Benzene is the parent member of Aromatic compounds. The compounds which resemble benzene in structure and properties are called aromatic compounds. Aroma means fragrant smell (pleasant smell).
General molecular formula of monocyclic aromatic hydro carbons is CnH2n-6.
Methods of Preparation of Benzene
1. Decarboxylation of sodium salt of benzoic acid
2. Reduction of phenol
3. Hydrolysis of benzene sulphonic acid
4. Polymerisation of Acetylene
5. Commercial preparation of Benzene: Benzene is prepared on large scale from coal.
Physical Properties of Benzene
- It is colorless liquid with pleasant smell.
- Its boiling point is 80°C.
- It is lighter than water.
- It is insoluble in water but soluble in alcohol, ether etc.
- It burns with sooty flame due to high percentage of carbon.
Chemical Reactions of Benzene
Benzene is highly unsaturated, but it is less reactive because the benzene ring is stabilised by resonance.
Benzene is expected to give normal addition reaction due to unsaturation. Because of unusual stability caused by resonance, it normally gives substitution reactions rather than addition reactions. Due to the presence of π - electron cloud, benzene ring easily attracts electrophilies.
Thus, benzene normally gives electrophilic substitution reactions and under special conditions it also gives certain addition reactions.
Electrophilic substitution reactions: In these reactions, benzene acts as nucleophile. Electrophile can attack any of six positions of Benzene. When, electrophile attacks the benzene, positive charge is developed at ortho position and then delocalises to para position thus, ortho and para positions carry the charge in case of electrophilic attack.
Uses of Benzene
- As solvent for fats, resins.
- As motor fuel
- In dry cleaning
- As starting material in the manufacture of many compounds like aniline, oil of mirbane, insecticides, BHC, phenol, styrene, etc.
Structure of Benzene
Its molecular formula is C6H6 and empirical formula is CH. The ratio of carbon to hydrogen indicates, it is highly unsaturated. But the behaviour of benzene resembles saturated compounds. Thus, benzene looks like unsaturated but behaves like saturated.