Question

21. Which is not one of the limitations usually associated with the Friedel-Crafts alkylation reaction. a. Reaction fails when strongly deactivating substituents are present. b. Rearrangements of the carbocation intermediate are possible; n-alkyl derivatives are unavailable. c. Multiple alkylations frequently occur. d. Long branched chains of carbons cannot be used. e. An amine nitrogen cannot be present on the ring.

And can you please explain why?

Answer 1

Long branched chains of carbaons cannot be used is not the limitation of Friedel-Crafts alkylation reaction.

Because, long branched chains of carbons can be added to the benzene ring by using Friedel-Crafts alkylation.. But, straight chains of carbons cannot be added to the benzene ring by using the above mentioned reaction.... because, straight chains of carbons form a primary carbocation which is unstable when compared to the secondary or teritiary carbocations formed from branched chains... So, addition of stable carbocations formed from branched chains of carbons can be done by Friedel-Crafts alkylation. For adding straight chains of carbons Friedel-Crafts acylation is used.