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Chemical Properties of Benzene | chemca

Hydrocarbons

Chemical Properties of Benzene

Electrophilic Substitution, Addition, and Combustion Reactions.

By chemca Team • Updated Jan 2026

Due to the high stability of the delocalized $\pi$-electron system, Benzene resists addition reactions. Its characteristic property is Electrophilic Aromatic Substitution (EAS), where a hydrogen atom is replaced by an electrophile.

1. Electrophilic Substitution Reactions

A. Nitration

Benzene reacts with a nitrating mixture (Conc. $HNO_3$ + Conc. $H_2SO_4$) at 323-333 K to form Nitrobenzene.

$$C_6H_6 + HNO_3 \xrightarrow{H_2SO_4, \Delta} \underset{\text{Nitrobenzene}}{C_6H_5NO_2} + H_2O$$

Electrophile: Nitronium ion ($NO_2^+$)

B. Halogenation

Benzene reacts with halogens ($Cl_2, Br_2$) in the presence of a Lewis acid catalyst ($FeCl_3, FeBr_3, AlCl_3$) to form Haloarenes.

$$C_6H_6 + Cl_2 \xrightarrow{Anhyd. AlCl_3} \underset{\text{Chlorobenzene}}{C_6H_5Cl} + HCl$$

Electrophile: Chloronium ion ($Cl^+$)

C. Sulphonation

Reaction with fuming sulphuric acid (Oleum) forms Benzene Sulphonic Acid.

$$C_6H_6 + H_2SO_4 (SO_3) \rightarrow \underset{\text{Benzene Sulphonic Acid}}{C_6H_5SO_3H}$$

Electrophile: Sulphur Trioxide ($SO_3$)

2. Friedel-Crafts Reactions

A. Alkylation

Treatment with alkyl halides in the presence of anhydrous $AlCl_3$ introduces an alkyl group.

$$C_6H_6 + CH_3Cl \xrightarrow{Anhyd. AlCl_3} \underset{\text{Toluene}}{C_6H_5CH_3} + HCl$$

B. Acylation

Treatment with acyl halides ($RCOCl$) or acid anhydrides introduces an acyl group.

$$C_6H_6 + CH_3COCl \xrightarrow{Anhyd. AlCl_3} \underset{\text{Acetophenone}}{C_6H_5COCH_3} + HCl$$

Mechanism Note: The Lewis Acid ($AlCl_3$) generates the electrophile (Carbocation $R^+$ or Acylium ion $RCO^+$) required to attack the ring.

3. Addition Reactions

Under drastic conditions (high temp/pressure/UV light), benzene can lose its aromaticity and undergo addition.

A. Addition of Hydrogen:

$$C_6H_6 + 3H_2 \xrightarrow{Ni, \Delta} \underset{\text{Cyclohexane}}{C_6H_{12}}$$

B. Addition of Chlorine: In the presence of UV light (h$\nu$), benzene adds 3 molecules of Chlorine.

$$C_6H_6 + 3Cl_2 \xrightarrow{UV, 500K} \underset{\text{Benzene Hexachloride (BHC)}}{C_6H_6Cl_6}$$

BHC is also known as Gammaxene (Insecticide).

4. Combustion

Benzene burns with a luminous, sooty flame due to its high carbon content. Complete combustion yields $CO_2$ and $H_2O$.

$$C_6H_6 + \frac{15}{2}O_2 \rightarrow 6CO_2 + 3H_2O$$

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