Organomagnesium halides, better known as Grignard reagents, are formed by the exothermic reaction of metallic magnesium (usually turnings or powder) with a halide in an anhydrous, etheric solvent. Grignard reagents can be used for the formation of new carbon-carbon bonds and are one of the most important intermediates in such reactions. Both on a small scale in a glass flask in chemical laboratories, and on a larger scale in stirred vessels in the chemical industry, Grignard reagents are prepared in a dosing-controlled semi batch process based on placing a certain mass of magnesium together with a small amount of the halide in the solvent in the stirred vessel and then slowly adding the remaining halide-solvent mixture . Because of the exothermic nature of the reaction, the reaction temperatures must be closely controlled to avoid a thermal runaway and hazardous situations. A challenge of the synthesis of Grignard reagents is the Wurtz coupling product. This byproduct is formed by reaction of already-formed Grignard reagent with the halide reactant (figure ). The formation of this byproduct decreases the yield of the Grignard reagent and the purity of the product obtained. Thus, the formation of the Wurtz coupling product must be avoided as much as possible. The formation of the Wurtz coupling product can be reduced by various means: low reaction temperatures, low halide concentrations and low addition rates, as well as a high magnesium surface area and therewith mass transport . However, these approaches contradict the requirements of sustainability. A reduction of the halide concentration leads to an increase of the solvent content. Lowering of the cooling temperature causes reduced reaction rates and increasing operating costs. Some publications refer to changing the solvent from the commonly used tetrahydrofuran to the more expensive 2-methyltetrahydrofuran to minimize the amount of Wurtz coupling product. Considering the principles of green chemistry, the use of 2-methyltetrahydofuran is favourable, since the solvent can be made from renewable raw materials . But changing the solvent is not only more cost-intensive but also accompanied by a change in the whole reaction system. Since Grignard reagents are still purchasable mostly in tetrahydrofuran, the use of 2-methyltetrahydrofuran is not further considered in this publication. Alternatively, continuously operated, magnesium-filled plug flow reactor systems can be used to ideally keep contact times of the already-made Grignard reagent and the not yet reacted halide educt as short as possible. High surface-to-volume ratios, as they can be realized in micro-and millistructured reactors, ensure beneficial heat transfer to avoid hotspots within the magnesium bed which would enhance Wurtz coupling. Reaction networks involving consecutive reactions, like the Grignard reaction system, can be controlled by providing the required processing experience to the molecules. Thus, controlling the residence time distribution represents a meaningful strategy to achieve high yields and supress undesired byproduct formation. Despite the laminar flow conditions in small-sized channels, micro-and millistructured reactors frequently possess narrow residence time distributions, potentially approaching plug flow behaviour and the impact of this reactor technology to achieve a high degree of reaction control has been reported earlier. The residence time distribution in packed-bed reactors is more complicated than in empty channels. In part 1 of this contribution, the influence of the pumping characteristics and the properties of magnesium turnings on the residence time distribution in a reactor cartridge filled with metal turnings, has been reported. It was found that application of an oscillating flow influences the residence time distribution and axial a. Fraunhofer IMM, Carl-Zeiss-Strasse 18-20, 55129 Mainz/Germany. b. Ulm University, Helmholtz-Strasse 16, 89081 Ulm/Germany. * Corresponding authors: Gabriele Menges-Flanagan, gabriele.menges-flanagan@imm.fraunhofer.de Dirk Ziegenbalg, dirk.ziegenbalg@uni-ulm.de Electronic Supplementary Information (ESI) available: Distribution functions for residence time measurements, pictures of Grignard reagent synthesis in a stirred tank (semi-continuous batch mode). dispersion can be reduced by adjusting the amplitude and frequency of the oscillation. Since the magnesium reacts during the Grignard reagent synthesis, the entire magnesium bed is subject to constant changes. The impact of such changes on the residence time distribution and finally on the selectivity of the benchmark synthesis of benzylmagnesium bromide are studied in this work.