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lect 2.pptx
1. Organic Chemistry (II)
(Code: PE 213)
Instructor: Dr.Ehssan Nassef
e-mail: Ehssan.Nassef@Pua.edu.org.
Office Hours: Monday 1:30-3:30
Pharos University in Alexandria
Faculty of Engineering.
Petrochemical Department
2. Hydrocarbons
• Hydrocarbons can be classified to:
• Aliphatic Hydrocarbons (Alkane,Alkene and
alkynes) Organic I
• Alyciclic Hydrocarbons.
• Aromatic Hydrocarbons. Organic II
5. 5
The Term Aromatic
Arenes and Aromaticity
• Aromatic term is used to describe a fragrant substance.
• Today the ward aromatic refers to benzene and its structural
relatives.
• Benzaldehyde, toluene, and benzene are all aromatic
compounds.
• Arenes are hydrocarbons based on the bz ring as the
structural unit.
H
O
CH3
Benzene Benzaldehyde Toluene
6. 6
Structure and Stability of Benzene
• Benzene is more stable than typical
alkenes.
• All C-C-C bond angles are 120 degrees,
all six carbon atoms are sp2 hybridized,
and each carbons has a p orbital
perpendicular to the plane of the six-
member ring.
7. 7
We can see why benzene is stable:
According to resonance theory , the more
resonance forms a substance has ,the more
stable it is.
Benzene with two resonance forms of equal
energy ,is therefore more stable and less reactive
than alkene.
8. Structure of benzene
(The kekule Proposal)
• August Kekule proposed that:
1- benzene contains a ring of carbon atoms and
can be formulated as cyclohexa-1,3,5-triene.
2- Benzene is a planner structure.
BUT the proposal failed in explaining :
1- why benzene is uncreative compared to
alkene.
2- why does benzene give a substitution product
rather than an addation product on reaction with
broumine(Br2)
9. 9
Bonds of Benzene Rings
• Each π bond overlaps equally well with both
neighboring p orbitals; six π electrons are
delocalized around the ring.
1.5 bonds on average
10. 10
Molecular Orbital Description of
Benzene
• Planar molecule with
the shape of a regular
hexagon.
• All C-C-C bond angles
are 120 degrees
• All six carbons are sp2
hybridized.
• Each carbon atom has a
p orbital perpendicular
to the plane of the six-
membered ring.
• All six carbon atoms and
six p orbitals in benzene
are equivalent.
11. 11
The resonance Proposal of benzene
and the orbital structure
The six pi electrons are completely delocalized
around the ring.
12. 12
Let’s review…
• Benzene is a cyclic conjugated molecule
• It is unusually stable, with a heat of hydrogenation of -
206 kJ/mol
• It is planar with the shape of a regular hexagon; bond
angles 120
• It undergoes substitution reactions that retain the
cyclic conjugation
• It is a resonance hybrid
• Generally less dense than water
• Insoluble in water but soluble in many organic
solvents
13. 13
The Huckel 4n + 2 Rule
To be aromatic, the molecule must also follow the
Huckel 4n + 2 rule:
A molecule must have 4n + 2 pi electrons
where n is an integer
(0, 1, 2, 3, etc…).
The number of pi electrons must be odd.
15. 15
1. Destructive Distillation of Coal
• Gives coal gas, ammoniacal liquor, coal tar
and coke as products
• The coal tar produced is a mixture of many
organic compounds (mainly aromatic ones)
• benzene and methylbenzene can be
obtained by fractional distillation
16. New Way Chemistry for Hong Kong A-Level
3A
16
1. Destructive Distillation of Coal
A laboratory
set-up of the
destructive
distillation of
coal
20. 20
2. Reduction of Phenol
• Passing phenol vapour over heated zinc dust
produce benzene and zinc(II) oxide
Benzene is separated by fractional distillation
