Pyrazole

Pyrazole  | Full structural formula | Skeletal formula with numbers

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| Ball-and-stick model | Space-filling model

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Names
Preferred IUPAC name

1H-Pyrazole[1]

Systematic IUPAC name

1,2-Diazacyclopenta-2,4-diene

Other names

1,2-Diazole

Identifiers

CAS Number

* 288-13-1 Y

3D model (JSmol)

* Interactive image

ChEBI |

* CHEBI:17241 Y

ChEMBL |

* ChEMBL15967 Y

ChemSpider |

* 1019 Y

DrugBank |

* DB02757 Y

KEGG |

* C00481 Y

PubChem CID

* 1048

UNII |

* 3QD5KJZ7ZJ Y

InChI

* InChI=1S/C3H4N2/c1-2-4-5-3-1/h1-3H,(H,4,5) Y

Key: WTKZEGDFNFYCGP-UHFFFAOYSA-N Y

* InChI=1/C3H4N2/c1-2-4-5-3-1/h1-3H,(H,4,5)

Key: WTKZEGDFNFYCGP-UHFFFAOYAW

SMILES

* c1cn[nH]c1

Properties

Chemical formula

| C3H4N2
Molar mass | 68.079 g·mol−1
Melting point |  66 to 70 °C (151 to 158 °F; 339 to 343 K)
Boiling point |  186 to 188 °C (367 to 370 °F; 459 to 461 K)
Basicity (pKb)  | 11.5

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Y verify (what is YN ?)
Infobox references

Pyrazole is an organic compound of azole group with the formula C3H3N2H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen atoms, which are in ortho-substitution. Pyrazole is a weak base, with pKb 11.5 (pKa of the conjugate acid 2.49 at 25 °C).[2] Pyrazoles are also a class of compounds that have the ring C3N2 with adjacent nitrogen atoms.[3] Notable drugs containing a pyrazole ring are celecoxib (celebrex) and the anabolic steroid stanozolol.

## Contents

* 1 Preparation and reactions
* 1.1 History
* 1.2 Conversion to scorpionates
* 1.3 3,5-Diphenyl-1H-pyrazole
* 2 Occurrence and uses
* 3 See also
* 4 References
* 5 Further reading

## Preparation and reactions

Pyrazoles are synthesized by the reaction of α,β-unsaturated aldehydes with hydrazine and subsequent dehydrogenation:[4]

Substituted pyrazoles are prepared by condensation of 1,3-diketones with hydrazine (Knorr-type reactions).[5] For example, acetylacetone and hydrazine gives 3,5-dimethylpyrazole:[6]

CH3C(O)CH2C(O)CH3 \+  N2H4 →  (CH3)2C3HN2H  \+  2 H2O

File:Novel pyrazole ligands.tif

### History

The term pyrazole was given to this class of compounds by German Chemist Ludwig Knorr in 1883.[7] In a classical method developed by German chemist Hans von Pechmann in 1898, pyrazole was synthesized from acetylene and diazomethane.[8]

### Conversion to scorpionates

Pyrazoles react with potassium borohydride to form a class of ligands known as scorpionate. Pyrazole itself reacts with potassium borohydride at high temperatures (~200 °C) to form a tridentate ligand known as Tp ligand:

### 3,5-Diphenyl-1H-pyrazole

3,5-Diphenyl-1H-pyrazole is produced when (E)-1,3-diphenylprop-2-en-1-one is reacted with hydrazine hydrate in the presence of elemental sulfur[9] or sodium persulfate,[10] or by using a hydrazone in which case an azine is produced as a by-product.[11]

## Occurrence and uses

Celecoxib, a pyrazole derivative used as an analgesic

In 1959, the first natural pyrazole, 1-pyrazolyl-alanine, was isolated from seeds of watermelons.[12][13]

In medicine, derivatives of pyrazole are widely used,[14] including celecoxib and similar COX-2 inhibitors, zaleplon, betazole, and CDPPB.[15]

The pyrazole ring is found within a variety of pesticides as fungicides, insecticides and herbicides,[14] including fenpyroximate, fipronil, tebufenpyrad and tolfenpyrad.[16] Pyrazole moieties are listed among the highly used ring systems for small molecule drugs by the US FDA[17]

3-(Difluoromethyl)-1-methyl-1H-pyrazole-4-carboxylic acid is used in the manufacture of six commercial fungicides which are inhibitors of succinate dehydrogenase.[18][19]

## See also

* 3,5-dimethylpyrazole
* Pyrazolidine, fully saturated analogue
* imidazole, structural analogue of pyrazole with two non-adjacent nitrogen atoms.
* isoxazole, another analogue, the nitrogen atom in position 1 replaced by oxygen.

