Which reagent acts as the Lewis acid catalyst in the Friedel–Crafts reactions described?

Master NCEA Level 3 Organic Chemistry Reaction Schemes. Prepare with multiple choice questions, complete with hints and detailed explanations. Ace your exam with our comprehensive tools!

Multiple Choice

Which reagent acts as the Lewis acid catalyst in the Friedel–Crafts reactions described?

The basic idea being tested is how Friedel–Crafts reactions are driven by a Lewis acid that activates the electrophile. The Lewis acid coordinates to the electrophile’s leaving group (in an alkylation) or to the carbonyl of an acyl chloride (in an acylation), forming a highly reactive electrophile such as a carbocation-like species or an acylium ion. This makes it much easier for the benzene ring to attack, generating the substituted product and regenerating the Lewis acid at the end. Aluminum chloride is the classic catalyst because of its strong Lewis acidity and its ability to stabilize the activated electrophile via complexation, producing the reactive species in situ and then being regenerated. That makes the reaction efficient in catalytic amounts. H2SO4, while a strong acid, is Brønsted acidic rather than a Lewis acid, and would not provide the same activation pathway for the typical Friedel–Crafts electrophile. Other Lewis acids like FeCl3 or ZnCl2 can serve as catalysts in some Friedel–Crafts systems, but the standard description and most classic examples use aluminum chloride, which is why it’s the best choice here.

The basic idea being tested is how Friedel–Crafts reactions are driven by a Lewis acid that activates the electrophile. The Lewis acid coordinates to the electrophile’s leaving group (in an alkylation) or to the carbonyl of an acyl chloride (in an acylation), forming a highly reactive electrophile such as a carbocation-like species or an acylium ion. This makes it much easier for the benzene ring to attack, generating the substituted product and regenerating the Lewis acid at the end.

Aluminum chloride is the classic catalyst because of its strong Lewis acidity and its ability to stabilize the activated electrophile via complexation, producing the reactive species in situ and then being regenerated. That makes the reaction efficient in catalytic amounts.

H2SO4, while a strong acid, is Brønsted acidic rather than a Lewis acid, and would not provide the same activation pathway for the typical Friedel–Crafts electrophile. Other Lewis acids like FeCl3 or ZnCl2 can serve as catalysts in some Friedel–Crafts systems, but the standard description and most classic examples use aluminum chloride, which is why it’s the best choice here.

More practice questions