3,5-Dimethylpyrazole

3,5‑Dimethylpyrazole is an organic heterocyclic compound with the molecular formula C₅H₈N₂ and a molar mass of 96.13 g·mol⁻¹. It belongs to the pyrazole family, comprising a five‑membered aromatic ring that contains two adjacent nitrogen atoms. The compound is one of several isomeric dimethyl‑substituted pyrazoles, bearing methyl groups at the 3‑ and 5‑positions of the ring.

Chemical and physical properties

• Appearance: white solid
• Density: 1.027 g·cm⁻³ (20 °C)
• Melting point: 107.5 °C
• Boiling point: 218 °C (at reduced pressure)
• Solubility: readily soluble in polar organic solvents such as ethanol and acetone

The unsymmetrical substitution pattern gives the free base C₂ᵥ symmetry, while its conjugate acid (pyrazolium) and conjugate base (pyrazolide) adopt the same symmetry.

Synthesis
A common laboratory preparation involves the condensation of acetylacetone with hydrazine:

$$ \text{CH}_3\text{COCH}_2\text{COCH}_3 + \text{N}_2\text{H}_4 ;\longrightarrow; \text{(CH}_3\text{C)}_2\text{CHN}_2\text{H} + 2,\text{H}_2\text{O} $$

This reaction proceeds via cyclization of the diketone with the diamine, affording the pyrazole ring and eliminating water. The method is described in the Organic Syntheses literature.

Applications
3,5‑Dimethylpyrazole serves as a versatile intermediate in the synthesis of ligands used in coordination chemistry. Notably, it is a precursor to:

• Trispyrazolylborate ligands, which bind metal centers in a facial tridentate fashion.
• Trispyrazolylmethane ligands, employed in the preparation of metal carbonyl complexes.
• Pyrazolyldiphosphine ligands, utilized in bimetallic complex construction.

Additionally, the compound has been employed as a blocking agent for isocyanates in polymer chemistry, where it temporarily caps reactive isocyanate groups to control polymerization processes.

Safety and handling
The substance is classified as hazardous under GHS criteria, with relevant hazard statements (e.g., H302 – harmful if swallowed, H315 – causes skin irritation). Standard laboratory safety practices, including the use of gloves, goggles, and fume hoods, are recommended when handling the material.

References

1. Reger, D. L.; Grattan, T. C.; Brown, K. J.; Little, C. A.; Lamba, J. J. S.; Rheingold, A. L.; Sommer, R. D. (2000). “Syntheses of tris(pyrazolyl)methane ligands and {[tris(pyrazolyl)methane]Mn(CO)₃}SO₃CF₃ complexes: Comparison of ligand donor properties”. Journal of Organometallic Chemistry 607 (1–2): 120–128. DOI:10.1016/S0022-328X(00)00290-4.

2. Schenck, T. G.; Downes, J. M.; Milne, C. R. C.; MacKenzie, P. B.; Boucher, T. G.; Whelan, J.; Bosnich, B. (1985). “Bimetallic reactivity. Synthesis of bimetallic complexes containing a bis(phosphino)pyrazole ligand”. Inorganic Chemistry 24 (15): 2334–2337. DOI:10.1021/ic00209a003.

3. Johnson, W. S.; Highet, R. J. (1951). “3,5‑Dimethylpyrazole”. Organic Syntheses 31: 43. DOI:10.15227/orgsyn.031.0043.

4. Lanxess (2020). “Blocked Isocyanates” (PDF). Retrieved from https://www.lanxess.com.