Friedel-Crafts Reaction - Practice Questions & MCQ

This reaction allowed for the formation of alkyl benzenes from alkyl halides. The reactivity of haloalkanes increases as we move up the periodic table and increase polarity. This means that an RF haloalkane is most reactive followed by RCl than RBr and finally RI. This means that the Lewis acids used as catalysts in Friedel-Crafts Alkylation reactions tend have similar halogen combinations such as BF₃, SbCl₅, AlCl₃, SbCl₅, and AlBr₃, all of which are commonly used in these reactions.

For example:

Mechanism

1. In this step a carbocation is formed that acts as the electrophile in the reaction. This step activates the haloalkane. Secondary and tertiary halides only form the free carbocation in this step.

2. In this step, electrophilic attack on the benzene occurs that results in multiple resonance forms. The halogen reacts with the intermediate and picks up the hydrogen to eliminate the positive charge. The aromaticity is regained by the substrate after this step.

This electrophilic aromatic substitution allows the synthesis of monoacylated products from the reaction between arenes and acyl chlorides or anhydrides. The products are deactivated, and do not undergo a second substitution.

For example: