Benzene is the simplest aromatic hydrocarbon with the molecular formula C₆H₆. It was first discovered by Michael Faraday in 1825. Benzene is an unsaturated cyclic compound containing a ring of six carbon atoms with delocalized π-electrons. It is the parent compound of all aromatic hydrocarbons.

Benzene has a hexagonal ring structure with alternating double bonds (Kekulé structure), but in reality, electrons are delocalized, forming a resonance-stabilized ring.

     CH2
    //  \
   C     CH
  ||     ||
  HC     C
    \   /
     CH2

Because of resonance, all C–C bond lengths in benzene are equal (0.139 nm).

Benzene is aromatic because it satisfies Hückel’s rule (4n + 2 π-electrons). It contains 6 π-electrons (n = 1), which makes it highly stable compared to other unsaturated compounds.

• Colourless liquid with a characteristic aromatic smell.
• Boiling point: 80°C.
• Melting point: 5.5°C.
• Insoluble in water but soluble in organic solvents like ether and ethanol.
• Burns with a very smoky flame due to high carbon content.
• Toxic and carcinogenic; prolonged exposure may cause cancer.

Due to resonance stability, benzene undergoes mainly substitution rather than addition reactions.

1. Combustion

   Benzene burns in air with a smoking and luminous flame due to its high carbon content.

   2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O (Complete)
   2C₆H₆ + 9O₂ → 12CO + 6H₂O (Incomplete)
     

2. Addition reactions
1. Halogenation

Occurs in presence of FeCl₃ or FeBr₃. Benzene reacts with chlorine to form chlorobenzene. With bromine, it yields bromobenzene

C₆H₆ + Br₂ → C₆H₅Br + HBr
             (bromobenzene)
C₆H₆ + Cl₂ → C₆H₅Cl + HCl
.             (chlorobenzene)

  

2. Nitration

Benzene reacts wit conc. HNO₃ to yield nitrobenzene

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
  

3. Sulphonation

When benzene is reflux with conc H₂SO₄ for several hours, sulphobenzene is formed

C₆H₆ + H₂SO₄(conc.) → C₆H₅SO₃H + H₂O
  

Note: Nitration and sulphonation distinguishes benzene from cyclohexane.

4. Friedel–Crafts Alkylation

Friedel–Crafts alkylation is a reaction in which an alkyl group is introduced into the benzene ring. Anhydrous AlCl₃ acts as a Lewis acid catalyst by generating a strong electrophile that benzene attacks to form an alkylbenzene. This reaction is important in producing aromatic hydrocarbons such as toluene.

C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl
(benzene)     (methylbenzene / toluene)

5. Friedel–Crafts Acylation

In Friedel–Crafts acylation, an acyl group (RCO–) is introduced into the benzene ring using an acyl chloride in the presence of anhydrous AlCl₃. This produces an aromatic ketone. Acylation avoids rearrangements and gives a more predictable product compared to alkylation.

C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl
(benzene)        (acetophenone)

6. Reaction of Benzene with Ethene

Benzene reacts with ethene in the presence of anhydrous AlCl₃ or H⁺ to form ethylbenzene. Ethene is first converted to an electrophile (CH₃–CH₂⁺), which then attacks benzene. This reaction is used industrially because ethylbenzene is the main precursor for manufacturing styrene.

C₆H₆ + CH₂=CH₂ → C₆H₅CH₂CH₃
(benzene)       (ethylbenzene)

3. Addition Reactions
1. Hydrogenation

Benzene in the presence of finely divided nickel as catalyst, reacts with hydrogen to form cyclohexane

C₆H₆ + 3H₂ → C₆H₁₂ (cyclohexane)
  

2. Chlorination (UV light)

Benzene in the presence of sunlight as catalyst reacts with chlorine to form 1,2,3,4,5,6 hexachlorocyclohexane(benzene hexachloride)

C₆H₆ + 3Cl₂ → C₆H₆Cl₆ (benzene hexachloride)
  

1. From Sodium Benzoate

Dry distillation with soda-lime.

C₆H₅COONa + NaOH → C₆H₆ + Na₂CO₃
  

2. From Coal Tar

Benzene is obtained from the fractional distillation of coal tar.

3. From Petroleum

Catalytic reforming of petroleum fractions yields benzene.

• Starting material for making drugs, plastics, dyes, detergents, and explosives.
• Used in the manufacture of styrene, phenol, and nylon.
• Used as a solvent (restricted due to toxicity).
• Used to make cyclohexane, the precursor to nylon-6 and nylon-66.