Synthesis of Haloperidol with 4-chlorobutanoyl chloride and piperidin-4-one
The reaction mechanism for the step in question simply involves initial formation of a Grignard reagent followed by nucleophilic attack into the ketone. 



When forming Grignard reagents from halides, there is a general trend that the rates of formation are in the order  I > Br >> Cl, making it possible to get selectivity where multiple halides exist.