Alkylation of benzene is catalyzed by

Analysis of the Question

The question asks for the specific catalyst required for the alkylation of benzene. This is a classic reaction in organic chemistry known as the Friedel-Crafts Alkylation.

Concept Explanation

The alkylation of benzene involves replacing a hydrogen atom on the benzene ring with an alkyl group (such as a methyl or ethyl group). Because benzene is a stable aromatic compound, it does not react easily with simple alkyl halides. To make the reaction happen, we need a Lewis acid catalyst.

A Lewis acid is an electron-pair acceptor. In the Friedel-Crafts reaction, the catalyst interacts with the alkyl halide to generate a strong electrophile (a carbocation), which is then reactive enough to attack the benzene ring.

Step-by-Step Solution

1. Identify the reaction type: The reaction is Friedel-Crafts Alkylation.
2. Recall the reagents: The reaction typically involves benzene and an alkyl halide (e.g., $CH_3Cl$).
3. Identify the catalyst: The reaction requires a strong Lewis acid to polarize the C-Cl bond in the alkyl halide. Common catalysts used for this purpose include Aluminium chloride ($AlCl_3$) or Iron(III) chloride ($FeCl_3$).
4. Evaluate the options: • Palladium (A) and Nickel (B): These are typically used as catalysts for hydrogenation (adding hydrogen) to unsaturated hydrocarbons, not for alkylation. • Sunlight (C): This is used for free-radical substitution in alkanes, not for electrophilic substitution in benzene. • Aluminium chloride (D): This is the standard Lewis acid catalyst used in Friedel-Crafts reactions.

Conclusion

It acts by accepting a lone pair of electrons from the chlorine atom of the alkyl halide, creating a highly reactive carbocation that proceeds to substitute the benzene ring.

Exam Tip

Whenever you see "Friedel-Crafts" in an organic chemistry question, immediately look for $AlCl_3$ or $FeCl_3$. These are the "powerhouse" catalysts for aromatic substitution reactions. Don't confuse them with hydrogenation catalysts like Nickel or Palladium!