MUST CHEM 122-05,3- Reactions of aromatic hydrocarbons.ppt

Lecture 5: Introduction to Organic Chemistry – A Comprehensive Guide for Undergraduate Students
This PowerPoint presentation, Lecture 5: Introduction to Organic Chemistry, is an essential resource designed for undergraduate students pursuing chemistry, biology, pharmacy, and other science-related disciplines. It provides a structured and detailed introduction to the fundamental concepts of organic chemistry, ensuring a solid foundation for further studies in the field.

Organic chemistry is a crucial branch of chemistry that focuses on the structure, properties, composition, reactions, and synthesis of carbon-containing compounds. It plays a pivotal role in various scientific fields, including medicine, pharmaceuticals, biochemistry, environmental science, and materials science. This lecture presentation covers the core principles necessary to understand organic molecules and their transformations, making it an invaluable tool for students and educators alike.

Key Topics Covered in this Lecture:
1. Introduction to Organic Chemistry
Definition and significance of organic chemistry in the scientific world.
Historical development of the field and major contributors.
The distinction between organic and inorganic compounds.
2. Structure and Bonding in Organic Compounds
Atomic structure and electronic configuration.
Covalent bonding and hybridization (sp, sp², sp³).
Molecular orbital theory and resonance structures.
The concept of electronegativity and polarity in organic molecules.
3. Functional Groups and Their Importance
Overview of key functional groups: alkanes, alkenes, alkynes, alcohols, ethers, aldehydes, ketones, carboxylic acids, esters, amines, amides, and aromatic compounds.
How functional groups determine the chemical properties and reactivity of organic molecules.
The role of functional groups in biological and industrial applications.
4. Nomenclature of Organic Compounds
Systematic IUPAC naming conventions for aliphatic and aromatic compounds.
Common names vs. IUPAC names.
Rules for naming branched-chain hydrocarbons, cyclic compounds, and complex organic molecules.
5. Isomerism: The Different Forms of Organic Molecules
Structural isomerism: Chain, positional, and functional group isomers.
Stereoisomerism: Geometrical (cis-trans) and optical isomerism.
Chirality and enantiomers in biological systems.
6. Reaction Mechanisms in Organic Chemistry
Introduction to reaction mechanisms and their importance in predicting product formation.
Classification of organic reactions:
Substitution reactions (SN1, SN2)
Addition reactions (electrophilic and nucleophilic addition)
Elimination reactions (E1, E2 mechanisms)
Rearrangement reactions
Oxidation and reduction reactions in organic synthesis
The role of catalysts and reaction conditions in determining reaction outcomes.
7. Organic Chemistry in Everyday Life
The impact of organic chemistry on medicine and drug development.
The significance of polymers and plastics in modern materials.

1. REACTIONS OF AROMATIC REACTIONS OF AROMATIC
HYDROCARBONSHYDROCARBONS
Lecturer:
Chemistry Section
MUSTFebruary, 2017

2. Reactivity of ArenesReactivity of Arenes
•Chemical formula of benzene C
6
H
6
shows that
benzene has a low carbon to hydrogen ratio.
•One would expect benzene to be unsaturated
and undergo addition reactions like alkenes
•However most reactions of benzene are
typically substitution reactions. They are known
as electrophilic substitution reactions.
•Why? Lets understand the structure of benzene
first!

3. Structure of benzeneStructure of benzene
•Benzene has the formula C
6H
6. This formula suggests
that this is unsaturated compound. Hence we expect
structure of benzene to have either double or triple
bonds.
•In 1865, Kekulè proposed that benzene contains six
carbon atoms with a series of alternating carbon-
carbon single and double bonds which were in rapid
oscillation.

4. Problems with proposed Kekulè Problems with proposed Kekulè
ModelModel
•With the presence of the double bonds, you would expect
benzene to undergo addition reactions like other unsaturated
compounds.
•However it was found out that most reactions of benzene
involved substitution and addition reactions could not occur
very easily.
•As benzene contains single and double bonds you would
expect it to give two values of bond lengths when measured,
one for the single bond and the other for the double bond.

5. •However upon measuring, benzene gave only one value
indicating that all bonds are the same.
•Calculating the enthalpy change of formation of benzene
from its elements, the value is +252kJ/mol. But measured
enthalpy is +82kJ/mol.
•Why the difference? From the values it indicated that the
real structure of benzene was more energetically stable
than the proposed model.
•Hence one would say that the real structure is not the
one suggested by Kekulè.

