Aromatic Compounds discussed comprehensively

1. Course: 0738―ORGANIC MATERIAL STRUCTURES
AND CHARACTERISTICS
Presenter: Sana Jamshaid

2. “LET US LEARN TO DREAM
GENTELMAN, AND THEN
PERHAPS, SHALL LEARN THE
TRUTH”.
(KEKULE)
2

3.  Introduction
 Discovery of Benzene
 Structure of Benzene
 Classification of aromatic compounds
 Criteria for aromaticity (Huckel’s Rule)
 Difference between aromatic, antiaromatic, nonaromatic
 Aromatic heterocyclic compound
 Annulenes
 Nomenclautre
 Sources of aromatic compounds
 Chemistry of Benzene
 Stability of Benzene
 Synthesis of Benzene
 Physical Properties of benzene
 Spectroscopic characteristics of aromatic compounds
 Chemical properties of Benzene
 Aromatic compounds in our daily life
 Scope of aromatic compounds and its derivatives in recent research trends
 Conclusion

4.  The term aromatic compounds was 1st used by
Kekule to classify benzene & its derivatives.
 Benzene and its derivatives, many of which possess
fragrance/ aroma.
 According to new fact, aromatic is associated with
“chemical stability” rather than aroma.
 So aromatic is used for series containing benzene
ring.
 May be called as “ Arenes and derivatives.”
 Include H-carbons, ketones, aldehydes, amines,
ethers etc.
4

5.  The word "benzene" derives
historically from "gum
benzoin", sometimes called
"benjamin" an aromatic resin
known to European
pharmacists and perfumers
since the 15th century as a
product of southeast Asia.
 Michael Faraday first isolated
and identified benzene in 1825
from the oily residue derived
from the production of
illuminating gas, giving it the
name bicarburet of hydrogen.

6. Structure of Benzene
KEKULE fall asleep while sitting in front of fire,
dreamed about chains of atoms in form of twisting
snakes. one of snake caught hold of its own tail ,
forming a whirling ring.
KEKULE awoke , freshly inspired , spent remainder
night working on his now-famous hypothesis.
6

7. 7
• The resonance description of benzene consists of two equivalent
Lewis structures, each with three double bonds that alternate with
three single bonds.
• The true structure of benzene is a resonance hybrid of the two Lewis
structures, with the dashed lines of the hybrid indicating the position of
the  bonds.
• We will use one of the two Lewis structures and not the hybrid in
drawing benzene. This will make it easier to keep track of the electron
pairs in the  bonds (the  electrons).

8. A structure of benzene, containing 3 cyclic conjugated
double bonds which systematically called 1,3,5-
cyclohexatriene

10. 10
Four structural criteria must be satisfied for a compound to be aromatic.
The Criteria for Aromaticity
[1] A molecule must be cyclic.
To be aromatic, each p orbital must overlap with p orbitals on adjacent
atoms.

11. 11
[2] A molecule must be planar.
All adjacent p orbitals must be aligned so that the  electron density can be
delocalized.
Aeromatic compound (landscape)

12. 12
[3] A molecule must be completely conjugated.
Aromatic compounds must have a p orbital on every atom.

13. 13
[4] A molecule must satisfy Hückel’s rule, and contain
a particular number of  electrons.
Benzene is aromatic and especially stable because it contains 6 
electrons. Cyclobutadiene is antiaromatic and especially unstable because
it contains 4  electrons.
Hückel's rule:

15. 1. Aromatic—A cyclic, planar, completely conjugated compound with 4n +
2  electrons.
2. Antiaromatic—A cyclic, planar, completely conjugated compound with
4n  electrons.
3. Not aromatic (nonaromatic)—A compound that lacks one (or more) of
the following requirements for aromaticity: being cyclic, planar, and
completely conjugated.
https://www.youtube.com/watch?v=nrRH_xYSjE4&t=138s
Considering aromaticity, a compound can be classified in one
of three ways:

17. Aromatic Heterocyclic Compounds
A heterocyclic compound has hetero atom in the ring system other than carbon
The heteroatom donates either one or two electrons to the  system
Heteroatom donates two electronsHeteroatom donates
one electron

18.  A six-membered heterocycle with a nitrogen
atom in its ring
  electron structure resembles benzene (6
electrons)
 The nitrogen lone pair electrons are not part of
the aromatic system (perpendicular orbital)
 Is it aromatic, anti-aromatic or non-aromatic?

19.  A five-membered heterocycle with one nitrogen
  electron system similar to that of cyclopentadienyl anion
 Nitrogen atom is sp2-hybridized, and lone pair of electrons occupies a p
orbital (6  electrons)
19 Is it aromatic, anti-aromatic or non-aromatic?

