Benzene is a colorless, highly flammable liquid with the formula C6H6, classified as an aromatic hydrocarbon due to its cyclic structure and delocalized pi electrons. It primarily serves as a precursor in chemical manufacturing but has limited consumer applications due to its toxicity. The structure of benzene has been explored through various methods including molecular orbital theory, spectroscopic analysis, and resonance, leading to its recognition as a staple in organic chemistry.

1. Unit-I
BENZENE AND ITS
DERIVATIVES

2. HELLO!
2
I am
Mr. ANSARI SHOAIB AHMED
Assistant Professor
Department of Pharmaceutical Chemistry
Oriental College of Pharmacy

3. BENZENE
Introduction
Benzene is an organic chemical compound with the molecular
formula C6H6. The benzene molecule is composed of six carbon
atoms joined in a planar ring with one hydrogen atom attached to
each. Because it contains only carbon and hydrogen atoms,
benzene is classed as a hydrocarbon.
3

4.  Benzene is a natural constituent of crude oil and is one of the
elementary petrochemicals.
 Due to the cyclic continuous pi bonds between the carbon
atoms, benzene is classed as an aromatic hydrocarbon.
 It is sometimes abbreviated PhH.
4
Source

5.  Benzene is a colorless and highly flammable liquid with a
sweet smell, and is partially responsible for the aroma around
petrol (gasoline) stations.
 It is used primarily as a precursor to the manufacture of
chemicals with more complex structure, such as ethylbenzene
and cumene, of which billions of kilograms are produced
annually.
 Although a major industrial chemical, benzene finds limited
use in consumer items because of its toxicity
5
Properties

6. NOMENCLATURE
Let’s start with the first set
of slides

7. “
Benzene and its homologues are frequently called by the Common
Names which are accepted by the IUPAC system.
The homologues of benzene having a single alkyl group are named
as Alkylbenzenes.
These are made of a benzene ring to which is attached the alkyl
group. The all group bonded to a ring carbon is designated as the
Side-Chain.
7

8. Monosubstitution
8
Benzene
Toluene
(Methylbenzene)
Ethylbenzene Cumene
(Isopropylbenzene)

9. Alkyl substitution
is larger than the
ring ?
9
3-Phenyldecane
5-Phenylpentanoic Acid
2
3
4
5
6
7
8
9
10
1
5
4
3
2
1

10. Disubstitution
10
o-Xylene m-Xylene p-Xylene
Dimethylbenzenes have the common name Xylenes. The three
isomeric xylenes are:
ortho
meta
para
o-bromotoluene m-dibromobenzene p-cloronitrobenzene

11. Polysubstituted
Derivatives
11
3,5-Dibromophenol
1-Bromo-3,5-Dichlorobenzene 2,4,6-Trinitrotoluene

12. Structure of
Benzene
12

13. Elemental analysis and molecular weight determination
showed that benzene had the molecular formula C6H6.
This indicated that benzene was a highly unsaturated
compound (compare it with n-hexane, C6H14).
Molecular
Formula
13
n-hexane
(C6H14)
Cyclo-hexane
(C6H12)
Cyclo-hexatriene
Benzene
(C6H6)

14. Molecular
Orbital Structure
of Benzene
14

15. 15

16. Hybridization
16
 The structure of benzene is best described in terms of the modern molecular orbital
theory.
 All six carbon atoms in benzene are sp² hybridized.
 The sp² hybrid orbitals overlap with each other and with s orbitals of the six hydrogen
atoms forming C-C and C-Hσ bonds

17. EVIDENCES
Analytical, synthetic and
other evidences in the
derivation of structure of
benzene

18. Synthetic Evidences
 Benzene could be constructed as a straight-chain or ring
compound having double (C=C) and/or triple (C≡C)
bonds.
 But benzene did not behave like alkenes or alkynes. It did
not decolorize bromine in carbon tetrachloride or cold
aqueous potassium permanganate. It did not add water
in the presence of acids.
Straight Chain not Possible
01

19. 19
NO REACTION
NO REACTION
NO REACTION
Cyclo-hexatriene
Benzene
(C6H6)
Br2/CCl4
H2O/H+
KMnO4
Dilute cold

20. Evidence of
Cyclic Structure
1) Substitution of Benzene
Benzene reacted with bromine in the presence of FeBr3 (catalyst) to form
monobromobenzene.
20
The fact that only one monobromo and no isomeric products were obtained
indicated that all six hydrogen atoms in benzene were identical. This could be
possible only if benzene had a cyclic structure of six carbons and to each carbon
was attached one hydrogen.
Monobromobenzene
Benzene
02

