydrocarbon
Step II: The teacher leads the students to draw the structure of benzene
Step III: The teacher states the physical and chemical properties of benzene.
Step IV: The teacher mentions the uses of benzene
EVALUATION
i. explain aromatic compound and their characteristics
ii. draw the structure of benzene
iii. state the physical properties of benzene
iv. state the similarities with an alkanes
v. mention the uses of benzene
ASSIGNMENT
What are the differences between benzene and alkenes
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