Structures of NH-pyrazoles bearing only C-methyl substituents: 4-methylpyrazole is a hydrogen-bonded trimer in the solid (100 K)

The X-ray molecular structure of 4-methylpyrazole (3) at 100 K has been determined. The compound crystallizes as a trimer with the N–H protons localized. 4-Methylpyrazole belongs to the series of eight NH-pyrazoles bearing only H and CH3 substituents on the carbon atoms. There are two pairs of tautomers so the number of pyrazoles existing in solution is only six. The thermal behavior of the six pyrazoles was investigated by differential scanning calorimetry and their melting points analyzed on the basis of their solid state structures and number and position of C-methyl substituents.