Being guided by results reported by Knochel and co-workers on "Turbo-Grignard reagents", the effect of lithium chloride as additive in the first grinding step was tested. To our delight, this protocol modification increased the yield of 2a from 25% to 60% (Table , entries 4 and 6). A surprising discovery was then made by screening a range of lithium salts, which revealed LiOH to be superior over LiCl (63% of 2a, Table , entry 1). To the best of our knowledge, this observation is unprecedented, and we attribute this unexpected improvement by hydroxide to the unusual reaction conditions lacking standard interactions between a possible magnesiate and the surrounding solvent. Drying of the grinding vessel and the balls (apart from the lid and valve material containing low-melting plastics) by keeping it at 100 °C overnight prior to use had almost no effect on the yield of 2a (56%; Table ). In contrast, flushing the milling vessel with argon after the addition of 1a and the ethereal solvent proved crucial. Omitting this step and performing the magnesium insertion reaction in ambient atmosphere caused the formation of various side products, and the yield of 2a dropped to 14% (Table , entry 8). Table : Effect of changing the optimal reaction conditions of the mechanochemical Grignard reaction of 1a with CO2 in a planetary ball mill. Entry Modified conditions Yield [%] of 2a 1 None 63 Next, the substrate scope was investigated, and the results are shown in Scheme 2. Applying the optimal reaction conditions on aryl bromides other than 2a led to analogous yields for the sterically and electronically related 2-methyl-and 4-trifluoromethyl-substituted acids 2b and 2c, which were obtained in 67% and 71% yield, respectively. Interestingly, the number of electron-withdrawing halogen substituents on the phenyl ring had little to no effect on the yield of the acids. Thus, 4-chlorobenzoic acid (2d) was obtained in 49% yield, compared to 51% for both 4-fluoro-and perfluorobenzoic acids (2e and 2h, respectively). In the series of compounds with electron-donating substituents, sterically hindered 2,4,6-tri-isopropylphenyl bromide (1i) underwent the carboxylation to the respective acid 2i in 52% yield, while 4-tert-butylphenyl bromide (1f) formed 42% of 4-tert-butylbenzoic acid (2f). Using a naphthyl group instead of substituted phenyls generated 2-naphthoic acid (2l) in 44% yield. In the case of methyl 3-bromobenzoate (1g), only starting material was recovered after workup and drying in vacuo, suggesting that the magnesium insertion reaction had not occurred.