A 1-bromoalkylbenzene derivative is prepared by reacting a phenylalkene derivative with hydrogen bromide in the presence of a non-polar solvent. The phenylalkene derivative is prepared by reacting an alkenyl halide with metal magnesium to form a Grignard reagent, and then reacting the Grignard reagent with a benzyl halide derivative. An allyl Grignard reagent is prepared by reacting continuously an allyl halide derivative with metal magnesium in an organic solvent, in which the allyl halide derivative and metal magnesium are continuously added to the reaction system and the allyl Grignard reagent formed is continuously removed from the reaction system. The processes provide the intended compounds in high yields, high selectivities and high purities.