NEET Chemistry Notes Hydrocarbons Aromatic Hydrocarbons
Aromatic Hydrocarbons
Aromatic Hydrocarbons
Aromatic hydrocarbons are specialised cyclic compounds which are known for their characteristic unique smell (Aroma = fragrance). The group is also called arenes.
Aromaticity
- Ring systems whether having benzene ring or not with the following characteristics are said to be aromatic.
- Planarity.
- Complete delocalisation of the
-electrons in the ring. - Presence of (4n + 2) electrons in the ring, where n is an integer (n =0,1,2).
Certain compounds such as pyrrole, furan, etc., although do not contain benzene ring, but behaves like benzene. These compounds are called non-benzenoid aromatic compounds.
Structure of Benzene
Benzene is said to be resonance hybrid of the following forms.
There is complete delocalisation of -electrons.
It is aromatic, (4n + 2) =6, Here n = number of-electrons delocalised (Huckel’s rule is followed). All the C— C bond lengths are equal as confirmed by X-ray diffraction study.
A compound with delocalised electrons is more stable and benzene has six delocalised -electrons.
Preparation of Benzene
Chemical Properties of Benzene
The benzene exhibit the following chemical properties may be given as
Electrophilic Substitution Reactions
Benzene shows electrophilic substitution reactions due to 6 -electrons and resonance. Mechanism of electrophilic substitution involves following steps :
Addition Reactions
Directive Influence of Functional Group in Monosubstituted Benzene
- The substituent already present in the benzene ring directs the incoming group either to ortho and para or to meta position.
- o, p-directing groups activate the benzene ring.
- m-directing groups deactivate the benzene ring, e.g. —OH group is ortho-para directing.
m-directing groups
Carcinogenicity and Toxicity
Benzene and polynuclear hydrocarbons containing more than two -benzene rings fused together, are toxic and said to possess cancer producing (carcinogenic) property.
Such polynuclear) -hydrocarbons are formed on incomplete combustion of organic material like tobacco, reel and petroleum.