21. 21
2) Addition of Hydrogen to Benzene
Benzene added three moles of hydrogen in the presence of nickel catalyst to give
cyclohexane.
This confirmed the cyclic structure of benzene and also showed the presence of three
carbon-carbon double bonds.
Cyclo-hexane
(C6H12)
Cyclo-hexatriene
Benzene
(C6H6)
Ni
150o/Pressure
+ 3 H2

22. Analytical Evidence
1) IR SPECTROSCOPY
Aromatic compounds produce a large number of characteristic bands in
the infra-red region. The following regions are particularly useful for
recognizing the presence of benzene (and polynuclear aromatics) and
benzene derivatives: the C-H (stretch) absorption region is 3080-3030
cm¹ (w), and the bands for C=C (in-plane vibration) are: 1625-1 600 cm-¹
(v), 1 590-1 575 cm (v) and 1 525-1475 cm¯¹ (v)..
22
03

23. 23
IR Spectra
of
Benzene

24. 24
2) NMR SPECTROSCOPY
NMR spectroscopy is an analytical technique used in structural elucidation. NMR spectroscopy used
to identify total number of carbon and total number of hydrogen. There are two types of NMR
spectroscopy.
I. H1-NMR SPECTROSCOPY
II. C13-NMR SPECTROSCOPY
Benzene contains six carbon and six hydrogen. H1 NMR spectra shows a single peak with chemical
shift of 7.34, while C13 NMR spectra shows a single peak with a chemical shift of 128.5 . It confirms
that all the carbon in benzene are identical in nature and there is one hydrogen attached to each
carbon.

25. 25
3) MASS SPECTROSCOPY
In mass spectrometry, a substance is bombarded with an electron beam having sufficient energy to
fragment the molecule. The positive fragments which are produced (cations and radical cations) are
accelerated in a vacuum through a magnetic field and are sorted on the basis of mass-to-charge
ratio. Since the bulk of the ions produced in the mass spectrometer carry a unit positive charge, the
value m/e is equivalent to the molecular weight of the fragment. The analysis of mass spectroscopy
information involves the re-assembling of fragments, working backwards to generate the original
molecule.

26. 26
MASS
Spectra of
Benzene

27. “
KEKULE’ STRUCTURE OF
BENZENE
27

28. 28
In 1865, August Kekule suggested that benzene consist of a cyclic planar structure of six carbons
with alternate double and single bonds. To each carbon attached one hydrogen. Benzene according
to this proposal, was simple, 1,3,5-cyclohexatriene.
There were two objections:
i. If the Kekule's structure was correct, there should exist two or isomers of dibromobenzene. In
one isomer, the two bromine atoms should be on carbons that are connected by a double bond,
as shown in structure (a). In the other isomer, the bromines should be carbons connected by a
single bond, as shown in structure (b). In fact, only one ortho-dibromobenezene could be
prepared.
Usually written as

29. 29
2 Br2
FeBr3
(a) (b)
+
To overcome this problem, Kekule’ further suggested that benzene was a mixture of two forms (1 &
2) in rapid equilibrium state.
1 2
ii. Kekule’ structure failed to explain why benzene with three double bonds did not give addition
reactions like other alkenes. For example benzene does not react with HBr or Br2 in CCl4.

30. RESONANCE
Resonance description of
Benzene
30

31. Resonance
description of
Benzene
 The phenomenon in which two or more structures can be
written for a substance which involve identical positions of
atoms is called Resonance.
 The actual structure of the molecule is said to be Resonance
Hybrid of various possible alternative structures.

32. 32
 The alternative structures are referred to as the Resonance Structures or Contributing
Forms.
 A double-headed arrow (↔) between the resonance structures is used to represent the
resonance hybrid.
 Thus in the case of benzene, Kekule's structures (1) and (2) represent the resonance
structures. The actual structure of the molecule may be represented as a hybrid of these
two resonance structures, by the single structural formula (3).
1 2 3

33. 33
It should be clearly understood that the resonance structures (1) and (2) are not the actual
structures of the benzene molecule. They exist only in theory. None of these structures
adequately represents the molecule. In resonance theory, we view the benzene molecule
(which of course is a real entity) as being a hybrid of these two hypothetical resonance
structures.
Look at the structures (1 and 2) carefully. All single bonds in structure (1) are double bonds in
structure (2), which is considered a hybrid of them, then the carbon-carbon bonds in benzene
are neither single bonds nor double bonds. Rather, they are something halfway between. This is
exactly what we find experimentally. Spectroscopic measurements show that benzene is planar
and that all of its carbon-carbon bonds are of equal length, 1.40 Å. This value lies i between the
carbon-carbon single bond length (1.54 Å) and the carbon-carbon double bond length 1.34 Å).

