Suez Canal University
Faculty of science
Student name
Karim Nader Ibrahim Mohamed
Faculty
Science
Level
Two
Department
Chemistry
National ID
29909060201371
Student code
1801100165
Program
Chemistry
Subject
Organic Chemistry 2 (CH205)
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Suez Canal University
Faculty of science
Introduction
Discovery of Benzene
Benzene was first discovered by British Professor Michael Faraday in
1825, while he was heating whale oil. He found a colorless liquid. The
name benzene was given by German Chemist Eilhard Mitscherlich in
1833.
The cyclic structure of aromatic compounds remained unknown until
1865 when German Professor Friedrich August Kekulé studied it, Kekulé
did not discover the presence of interactions between the double
bonds.
We had to wait until 1931 for American Professor Linus Pauling to
found that benzene has a hybrid structure composed of delocalized
electrons.
Research objectives
 Conjugation and resonance.
 Aromaticity.
 Aromatic Hydrocarbons.
 Benefits of aromatic compounds.
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Suez Canal University
Faculty of science
Research
I.
Conjugation and resonance
Conjugation is the ability to resonate, so there is no difference
between them. this happens when two or more p orbitals overlap each
other. Overlap can extend beyond two p orbitals to include three, four,
five, and even more consecutive p orbitals.
The overlapping can include an empty p orbital or an orbital containing a
lone pair or p-orbitals of a pi bond or a half-filled orbital.
Resonance of Benzene
The carbon (C) atoms in the benzene ring are sp2 hybridized. One C atom
which have sp2 hybridized orbitals overlaps with the sp2 orbital of
adjacent carbon atom C-C sigma bond. Another sp2 hybridized orbitals of
the same Carbon atom combine with s orbital of hydrogen to C-H sigma
bond. The unhybridized p orbitals of carbon atoms form Pi bonds with
adjacent carbon atoms by lateral overlap.
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Suez Canal University
Faculty of science
II.
Aromaticity
Organic compounds are divided into: Aromatic compounds,
Nonaromatic compound and Anti-aromatic compounds.
Aromatic compound must be cyclic, planer, completely conjugated,
obeys Hückel’s rule.
 Cyclic: must all atoms be connected with each end.
 Planer: the overlapping must be Sp2, all atoms in the molecule lie
in the same plane.
 Completely conjugated: The Pi bond must be delocalized in all the
ring.
 Hückel’s rule: Hückel's Molecular Orbital Theory, a compound is
stable if all of its orbitals are filled or half filled with paired
electrons.
The 4n+2 rule, count the number of π electrons in the molecule.
Then, make this number equal to 4n+2 and solve for n. If is 0 or any
positive integer. For example, benzene has six π electrons:
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Suez Canal University
Faculty of science
Each π bond always have 2 π electrons. Benzene has 3 double bonds;
therefore, it has 6 π electrons.
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Suez Canal University
Faculty of science
III.
Aromatic Hydrocarbons
Aromatic hydrocarbons are a type of organic compounds. Aromatic
compounds contain a straight unsaturated ring of atoms that is stable
due to a delocalization of π electrons. The most important aromatic
compound is composed of six carbon atoms in the form of a ring.
Properties of Aromatic Hydrocarbons
The first aromatic hydrocarbon compound was benzene. In the benzene
ring each carbon has one H and each two alternative carbon have double
bond with alternative with single bond in the other two carbon C—C=C in
which the pi electron is delocalized.
The continuous delocalization of pi electrons in the benzene molecule is
represented by a circle inside the hexagon.
These compounds exhibit aromaticity (additional stability granted by
resonance), The ratio of carbon atoms to hydrogen atoms is relatively high
in these types of molecules, when burnt the aromatic hydrocarbons
display a strong and sooty flame which is yellow in color, these compounds
generally undergo electrophilic substitutions and nucleophilic aromatic
substitution reactions.
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Suez Canal University
Faculty of science
IV.
Benefits of aromatic compounds
Heme: it’s pigment that give the blood it’s red color, which it takes
important role in structure of hemoglobin in blood.
The molecule heme contains 8 rings connected with each other with 22
π electrons. According to Hückel's 4n + 2 rule, it is aromatic (n = 5).
The uses of benzene include making plastics, resins, synthetic
fibers, rubber lubricants, dyes, detergents, drugs and pesticides, glues,
adhesives, cleaning products, paint strippers and tobacco smoke.
Toluene is used in making of paints, alcoholic drinks, glues and nail polish
remover.
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Suez Canal University
Faculty of science
Dimethylbenzene (xylene) is used in medicine, dentistry and plastics,
solvent, clearing agent, lubricant, precursor to polyester.
Uses of Aromatic Hydrocarbons
1.
2.
3.
4.
5.
Methylbenzene is used as a solvent in some types of glues
Naphthalene is used in preservation.
For the synthesis of drugs and dyes.
TNT is widely used for explosives.
Plastic industry (PVC) and petrochemical industries.
Conclusion
The aromaticity plays an important role in understanding of organic
compounds, it makes the compound more stable by resonance of
electron in the ring, which make the compound used in several
industries.
Hückel’s rule make it easy for us to knew that the compound is
aromatic or non-aromatic or anti-aromatic.
Molecular orbital theory makes it possible to understand the
conjugation and the hybridization.
The aromatic compounds enter in several industry and in components
of our body.
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Suez Canal University
Faculty of science
References
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P. Schleyer, "Aromaticity (Editorial)", Chemical Reviews, 2001.
P. Schleyer, "Introduction: Delocalization- and (Editorial)",
Chemical Reviews, 2005.
Vollhardt, K. Peter C., and Neil E. Schore. Organic Chemistry:
Structure and Function. New York: W.H. Freeman and Company,
2007.
https://www.minichemistry.com
https://chem.libretexts.org
https://www.sciencedirect.com/
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