6. Benzene Structure proposed by Benzene Structure proposed by
Linus PaulingLinus Pauling
•In 1931, Pauling suggested that real benzene
structure is half way between the two structures
proposed by Kekulè.
•This structure is called a resonance hybrid.
–This structure explained the observed bond lengths and lack
of reactivity.
–It further explained why the structure is thermodynamically
stable, since resonance hybrid of the two structures will have
lower energy.
•This resonance hybrid is represented by drawing a
circle inside around the ring

7. Benzene Structure todayBenzene Structure today
•Benzene is a planar molecule with all carbon-carbon bonds
identical.
•Each carbon in the benzene is sp
2
hybridised and each bond
angle is 120 hence planar shape
•Each carbon forms three sigma () bonds, one with hydrogen
atom and the other two, one with each of the two carbon atoms
attached to it.
•Each carbon atom has one electron in the unused p orbital.
Since there are six carbon atoms, you have a total of six p
orbitals.
p orbital

8. Benzene Structure today……Benzene Structure today……
•The p orbitals overlap sideways to share the electrons resulting in
the formation of pi () bonds all over the entire ring. And with
these pi bonds electrons are free to move around the entire ring.
–This sharing of electrons all over the ring is called delocalisation.
Hence the electrons that are free to move are called delocalised
electrons.
–This delocalisation makes all bonds similar and is responsible for
the unexpected stability of the benzene molecule
–This stability is due to bonding that consists of sigma bonds and
delocalised pi bond, which extends all over the ring producing a
ring of high electron density around the molecule.
•The resulting bonds are intermediates between single and double
bonds.
•Both high electron density and unusual stability is known as
aromatic stability.

9. Benzene Structure today……Benzene Structure today……
Delocalised pi system
•Hence aromatic hydrocarbons are hydrocarbons that
have a stabilised ring of delocalised pi bonds/ electrons.

10. Why electrophilic Substitution Why electrophilic Substitution
reactionsreactions
•Benzene has a delocalised pi system of electrons. thus electrons
are free to move round the whole ring structure.
•Consequently the ring an area of high electron density, hence
susceptible to attack by an electrophile.
•To undergo additional reactions, it requires breaking the bonds
of the ring system. This this does not happen because the
aromatic ring has an unexpected stability.
•The high electron density makes the ring susceptible to
electrophilic attack while the unusual stability makes it almost
impossible to destroy the ring system.
•As a result most reactions are substitution rather than addition.

11. Mechanism for electrophilic Mechanism for electrophilic
Substitution reactionSubstitution reaction
•Electrophilic substitution reaction occurs in almost three stages.
Suppose you have an electrophile El
+
. The reaction will proceed
as follows:
–First the electrophile is attacked by high electron density of the aromatic
ring. Then the electrophile forms a bond with the cloud of pi electrons
resulting in a formation of a compound known as a pi complex as
shown below
+ El
+
El
+

12. Mechanism for electrophilic Mechanism for electrophilic
Substitution reaction…..Substitution reaction…..
–Secondly, the electrophile eventually attacks one carbon atom of the
ring and bonds to it.
–The carbon atom uses the p orbital that was originally part of the
delocalised pi system to form this bond.
–This results in the destruction of the delocalised pi system as shown
below.
El
+ El
+
H

13. Mechanism for electrophilic Mechanism for electrophilic
Substitution reaction…..Substitution reaction…..
•Thirdly, When the electrophile bonds to the carbon, the ring
loses its stability and therefore it will attempt to regain its
stability.
•This is achieved by ejection of hydrogen ion.
•Now the aromatic ring has regained its stability by ejecting the
hydrogen ion.
•Note that the net result is the substitution of a hydrogen atom by
an electrophile.
+
El
+ H
+
H
El

14. Types of electrophilic Substitution Types of electrophilic Substitution
reaction: Nitrationreaction: Nitration
•In the nitration reaction, benzene is reacted with a mixture of nitric
acid, HNO
3 and sulphuric acid, H
2SO
4 to produce nitrobenzene.
•The overall reaction equation is:
•The hydrogen atom on the benzene ring is replaced by the NO
2
+

group. The, NO
2
+
, known as the nitronium ion, is the electrophile
in this reaction.
•This electrophile is generated in the reaction mixture of nitric acid
and sulphuric acid known as nitrating mixture.
+ HNO
3
H
2SO
4
50
o
C
NO
2
+ H
3O
+
nitrobenzene

15. Types of electrophilic Substitution Types of electrophilic Substitution
reaction: Nitration…..reaction: Nitration…..
•In this nitrating mixture, nitric acid, which is a weaker acid
than sulphuric acid, gets protonated by sulphuric acid and
then loses water molecule to form nitronium ion.
•Further substitution occurs in this reaction to produce
dinitrobenzene but it is in very small amounts. This
dinitrobenzene is a 1, 3 product.
•Dinitrobenzene is a solid and can therefore be easily
separated from nitrobenzene, which is a liquid.
HNO
3 + 2H
2SO
4 NO
2
+
+ H
3O
+
+ 2HSO
4
-
NO
2
NO
2

16. Types of electrophilic Substitution Types of electrophilic Substitution
reaction: Sulphonationreaction: Sulphonation
•Sulphonation involves the replacement of one hydrogen atoms of
the benzene ring by an -SO
3H group.
•The sulphur atom in this species has a large partial positive charge
and attacks the electron rich benzene ring.
•Therefore
+
SO
3H acts as the electrophile.
•In a sulphonation reaction, benzene is refluxed with
concentrated sulphuric acid.
•Refluxing means having mixtures in a closed apparatus,
which allows mixtures to be heated for long periods while
condensing vapour that forms, so that it falls back into
heating mixture again.