34. Resonance
Energies of
Benzene.
Benzene's special stability is due to the formation of the
delocalized molecular orbital. The magnitude of this extra stability
can be estimated by measuring the changes in heat of
hydrogenations that are associated with reactions. Hydrogenation
of cyclohexene evolves 28.6 kcal/mole, a value typical for
hydrogenation of alkenes.
34
+ H2 + 28.6 kcal

35. 35
 Hydrogenation of both double bonds of 1,3-cyclohexadiene evolves 54.4
kcal/mole, approximately double the amount observed for cyclohexene.
+ 2 H2 + 55.4 kcal
 The molecule of 1,3,5-cyclohexatriene (Kekule structure), containing three ordinary
double bonds, is hypothetical-any efforts to produce it yields benzene. We would,
however, expect complete hydrogenation of the unknown 1.3.5-cyclohexatriene to evolve
approximately 3x 28.6 or 85.8 kcal/mole.

36. Actual
Resonance
Energy
36
Cyclo-hexane
(C6H12)
Hypothetical
Cyclo-hexatriene
+ 3 H2
+ 85.8 kcal
Cyclo-hexane
(C6H12)
Hypothetical
Cyclo-hexatriene
+ 3 H2
+ 49.8 kcal
Hydrogenation of benzene gives only 49.8 kcal/mole.

37. 37
Figure (PTO) makes these energy relationships more evident. The 36 kcal
difference between the heat evolved in the hydrogenation of benzene and
that estimated for hydrogenation of a compound with three ordinary
double bonds is the added stability. This added stability is sometimes
called Resonance Energy. Resonance energy is a measure of how much
more stable a resonance bybrid structure is than its extreme resonance
structures.

38. 38

39. AROMATICITY
Huckel Rule
39

40. Aromaticity
In chemistry, aromaticity is a property of cyclic (ring-shaped),
planar (flat) structures with pi bonds in resonance (those
containing delocalized electrons) that gives increased stability
compared to other geometric or connective arrangements
with the same set of atoms. Aromatic rings are very stable and
do not break apart easily. Organic compounds that are not
aromatic are classified as aliphatic compounds—they might be
cyclic, but only aromatic rings have enhanced stability.

41. 41
The aromatic compounds apparently contain alternate double and single bonds in a
cyclic structure and resemble benzene in chemical behavior. They undergo
substitution rather than addition reaction This characteristic behavior is called
Aromatic Character or Aromaticity. Aromaticity is, in fact, property of the sp²
hybridized planar rings in which the p orbitals (one on each atom) allow cycle
delocalization of electrons.

42. 42
Criteria for Aromaticity
On the basis of the above considerations, can be laid down criteria or rules which
help us in knowing whether a particular compound is aromatic or non-aromatic.
Rule 1. An aromatic compound is cyclic and planar.
Rule 2. Each atom in an aromatic ring has a p orbital. These p orbitals must be
parallel so that a continuous overlap is possible around the ring.
Rule 3. The cyclic molecular orbital (electron cloud) formed by overlap of p
orbitals must contain (4n+2) electrons, where n-integer 0, 1, 2, 3 etc. This is
known as Huckel Rule
Let us apply these rules to the following examples:

43. Aromaticity
Aromatic
 Cyclic structure
 Complete resonance
 Odd pair of electrons
Anti-Aromatic
 Cyclic structure
 Complete resonance
 Even pair of electrons
43

44. 44
Cyclic
?
Conjugated
?
Pi bonds? Lone pairs of
electrons?
Total  E’s (4n+)
E
Aromatic?
Yes Yes 3 0 6 Yes Yes
Cyclic
?
Conjugated
?
Pi bonds? Lone pairs of
electrons?
Total  E’s (4n+)
E
Aromatic?
Yes Yes 3 0 6 Yes Yes

45. REACTIONS OF
BENZENE
(Chemical Properties)
45

46. Electrophilic
Aromatic
Substitution
Benzene undergoes electrophilic substitution reactions. The
benzene ring with its delocalized electrons is an electron-rich
system. It is attacked by electrophiles, giving substitution
products.
These reactions can be represented as:
Substitution
Product
+ E-Nu + H-Nu

47. 47
01
02
03
04
05
Reactions of
benzene
Halogenation
Nitration
Sulphonation
Alkylation
Acylation

48. REACTIONS
OF
BENZENE
48

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