We first set out to investigate the coupling of benzoic acid (1) and p-toluidine (2). After neat grinding equimolar amounts of 1 and 2 at room temperature for 1 h, powder X-ray diffraction (PXRD) analysis showed the formation of a new crystalline product (3, ESI Fig. ). However, 1 H Nuclear Magnetic Resonance (NMR) analysis indicated that the amide bond formation did not occur (ESI Fig. ). We monitored the assembly of 3 by in situ synchrotron PXRD which showed that 3 formed within the first minute of milling and persisted until 60 min of milling (ESI Fig. ). Crystal structure of 3 was solved from single-crystal X-ray diffraction data (SCXRD) and revealed the formation of an ammonium carboxylate salt between 1 and 2 in a 2:1.5 stoichiometry (ESI Table ). The asymmetric unit of 3 is composed of two molecules of 1 and one and a half molecules of 2 (ESI Fig. ). Interestingly, one molecule of protonated 2 forms hydrogen bonds with three deprotonated molecules of 1 (Fig. ), while a second molecule of 2 lies in the inversion centre and has positional disorder involving its amino and methyl groups (details can be found in ESI). Motivated by the formation of ammonium carboxylate salt in an unexpected stoichiometric ratio favouring an excess of 1, we performed a neat grinding experiment by milling 1 and 2 in a 2:1 stoichiometric ratio. Pleasingly, this resulted in a different crystalline product according to the PXRD analysis (4, ESI Fig. ).