20. Furan structure
Furan and Thiphene structure are similar
 Are these aromatic, anti-aromatic or non-aromatic?

21. N
N
H
O S

23. 23
• Hydrocarbons containing a single ring with
alternating double and single bonds are called
annulenes.
• Completely conjugated rings larger than benzene
are also aromatic if they are planar and have 4n + 2
 electrons.
• To name an annulene, indicate the number of
atoms in the ring in brackets and add the word
annulene.

24. 24
• [10]-Annulene has 10  electrons, which satisfies
Hückel's rule, but a planar molecule would place the
two H atoms inside the ring too close to each other.
Thus, the ring puckers to relieve this strain.
• Since [10]-annulene is not planar, the 10  electrons
can’t delocalize over the entire ring and it is not
aromatic.

25. https://www.youtube.com/watch?v=-
U3tcz5anWU&t=165s

30.  Simple aromatic hydrocarbons come from two main
sources: coal and petroleum.
 Coal is an enormously complex mixture made up
primarily of large arrays of benzene-like rings joined
together.
 Thermal breakdown of coal occurs when it is heated
to 1000 °C in the absence of air, and a mixture of
volatile products called coal tar boils off.
 Fractional distillation at coal tar yields benzene,
toluene, xylene (dimethylbenzene), naphthalene,
and a host of other aromatic compounds.

31.  Benzene is a chemical that is a
colourless or light yellow liquid at
room temperature. It has a sweet
odour and is highly flammable.
 Natural sources of benzene include
volcanoes and forest fires. Benzene is
also a natural part of crude oil,
gasoline, and cigarette smoke.
 https://www.youtube.com/watch?v=oA
75KZLzRxQ&t=116s

33.  With reduction of benzene diazonium chloride with sodium
stannite or hypophosphorus acid.
 Benzene sulphonic acid on hydrolysis with superheated
steam gives benzene.
 C6H5.SO3H + H2O —> C6H6 + H2SO4
 Sodium benzoate is heated with soda-lime (NaOH) and
when it gets decarboxylated benzene is obtained.
 https://www.youtube.com/watch?v=X10VZeQmFDo

34.  Catalytic reforming
 Toluene hydrodealkylation
 Toluene disproportionation
 Pyrolysis gasoline
 Production from coal tar

37. Aromatic Rings
C=C 1600-1500 (stretching)
=C-H 3100-3000 (stretching),
2000-1600 (phenyl ring substitution overtones)
U S
overtones
Double bonds
/hydrocarbons

42. Principle types of reactions of BENZENE:
• Electrophilic Substitution reaction
https://www.youtube.com/watch?v=nhpXRjoWVHU
• Nucleophilic Aromatic substitution
https://www.youtube.com/watch?v=NsD8Q13ryHk

44. USES
• Important industrial solvent and precursor in the
production of drugs, plastics, synthetic rubber, dyes,
resins, nylon and synthetic fibers.
*Aromatic hydrocarbons of commercial interest
are benzene, toluene,ortho-xylene and para-
xylene
*e.g. The use of polystyrene foam, produced from
benzene, contributes to make homes more
comfortable, keeping them warm in winter and cool in
summer. It also helps to reduce energy emissions
and prevents leakage.
* Synthetic rubbers, also derived from aromatics,
gives tyres better road-hugging ability, especially on

45. •In production of rubbers, lubricants, dyes, detergents, drugs,
and pesticides.
*e.g. Cumene and phenol, for example, both produced from
benzene, are used as a starting material to make aspirin and
penicillin, one of the first and still one of the antibiotic agents.
Benzene is also a natural part of crude oil, gasoline, and
cigarette smoke.
In a refrigertor for example interior panels are most often made
out of tough, corrosion-resistant plastics such as ABS or high-
impact polystyrene; clear, see-through drawers and
compartments are made out of polystyrene or polycarbonate.
More importantly, the foamed polyurethane, derived from
toluene.
Most of a CD consists of an injection-molded piece of clear
polycarbonate: a plastic derived from benzene, through
bisphenol A and cumene.

46.  In clothes : acrylic fibres, polyester, nylon, luxurious fibres
– linen, silk, cashmere wool – to give them more resistance
all are aromatics.
 Aromatics are used today in most of our sports equipment.
From polyurethane footballs to nylon parachutes, from light
running shoes to polyester swimwear, aromatics provide
the materials.
 Gas-burner, microwave oven, coffee machine, pots and
pans, milk bottles and yoghurt pots, meat trays, eggs tray
even the cutlery all rely on the input of the aromatics
industry

70. About 35 million tonnes aromatic compounds are
produced worldwide every year. They are extracted
from complex mixtures obtained by the refining of oil
or by distillation of coal tar, and are used to produce
a range of important chemicals and polymers,
including styrene, phenol, aniline, polyester and
nylon which are staring material of many organically
synthesised products.