Aromatic hydrocarbons

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.

Resonance structure of benzeneThe 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

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 angle of120˚ each.

ORBITAL OVERLAPP IN BENZENEThere 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.

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.

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 electronic structure of the molecule.

Cyclopentadienyl anion

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 equal to 0,1,2,3……..n . This is known as Huckel Rule.

AROMATIC HYDROCARBONS

Benzene, 6 π e- Naphthalene, 10 π e- Anthracene, 14 π e-

(n=1) (n=2) (n=3)

PREPARATION OF BENZENE

Decarboxylation of aromatic acidBenzene is prepared in the laboratory by heating sodium benzoate with soda lime.

Reduction of phenolBenzene can be prepared from phenol by distillation with

zinc.