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Chemical Properties of Alkyl Benzenes | chemca

Hydrocarbons

Chemical Properties of Alkyl Benzenes

Reactions of the Side Chain (Oxidation, Halogenation) and the Ring (Substitution).

By chemca Team • Updated Jan 2026

Alkyl benzenes (like Toluene) exhibit two types of chemical reactions: those involving the alkyl side chain and those involving the benzene ring. The alkyl group acts as an activating, ortho-para directing group for ring substitutions.

1. Oxidation of Side Chain

Strong Oxidation to Benzoic Acid

Strong oxidizing agents like alkaline $KMnO_4$ or acidic $K_2Cr_2O_7$ oxidize the entire alkyl side chain directly to the carboxylic acid group ($-COOH$), regardless of the chain length.

$$ \underset{\text{Toluene}}{C_6H_5-CH_3} \xrightarrow{KMnO_4/KOH, \Delta} C_6H_5-COOK \xrightarrow{H_3O^+} \underset{\text{Benzoic Acid}}{C_6H_5-COOH} $$

For Longer Chains (e.g., Ethylbenzene):

$$ C_6H_5-CH_2-CH_3 \xrightarrow{[O]} C_6H_5-COOH + CO_2 + H_2O $$

Critical Condition: Oxidation occurs ONLY if the alkyl group has at least one benzylic hydrogen (H on the carbon attached to the ring).
Example: Tert-butyl benzene ($C_6H_5-C(CH_3)_3$) does not undergo oxidation because it lacks benzylic hydrogen.

2. Free Radical Halogenation

Side Chain Substitution

Reaction with chlorine or bromine in the presence of UV light ($h\nu$) or heat leads to substitution at the benzylic position via a free radical mechanism.

$$ C_6H_5CH_3 + Cl_2 \xrightarrow{h\nu} \underset{\text{Benzyl Chloride}}{C_6H_5CH_2Cl} \xrightarrow{Cl_2} C_6H_5CHCl_2 \xrightarrow{Cl_2} \underset{\text{Benzotrichloride}}{C_6H_5CCl_3} $$

Note: In the absence of light and presence of a Lewis acid ($FeCl_3$), ring substitution occurs instead.

3. Electrophilic Substitution (Ring Reactions)

Ortho-Para Directing Nature

The alkyl group is electron-releasing (+I effect, Hyperconjugation). It activates the benzene ring and directs incoming electrophiles to the Ortho and Para positions.

A. Nitration

$$ \underset{\text{Toluene}}{C_6H_5CH_3} + HNO_3 \xrightarrow{H_2SO_4} \underset{\text{o-Nitrotoluene (Minor)}}{o-NO_2-C_6H_4CH_3} + \underset{\text{p-Nitrotoluene (Major)}}{p-NO_2-C_6H_4CH_3} $$

B. Halogenation

$$ C_6H_5CH_3 + Cl_2 \xrightarrow{FeCl_3, Dark} \text{o-Chlorotoluene} + \text{p-Chlorotoluene} $$

Note: While alkyl groups activate the ring, they are ortho-para directors. Usually, the Para isomer is the major product due to less steric hindrance compared to the Ortho isomer.

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