Benzene can be converted to its derivative toluene by the addition of a methyl group. The reaction is an example of

Analysis

The question asks for the classification of the reaction where benzene is converted to toluene (methylbenzene) by adding a methyl group. This specific process is known as the Friedel-Crafts alkylation.

Concept Explanation

Benzene is an aromatic hydrocarbon characterized by a stable, delocalized pi-electron cloud. Because of this stability, benzene does not typically undergo addition reactions (which would break the aromatic ring). Instead, it undergoes electrophilic aromatic substitution.

In the Friedel-Crafts alkylation:

1. A methylating agent (like chloromethane, $CH_3Cl$) reacts with a catalyst (like anhydrous $AlCl_3$) to generate a strong electrophile: the methyl carbocation ($CH_3^+$).
2. This electrophile is attracted to the electron-rich benzene ring.
3. The electrophile attacks the ring, temporarily forming an intermediate, and then a hydrogen atom is displaced (substituted) from the benzene ring to restore aromaticity.

Step-by-Step Solution

1. Identify the Reactants: Benzene ($C_6H_6$) + Methylating agent ($CH_3Cl$).
2. Identify the Catalyst: Anhydrous Aluminum Chloride ($AlCl_3$).
3. The Mechanism: The $CH_3^+$ electrophile replaces one of the hydrogen atoms on the benzene ring.
4. The Product: Toluene ($C_6H_5CH_3$) + Hydrogen Chloride ($HCl$).
5. Classification: Since a hydrogen atom on the benzene ring is replaced by a methyl group ($CH_3$) and the aromatic ring structure is preserved, the reaction is a substitution reaction. Because the incoming group is an electrophile, it is specifically an electrophilic substitution.

Conclusion

Option A is incorrect because benzene is electron-rich and repels nucleophiles. Option B is incorrect because we are not removing atoms to form a double or triple bond. Option D is incorrect because addition would destroy the aromatic stability of the benzene ring.

The correct classification is Electrophilic substitution.

Exam Tip

Whenever you see "Friedel-Crafts" (whether alkylation or acylation) in a Chemistry question, immediately associate it with Electrophilic Aromatic Substitution. Remember: Benzene loves to substitute, not add!