Acid-catalyzed additions and substitutions

The first step in the Friedel-Crafts synthesis is formation of an electrophile capable of electrophilic substitution in the aromatic ring. Such an electrophile (or cation) is generated from an alkyl halide by its reaction with a Lewis acid, in this case boron trifluoride. Boron trifluoride 

If the nonfluorinated compound 2-chloropropane reacted with benzene under similar conditions, the product would be isopropylbenzene. 

The parent compound, propylene, would react with benzene at the central carbon, because the protonation with the acid takes place at the carbon carrying two hydrogens, and the product would be isopropylbenzene. 

aluminum chloride abstracts the reactive benzylic fluorine from 1-phenylperfluoropropene and leaves a positively charged species that cyclizes to an indene derivative. The reactive benzylic fluorine atoms in this intermediate are replaced by chlorine [55] in a way similar to the replacement of fluorine by halogens in Explanations 22,23, and 24. 

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