Benzene -Classification, Structure & its Stability
What Is Benzene?
A cloud of questions can be solved by this article about benzene.E.g What is Benzene? How is it stable? What is the role of electronic cloud in structure of Benzene? What is the role of Kekule's Structure in Benzene structure?
The term aromatic was derived from the Greek word ‘aroma’ meaning
“fragrant” and was used in Organic Chemistry for a special class of
These compounds have a low hydrogen to carbon ratio
in their molecular formula and have a characteristic odour.
However,it was soon realized that many aromatic compounds are odourless
whereas many others are fragrant though they are not aromatic.
Further, when aromatic compounds of higher molecular mass were subjected
to various methods of degradation, they often produced benzene or derivatives
of benzene. It was observed that almost all the aromatic compounds have
a six carbon unit in their molecules like benzene. Hence, benzene was
recognized as the simplest and the parent member of this class of compounds.
So aromatic hydrocarbons include benzene and all those compounds that
are structurally related to benzene.
It appears from the definition of aromatic hydrocarbons that any study
of this class of compounds must begin with the study of benzene.
Benzene has characteristic structural features. It has a regular planar
hexagonal structure. On the basis of the number of benzene rings
aromatic hydrocarbons can be categorized into following classes.
a. Monocyclic Aromatic Hydrocarbons and their derivatives
b. Polycyclic Aromatic Hydrocarbons
A. Monocyclic Aromatic Hydrocarbons and their Derivatives:
Aromatic hydrocarbons containing one benzene ring in their molecules are
called Monocyclic Aromatic Hydrocarbons, e.g. benzene and its derivatives.
B. Polycyclic Aromatic Hydrocarbons:
Aromatic hydrocarbons containing two or more benzene
rings in their molecules are called Polycyclic Aromatic
Hydrocarbons. They may be divided into two main classes.
(i) Those in which benzene rings are isolated, e.g. bi
(ii) Those in which the benzene rings are fused together at ortho positions
so that the adjacent rings have a common carbon to carbon
bonds, e.g. naphthalene, phenanthrene and anthracene
Benzene was discovered by Michael Faraday in 1825 in the gas produced
by the destructive distillation of vegetable oil and twenty
years later it was also found in coal-tar by Hoffmann.
1. The empirical formula of benzene is determined by the elemental analysis.
2. Its molecular mass determined by the vapour density method is 78.108.
This is six times the empirical formula mass (CH = 12 + 1 = 13). Therefore, the
molecular formula of benzene is C6H6.
3. The molecular formula of benzene indicates that it is highly unsaturated
The structure of benzene continued to be a serious problem for chemists
for about 40 years. A German chemist, Kekule at last solved the problem
in 1865. Kekule proposed a cyclic regular hexagonal structure for benzene,
which contains three double bonds alternating with three single bonds.
He supported his theory by the following arguments.
(i) Benzene gives only one monosubstituted product.
(ii) Benzene gives only three disubstituted products.
These points confirm the regular hexagonal structure for benzene in which
all the carbon atoms are occupying identical positions in the molecule.
Therefore, benzene forms only one toluene, one phenol and one nitro
(iii) Benzene adds three hydrogen
molecules in the presence of a catalyst.
iv) Benzene adds three molecules of chlorine in the presence of sunlight.
These two reactions confirm the presence of three double bonds alternating
with three single bonds.
Objections to Kekule’s Formula
Kekule’s formula with three double bonds demands a high degree of
unsaturation from benzene while usually it exhibits a saturated character.
Thus benzene yields substitution products readily and forms addition products
reluctantly. Benzene is also a very stable compound. All these properties
of benzene can be easily explained using the modem theories about its
Modern Concepts About the Structure of Benzene
The hexagonal frame-work of benzene can be
conveniently explained using hybridization
approach. According to this, each carbon
in benzene is sp2 hybridized. The three sp2
hybrid orbitals on each carbon are utilized
to form three s-bonds, two. with adjacent
carbon atoms and one with hydrogen.
The unhybridized 2pz orbitals remain at right
angle to these sp2 orbitals. Since all the sp2
orbitals are in the same plane therefore all
the carbon and hydrogen atoms are coplanar.
All the angles are of 120° which confirms the
regular hexagonal structure of benzene. The
unhybridized 2pz orbitals partially overlap to
form a continuous sheath of electron cloud,
enveloping, above and below, the six carboncarbon
sigma bonds of the ring.
Since each 2pz orbital is overlapped by the 2pz orbitals of adjacent
carbon atoms, therefore, this overlapping
gives, 'diffused' or 'delocalized' electron