1. It is a well-established and frequently used Grignard synthesis (synthesis is among the 20 most cited Grignard reagents on the platform SciFinder 13 ). 2. It is known that this reaction does not tend to form the Wurtz coupling product 3. This is important for the RTD measurements since the Wurtz coupling product 3 exhibits bands in the IR spectrum at wavelengths very similar to the band positions of the IR spectrum of toluene, so the presence of 6 would thwart the calibration of the IR spectrometer for toluene. 3. The synthesis of benzylmagnesium bromide instead is prone to the formation of the Wurtz coupling product 6. Grignard reagent and Wurtz coupling product both exhibit bands in the IR spectrum at wavelengths similar to the band positions used to determine toluene concentration during RTD measurement. Again, this would thwart the calibration of the IR spectrometer. A typical temperature profile during the formation of 2 is shown in figure . Feeding of magnesium turnings into the reactor led to small temperature drops at sensor T4, indicating the contact of the thermoelement T4 with the colder magnesium turnings. If the boiling point of the chemicals involved is not exceeded, there is no formation of gas bubbles which would influence the flow behaviour within the reactor cartridge to a considerable extent. In contrast to the use of phenylmagnesium bromide synthesis for RTD measurements, influences of pump behaviour on the selectivity of Grignard reagent formation are most readily demonstrated with syntheses that exhibit a strong affinity for the formation of the Wurtz coupling product. It is reported that the synthesis of benzylmagnesium bromide 5 can sometimes yield the Wurtz product quantitatively 1 , making this synthesis a suited benchmark reaction. Using the same process parameters (flow rate, thermostatic bath temperature, magnesium mass, solvent) as for the synthesis of phenylmagnesium bromide 2 allows comparability of residence time distributions based on the Bodenstein numbers. Only a lower halide concentration was chosen (0.7 mol/L instead of 1.0 mol/L) to avoid precipitation of the Wurtz product 6 during the continuous syntheses. Benzyl alcohol, a typical product formed by reaction of the Grignard reagent with oxygen and subsequent hydrolysis , was detected in the reaction solution. The fraction of benzyl alcohol was found to be up to 19 % and could not be fully avoided. For the actual synthesis case, less active Grignard reagent is available for a subsequent synthesis (e.g. Grignard reaction) due to the presence of the benzyl alcohol. For studying the impact of the residence time distribution on the selectivity of the Grignard formation, the consecutive reaction is irrelevant.