Preparation of Alkyl Benzenes

Synthesis via Friedel-Crafts, Wurtz-Fittig, and Acylation-Reduction.

By chemca Team • Updated Jan 2026

Alkyl benzenes (Arenes) contain an alkyl group attached to the benzene ring. While Toluene ($C_6H_5CH_3$) is the simplest member, preparing longer straight-chain alkyl benzenes requires specific strategies to avoid rearrangement.

1. Friedel-Crafts Alkylation

Standard Alkylation

Benzene reacts with alkyl halides in the presence of a Lewis Acid catalyst (Anhydrous $AlCl_3$) to form alkyl benzenes.

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

Major Limitation (Rearrangement):
Reaction with n-Propyl chloride yields Isopropyl benzene (Cumene) instead of n-Propyl benzene. The primary carbocation rearranges to a more stable secondary carbocation via hydride shift.

$CH_3CH_2CH_2^+ \xrightarrow{H^- shift} CH_3C^+HCH_3$

2. Wurtz-Fittig Reaction

Coupling Reaction

A mixture of an aryl halide and an alkyl halide reacts with Sodium metal in dry ether to yield alkyl benzene. This method is useful for attaching a specific alkyl group without rearrangement.

$$ \underset{\text{Bromobenzene}}{C_6H_5Br} + 2Na + \underset{\text{Bromoethane}}{C_2H_5Br} \xrightarrow{\text{Dry Ether}} \underset{\text{Ethyl Benzene}}{C_6H_5-C_2H_5} + 2NaBr $$

3. Acylation followed by Reduction (Best Method)

For n-Alkyl Benzenes

To prepare straight-chain alkyl benzenes (like n-Propyl benzene) without rearrangement, benzene is first acylated to form a ketone, which is then reduced.

Step 1: Friedel-Crafts Acylation

$$ C_6H_6 + CH_3CH_2COCl \xrightarrow{AlCl_3} \underset{\text{Propiophenone}}{C_6H_5-CO-CH_2CH_3} $$

Step 2: Reduction (Clemmensen or Wolff-Kishner)

$$ C_6H_5-CO-CH_2CH_3 \xrightarrow{Zn-Hg/HCl} \underset{\text{n-Propyl Benzene}}{C_6H_5-CH_2CH_2CH_3} $$

Clemmensen Reduction: $Zn-Hg$ + Conc. $HCl$ (Acidic medium).
Wolff-Kishner Reduction: $NH_2NH_2$ + $KOH$/Glycol (Basic medium).

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