In conclusion, we developed mechanochemical Grignard reactions with CO2 and SMC as C1-synthons to prepare carboxylic acids from organobromides. In contrast to standard methods of this type, the protocol requires the presence of only 2 equiv. of an ethereal additive, which presumably promotes the Schlenk equilibrium of the in-situ generated organomagnesium reagents. Intriguingly, lithium hydroxide proved superior over lithium chloride, which is commonly applied in Grignard reactions as activating agent. With methoxysubstituted aryl bromides, significant amounts of ketones are formed, contrasting observations made in solution-based Grignard reactions. With SMC as C1 source, this reactivity is less pronounced.