Mechanical treatment of dry powder mixtures has often been reported to give new compounds. The assumption of neat mechanochemical conditions must, however, be made with caution even if the reactants are not crystal hydrates. The fluidised intermediate, which is often necessary for mechano-cocrystallisation of organic molecules, can readily form by interaction with atmospheric moisture. We illustrate this using "dry" mechanochemical synthesis of glycinium semi-malonate as an example. The α-glycine and malonic acid mixture is shown to be highly hygroscopic, which can be largely accredited to the hygroscopicity of malonic acid. Dissolution in atmospheric moisture is sufficient for the spontaneous crystallisation of the salt product. This is a prime example where liquid is both necessary for mechanosynthesis, while simultaneously hindering the reaction by affecting the rheology of the mixture: Liquid Hindered Grinding. These results shed new light on possible sources of regional, seasonal and temperature effects on mechanosynthesis, as well as a potential role for polymer additives. Fig. 1 Reaction scheme for the mechano-cocrystallisation of αGly + Mal to form glycinium semi-malonate.

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