17. Types of electrophilic Substitution Types of electrophilic Substitution
reaction: Sulphonation……reaction: Sulphonation……
•A solution of sulphur trioxide, SO
3
, in sulphuric acid may also be
used in place of concentrated sulphuric acid.
•This solution of sulphur trioxide in sulphuric acid is known as
fuming sulphuric acid.
•The product is benzene sulphonic acid.
conc. H
2SO
4
reflux
SO
3H
benzene sulphonic acid

18. Types of electrophilic Substitution Types of electrophilic Substitution
reaction: Halogenationreaction: Halogenation
•Halogenation involves the reaction between benzene
reacts with chlorine or bromine in the presence of a
catalyst.
•Why is the catalyst used in this reaction?
–Bromine or chlorine molecules are not good enough
electrophiles to attack the benzene ring though they have
instant dipoles. Therefore the catalyst is used to induce
polarisation of the halogens.
–Iron (e.g. FeCl
3 or FeBr
3) or aluminium halides (e.g. AlCl
3) are
used as catalysts. These induce polarisation by accepting a lone
pair of electrons from one of the halogen atoms. For example:
+
+ FeBr4
-Br Br:
Fe
3+
Br
Br
Br
Br
+
+ -
-
-
-

19. Types of electrophilic Substitution Types of electrophilic Substitution
reaction: Halogenation……reaction: Halogenation……
•Then the electrophile Br
+
will proceed to attack the benzene ring.
• The catalyst in this case is known as the halogen carrier.
•The products for the reaction are bromobenzene or
chlorobenzene depending on the halogen involved in the
reaction.
+ Br
2
Br
+ HBr
FeBr
3

20. Effects of Substituents on reactivity Effects of Substituents on reactivity
and further substitution of the ringand further substitution of the ring
•It is also possible to have further electrophilic substitution of the
arenes after monosubstitution as in the nitration reaction.
•However further substitution depends on the nature of the groups
on the benzene ring.
–Some groups withdraw electrons from the ring making it less
reactive. Such groups are known as electron withdrawing
groups.
–Some groups are electron donating. These electron donating
groups release electrons onto the ring making it more reactive.
–Electron withdrawing groups make electrophilic substitutions
to occur more slowly while electron withdrawing groups make
it occur faster than in benzene itself

21. Examples of Electron donating and Examples of Electron donating and
Electron withdrawing groupsElectron withdrawing groups
Electron Donating Electron withdrawing
Alkyl groupd COOH
OH NO
2
NH
2 SO
3H
OCH
3 X (halogen)
The groups above do not only affect the rate of substitution, but
also direct the position where substitution will take place. This is
with reference to the carbon in the ring.

22. Cont...
Electron donating groups directs further substitution
to 2,4 and 6 positions (ortho/para directing) while
electron-withdrawing groups directs further
substitution to 3 and 5 positions (meta directing).
Halogens are an exception.
Though they are electron withdrawing, they direct
further substitutions to 2 and 4 positions.

23. Further substitution of the ringFurther substitution of the ring
CH
3
CH
3
HNO
3
H
2SO
4
NO
2
HNO
3
H
2SO
4
CH
3
NO
2O
2N
COOH
COOH
HNO
3
H
2SO
4
NO
2

24. Reactions of Side Chain of Alkyl Reactions of Side Chain of Alkyl
BenzenesBenzenes
•Alkyl benzenes or arenes are compounds in which
alkyl groups have replaced one or more hydrogen
atoms.
•Arenes undergo chemical reactions involving the ring
as well as of the group attached to the ring.
•Reactions of the side chain are typical of the group
attached to the ring.
•These reactions include oxidation and halogenation

25. Reactions of Side Chain of Alkyl Benzenes: Reactions of Side Chain of Alkyl Benzenes:
OxidationOxidation
•Alkyl groups are more easily oxidised than benzene itself. In
the presence of strong oxidising agents such as acidified
manganate ion, MnO
4
-
, and acidified dichromate ion,
Cr
2O
7
2-
, they are oxidised to carboxylic acids.
•In the presence of weak oxidising agents, e.g. manganese
(IV) oxide, MnO
2, the alkyl side chain is oxidised to an
aldehyde.
C
2H
5
MnO
4
-
C
O
OH
CH
3
MnO
2
C
O
H

26. Reactions of Side Chain of Alkyl Benzenes: Reactions of Side Chain of Alkyl Benzenes:
HalogenationHalogenation
•Halogenation of alkyl benzene is done by bubbling a
halogen through alkyl benzene exposed to light. The
alkyl side chain becomes halogenated and a hydrogen
halide is also produced.
CH
3
Cl
2
CH
2Cl
UV
Cl
2
CHCl
2
UV
Cl
2
CCl
3
UV

27. Practice ExamplesPractice Examples
•Explain why benzene easily undergoes electrophilic
substitution reaction and not addition reaction.
•Benzene undergoes halogenation reaction in the
presence of a catalyst, Iron halide. State what type of
reaction this is and why the catalyst iron halide is used.
•Using appropriate reaction mechanism, explain how
chlorination of benzene occurs.
•Explain the effects of electron withdrawing and
electron donating groups on the reactivity of benzene
ring and further substitution on the ring.