Aromatic hydrocarbons are compounds containing benzene rings. Benzene, the parent aromatic hydrocarbon, has six carbon atoms arranged in a hexagonal ring with alternating single and double bonds between carbons. The stability of benzene is explained by resonance, where the double bond positions are continuously shifting so that the pi-electrons are delocalized across the whole ring. Key features of aromatic compounds include planar, conjugated ring structures with (4n+2) pi electrons that undergo substitution rather than addition reactions. Common aromatic hydrocarbons include benzene, naphthalene, and anthracene.

1. Aromatic hydrocarbons

2. What are aromatic hydrocarbons ?
• These hydrocarbons are also known as ‘arenes’. Since most of them
possess pleasant odour (Greek; aroma means pleasant smelling), the
class of compounds was named as ‘aromatic compounds’. The parent
member of the family is benzene having the molecular formula C6H6. it
has hexagonal ring of six carbon atoms with three double bond in
alternate position. Aromatic compounds containing benzene ring are
known as benzenoids and those not containing a benzene ring are
known as non-benzenoids.

3. Resonance structure of benzene
The stability of benzene can be explained on the basis of concept of resonance.
Kekule in1865 gave a ring structure for benzene in which the positions
of the three double bonds are not fixed. He suggested that the double
bond keep on changing their positions an this is called Resonance. The
resonance structure of benzene is supported by the following facts:
Carbon carbon single bond is the lengthiest as compared to double and
triple bonds.
ii)Due to resonance the π-electron charge in benzene gets distributed over
greater area i.e., it gets delocalised. As a result of delocalisation the
energy of the resonance hybrid decreases as compared to contributing
structure by about 50kJ mol-1. the decrease in energy is called
resonance energy. Therefore, it is stabilised and behaves as a saturated
hydrocarbon.
iii)If the positions of double bonds are fixed. We expect two isomers of 1,2-
dichlorobenzene as shown below (one having Cl atoms attached to C-C
bond and the other having Cl atoms attached to C=C bond).
KEKULE

4. ORBITAL CONCEPT OF
BENZENE
• According to the orbital concept, each
carbon atom in benzene is sp2-
hybridised and one orbital remains
unhybridised. Out of the three hybrid
orbitals, two overlap axially with the
orbitals of the neighbouring carbon
atoms on both side to form σ-bond.
The third hybridised orbital of the
carbon atom overlaps with the half-
filled orbital of the hydrogen atom
resulting in C-H bonds. Thus, benzene
has a planar structure –with bond

5. ORBITAL OVERLAPP IN BENZENE
There is still one unhybridised
2p-orbital left on each carbon
atom. Each one of these
orbitals can overlap sidewise
with similar orbital of the
carbon atoms on either sides
to form two sets of π-bonds.

6. ELECTRON CLOUD
• The resultant π-orbital cloud
is spread over all the six
carbon atoms . As a result,
there are two continuous
rings of π-electron clouds,
one above and the other
below the plane of the
carbon atoms.

7. AROMATICITY
• Aromatic compounds are those
which resembles benzene in
chemical behaviour. These
compounds contain alternate
double and single bonds in a
cyclic structure. They undergo
substitution reaction rather than
addition reaction. This
characteristic be behaviour is
called Aromaticity. The
Aromaticity depends upon the
Cyclopentadienyl
anion

8. CONDITIONS FOR AROMATICITY
The main essential for Aromaticity are:
Delocalisation: the molecule should contain a cyclic cloud
of delocalized πelectron above and below the plane of the
molecule
Planarity: for the delocalisation of π-electron the ring must
be planar to allow cyclic overlap of p-orbitals. Therefore, for a
molecule to be aromatic, the ring must be planar.
(4n+2)π electron: for Aromaticity, the π-electron could
must contain a total of (4n+2)π electrons where n is an integer

9. AROMATIC HYDROCARBONS
Benzene, 6 π e- Naphthalene, 10 π e- Anthracene, 14 π e-
(n=1) (n=2) (n=3)

10. PREPARATION OF BENZENE
Decarboxylation of aromatic acid
Benzene is prepared in the laboratory by heating sodium
benzoate with soda lime.

11. Reduction of phenol
Benzene can be prepared from phenol by distillation with zinc.