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The formation of Grignard reagents from metallic magnesium and a halide is often accompanied by the formation of the Wurtz coupling product, an undesired side product formed by the reaction of a Grignard reagent molecule with a halide molecule. By using a scale-up approach from semibatch type synthesis to continuous lab and pilot scale for various Grignard reagents, it is demonstrated that a continuous production process can improve Grignard reagent selectivity and reduce Wurtz coupling.

Grignard reagent formation in continuously operated magnesium packed-bed reactors can be influenced by fine tuning the residence time distribution within the magnesium packing. By decreasing the magnesium turning size and increasing the packing density, narrower residence time distributions and therefore improved Bodenstein numbers can be obtained. The utilized pump system and its induced flow behaviour also have an impact on the residence time distributions in packed-bed reactors. By using oscillatory flow rates instead of pulsation-free pumps, Bodenstein numbers within a magnesium filled reactor cartridge can be increased by 25% for fine magnesium turnings and by 70% for coarse magnesium turnings, resulting in minimized backmixing and approaching plug flow behaviour.

Grignard reagents are used as intermediates in the production of complex molecules, since they can be used to form new carbon–carbon bonds, e.g. in the formation of active pharmaceutical ingredients. Side product formation like Wurtz coupling diminishes the selectivity in Grignard reagent formation and therefore side product formation needs to be reduced. It was found that the different pumping behaviours of a syringe pump, a valveless rotary piston pump and a micro annular gear pump and the obtained residence time distributions have an impact on the selectivity of the Grignard reagent formation. Selectivity can also be enhanced by the available magnesium surface and by choosing a tubular flow reactor instead of a batch reactor, showing the importance of choosing the right equipment and parameters for the specific reaction system.