## References

1. ↑ "Front Matter". Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 141. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
2. ↑ "Dissociation constants of organic acids and bases". http://sites.chem.colostate.edu/diverdi/all_courses/CRC reference data/dissociation constants of organic acids and bases.pdf.
3. ↑ Eicher, T.; Hauptmann, S. (2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications (2nd ed.). Wiley-VCH. ISBN 3-527-30720-6.
4. ↑ Schmidt, Andreas; Dreger, Andrij (2011). "Recent Advances in the Chemistry of Pyrazoles. Properties, Biological Activities, and Syntheses". Curr. Org. Chem. 15 (9): 1423–1463. doi:10.2174/138527211795378263.
5. ↑ Nozari, M., Addison, A., Reeves, G.T, Zeller, M., Jasinski, J.P., Kaur, M., Gilbert, J. G., Hamilton, C. R., Popovitch, J. M., Wolf, L. M., Crist, L. E., Bastida, N., (2018) Journal of heterocyclic Chemistry 55, 6, 1291-1307. https://doi.org/10.1002/jhet.3155.
6. ↑ Johnson, William S.; Highet, Robert J. (1963). "3,5-Dimethylpyrazole". Organic Syntheses. http://www.orgsyn.org/demo.aspx?prep=CV4P0351. ;  Collective Volume, 4, pp. 351
7. ↑ Knorr, L. (1883). "Action of ethyl acetoacetate on phenylhydrazine. I". Chemische Berichte 16: 2597–2599. doi:10.1002/cber.188301602194. https://zenodo.org/record/1425305.
8. ↑ von Pechmann, Hans (1898). "Pyrazol aus Acetylen und Diazomethan" (in German). Berichte der deutschen chemischen Gesellschaft 31 (3): 2950–2951. doi:10.1002/cber.18980310363. https://zenodo.org/record/1425922.
9. ↑ Outirite, Moha; Lebrini, Mounim; Lagrenée, Michel; Bentiss, Fouad (2008). "New one step synthesis of 3,5-disubstituted pyrazoles under microwave irradiation and classical heating". Journal of Heterocyclic Chemistry 45 (2): 503–505. doi:10.1002/jhet.5570450231.
10. ↑ Zhang, Ze; Tan, Ya-Jun; Wang, Chun-Shan; Wu, Hao-Hao (2014). "One-pot synthesis of 3,5-diphenyl-1H-pyrazoles from chalcones and hydrazine under mechanochemical ball milling". Heterocycles 89 (1): 103–112. doi:10.3987/COM-13-12867.
11. ↑ Lasri, Jamal; Ismail, Ali I. (2018). "Metal-free and FeCl3-catalyzed synthesis of azines and 3,5-diphenyl-1H-pyrazole from hydrazones and/or ketones monitored by high resolution ESI+-MS". Indian Journal of Chemistry, Section B 57B (3): 362–373. http://nopr.niscair.res.in/handle/123456789/43824.
12. ↑ Fowden; Noe; Ridd; White (1959). Proc. Chem. Soc.: 131.
13. ↑ Noe, F. F.; Fowden, L.; Richmond, P. T. (1959). "alpha-Amino-beta-(pyrazolyl-N) propionic acid: a new amino-acid from Citrullus vulgaris (water melon)". Nature 184 (4688): 69–70. doi:10.1038/184069a0. PMID 13804343. Bibcode: 1959Natur.184...69B.
14. ↑ 14.0 14.1 Kabi, Arup K.; Sravani, Sattu; Gujjarappa, Raghuram et al. (2022). "Overview on Biological Activities of Pyrazole Derivatives". Nanostructured Biomaterials. Materials Horizons: From Nature to Nanomaterials. pp. 229–306. doi:10.1007/978-981-16-8399-2_7. ISBN 978-981-16-8398-5.
15. ↑ Faria, Jéssica Venância; Vegi, Percilene Fazolin; Miguita, Ana Gabriella Carvalho; dos Santos, Maurício Silva; Boechat, Nubia; Bernardino, Alice Maria Rolim (1 November 2017). "Recently reported biological activities of pyrazole compounds". Bioorganic & Medicinal Chemistry 25 (21): 5891–5903. doi:10.1016/j.bmc.2017.09.035. ISSN 0968-0896. PMID 28988624.
16. ↑ FAO
17. ↑ Taylor, R. D.; MacCoss, M.; Lawson, A. D. G. J Med Chem 2014, 57, 5845.
18. ↑ Walter, Harald (2016). "Fungicidal Succinate-Dehydrogenase-Inhibiting Carboxamides". Bioactive Carboxylic Compound Classes: Pharmaceuticals and Agrochemicals. Wiley. pp. 405–425. doi:10.1002/9783527693931.ch31. ISBN 9783527339471.
19. ↑ Jeschke, Peter (2021). "Current Trends in the Design of Fluorine‐Containing Agrochemicals". Organofluorine Chemistry. Wiley. pp. 363–395. doi:10.1002/9783527825158.ch11. ISBN 9783527347117.

## Further reading

A. Schmidt; A. Dreger (2011). "Recent Advances in the Chemistry of Pyrazoles. Part 2. Reactions and N-Heterocyclic Carbenes of Pyrazole". Curr. Org. Chem. 15 (16): 2897–2970. doi:10.2174/138527211796378497.

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