Hydrogen bonding plays a key role in molecular recognition and crystal engineering research.[1] Aminopyrimidine carboxylate interactions are of fundamental importance since they are involved in protein-nucleic acids recognition and protein-drug binding.[2,3] 2-Amiopyrimidine forms 1:1 adduct with different mono and dicarboxylic acid rather than individual self assemble compounds.[4] The adduct of carboxylic acid with 2-Aminoheterocyclic ring system form a graph-set motif of R22(8) . [5] This motif is very robust in aminopyrimidine carboxylic acid /carboxylate systems. The hydrogen bond patterns in the crystal structure of 2-amino-4-methylpyrimidine 2-nitrobenzoic acid and 2-amino-4-methylpyrimidine 2- chlorobenzoic acid have been investigated by X-ray crystallography studies, NMR and FT-IR spectroscopy. We have prepared and determined two crystal structures of the acid-base co crystal consisting of 2-amino-4-methlpyrimidine, 2-nitrobenzoic acid compound (I) and 2-amino-4-methylpyrimidine, 2-chlorobenzoic acid compound (II). In the compound (I) the asymmetric unit consists of one 2-amino-4-methylpyrimidie and one 2-nitrobenzoic acid molecules. In the compound (II) the asymmetric unit consists of one 2-amino-4-methylpyrimidine and two 2-chlorobenzoic acid. In the compounds I & II, two of intermolecular N---H…O and O---H…N hydrogen bonds to form a cyclic hydrogen-bonded R22(8) ring motif.[5] In the crystal structure compound I , another type of R22(8) motif is formed by centrosymmetrically related 2-amino-4-methylpyrimidine molecules via N---H…N hydrogen bonds. Further the crystal structure is stabilized by intermolecular weak C---H…O hydrogen bonds and 𝜋 − 𝜋 stacking interactions.In compound II, these motifs centrosymmetrically connected via C---H…O hydrogen bond to form R64 (20) ring motifs. Further, the crystal structure stabilized by weak C---H…𝜋 and 𝜋 − 𝜋 stacking interactions [cg1-cg1=3.5284(9); cg1=(C6A-C11A)], [cg2-cg3=3.5591(8); cg2=( C6B-C11B); cg3=(C1-C
