**3. Results and Discussion**

The cocrystallization processes were carried out considering the p*K*a of the mandelic acid and the pyridinecarboxamide isomers as coformers, having the p*K*<sup>a</sup> values of 3.85 (**DL-H2ma**), based on the carboxylic group [34], 2.10 (**pic**), 3.35 (**nam**) and 3.61 (**inam**), based on pyridine nitrogen [35]. These compounds were chosen to evaluate the degree of acid proton transfer to the coformers, according to the Δp*K*<sup>a</sup> rule, which can contribute to the study of the salt/cocrystal continuum and provide information related to the ability to predict and control synthesis of cocrystals that contain mandelic acid [36]. According to this rule, it is generally accepted that a salt is formed when the value of Δp*K*<sup>a</sup> is greater than 3, while a value of Δp*K*<sup>a</sup> less than 0 should lead to the formation of cocrystals [37]. The values of Δp*K*<sup>a</sup> (p*K*a(protonated base)-p*K*a(acid)) calculated for **pic**, **nam** and **inam** are −1.75, −0.50 and −0.24, respectively, so the formation of cocrystals should be expected.

The binary solid forms were characterized using NMR (Supplementary Information, Figures S1–S11) and IR (Supplementary Information, Figures S13 and S14) spectroscopy, powder X-ray diffraction (Supplementary Information, Figure S12), and thermal DSC technique. The (**pic**)-(**D-H2ma**) (**1**) and (**inam**)-(**L-H2ma**) (**3**) crystal structures were established using the single crystal X-ray diffraction technique. The crystal data, experimental details and refinement results are summarized in Table 1. Data of (**nam**)-(L-H2ma) (**2**) and its crystal structure was taken from the bibliography (JILZOU01 [32]).
