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Piperidine
IUPAC name	Piperidine
Other names	Hexahydropyridine Azacyclohexane, pentamethyleneamine
Identifiers
CAS number	[110-89-4]
RTECS number	TM3500000
SMILES	C1CCCCN1
InChI	1/C5H11N/c1-2- 4-6-5-3-1/h6H,1-5H2
Properties
Molecular formula	C5H11N
Molar mass	85.15 g/mol
Appearance	colourless liquid
Density	0.862 g/ml, liquid
Melting point	−7 °C
Boiling point	106 °C
Solubility in water	miscible
Acidity (pKa)	11.24
Viscosity	1.573 cP at 25 °C
Hazards
EU classification	Flammable (F) Toxic (T)
NFPA 704	3 3 3
R-phrases	R11, R23/24, R34
Related compounds
Related compounds	pyridine pyrrolidine piperazine
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Piperidine is an organic compound with the molecular formula (CH₂)₅NH. This heterocyclic amine consists of a six-membered ring containing five methylene units and one nitrogen atom. It is a colorless fuming liquid with an odor described as ammoniacal, pepper-like;^[1] the name comes from the genus name Piper, which is the Latin word for pepper.^[2] Piperidine is a widely used building block and chemical reagent in the synthesis of organic compounds, including pharmaceuticals.

Production

Industrially, piperidine is produced by the hydrogenation of pyridine, usually over a molybdenum sulfide catalyst:^[3]

C₅H₅N + 3 H₂ → C₅H₁₀NH

Pyridine can also be reduced to piperidine by sodium in ethanol.^[4]

Natural occurrence of piperidine and derivatives

Piperidine itself has been obtained from pepper^[5], from Psilocaulon absimile N.E.Br (Aizoaceae)^[6], and in Petrosimonia monandra.^[7]

The piperidine structural motif is present in numerous natural alkaloids. These include piperine, which gives black pepper the hot taste. This gave the compound its name. Other examples are the fire ant toxin solenopsin^[8], the nicotine analog anabasine of the Tree Tobacco (Nicotiana glauca), lobeline of the indian tobacco, the toxic alkaloid coniine from poison hemlock, which was used to put Socrates to death.^[9]

Conformation

Piperidine prefers a chair conformation, similar to cyclohexane. Unlike cyclohexane, piperidine has two distinguishable chair conformations: one with the N–H bond in an axial position, and the other in an equatorial position. After much controversy during the 1950s–1970s, the equatorial conformation was found to be more stable by 0.72 kcal/mol in the gas phase.^[10] In nonpolar solvents, a range between 0.2 and 0.6 kcal/mol has been estimated, but in polar solvents the axial conformer may be more stable.^[11] The two conformers interconvert rapidly through nitrogen inversion; the free energy activation barrier for this process, estimated at 6.1 kcal/mol, is substantially lower than the 10.4 kcal/mol for ring inversion.^[12] In the case of N-methylpiperidine, the equatorial conformation is preferred by 3.16 kcal/mol,^[10] which is much larger than the preference in methylcyclohexane, 1.74 kcal/mol.

axial conformation	equatorial conformation

Reactions

Piperidine is a widely used secondary amine. It is widely used to convert ketones to enamines.^[13] Enamines derived from piperidine can be used in the Stork enamine alkylation reaction.^[14]

Piperidine can be converted to the chloramine C₅H₁₀NCl with calcium hypochlorite. The resulting chloramine undergoes dehydrohalogenation to afford the cyclic imine.^[15]

Uses

Piperidine is used as a solvent and as a base. The same is true for certain derivatives: N-formylpiperidine is a polar aprotic solvent with better hydrocarbon solubility than other amide solvents, and 2,2,6,6-tetramethylpiperidine is highly sterically hindered base, useful because of its low nucleophilicity and high solubility in organic solvents.

A significant industrial application of piperidine is for the production of dipiperidinyl dithiuram tetrasulfide, which is used as a rubber vulcanization accelerator.^[3]

Otherwise piperidine and its derivatives are ubiquitous building blocks in the synthesis of pharmaceuticals and fine chemicals. The piperidine structure is e.g. found in the pharmaceuticals risperidone, raloxifene, minoxidil, thioridazine, haloperidol, droperidol, mesoridazine, meperidine, melperone the psychochemical agents Ditran-B (JB-329), N-methyl-3-piperidyl benzilate (JB-336) and in many others.

Piperidine is also commonly used in chemical degradation reactions, such as the sequencing of DNA in the cleavage of particular modified nucleotides. Piperidine is also commonly used as a base for the deprotection of Fmoc-amino acids used in solid-phase peptide synthesis.

Piperidine is listed as a Table II precursor under the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances due to its use (peaking in the 1970s) in the clandestine manufacture of PCP (also known as angel dust).^[16]

References

1. ^ Frank Johnson Welcher (1947). Organic Analytical Reagents. D. Van Nostrand, 149. 
2. ^ Alexander Senning (2006). Elsevier's Dictionary of Chemoetymology. ISBN 0444522395. 
3. ^ ^{a b} Karsten Eller, Erhard Henkes, Roland Rossbacher, Hartmut Höke “Amines, Aliphatic” Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH. doi:10.1002/14356007.a02_001
4. ^ C. S. Marvel and W. A. Lazier (1941). "Benzoyl Piperidine". Org. Synth.; Coll. Vol. 1: 99. 
5. ^ Spaeth and Englaender, Ber.1935,68, 2218; cf. Pictet and Pictet, Helv. Chim. Acta, 1927, 10, 593
6. ^ Rimington, S. Afr. J. Sci, 1934, 31, 184
7. ^ Juraschewski and Stepanov, J. Gen. Chem., U.R.S.S., 1939, 9, 1687
8. ^ Arbiser JL, Kau T, Konar M et al. (2007). "Solenopsin, the alkaloidal component of the fire ant (Solenopsis invicta), is a naturally occurring inhibitor of phosphatidylinositol-3-kinase signaling and angiogenesis". Blood 109 (2): 560–5. doi:10.1182/blood-2006-06-029934. PMID 16990598. 
9. ^ The Plant Alkaloids, Thomas Anderson Henry, 4th ed. 1949, The Blakiston Company
10. ^ ^{a b} Luis Carballeira, Ignacio Pérez-Juste (1998). "Influence of calculation level and effect of methylation on axial/equatorial equilibria in piperidines". Journal of Computational Chemistry 19 (8): 961–976. doi:10.1002/(SICI)1096-987X(199806)19:8<961::AID-JCC14>3.0.CO;2-A. 
11. ^ Ian D. Blackburne, Alan R. Katritzky, Yoshito Takeuchi (1975). "Conformation of piperidine and of derivatives with additional ring hetero atoms". Acc. Chem. Res. 8 (9): 300–306. doi:10.1021/ar50093a003. 
12. ^ F.A.L. Anet, Issa Yavari (1977). "Nitrogen inversion in piperidine". J. Am. Chem. Soc. 99 (8): 2794–2796. doi:10.1021/ja00450a064. 
13. ^ Vinayak V. Kane and Maitland Jones Jr (1990). "Spiro[5.7]trideca-1,4-dien-3-one". Org. Synth.; Coll. Vol. 7: 473. 
14. ^ March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure Michael B. Smith, Jerry March Wiley-Interscience, 5th edition, 2001, ISBN 0-471-58589-0
15. ^ George P. Claxton, Lloyd Allen, and J. Martin Grisar (1988). "2,3,4,5-Tetrahydropyridine trimer". Org. Synth.; Coll. Vol. 6: 968. 
16. ^ List of Precursors and Chemicals Frequently Used in the Illicit Manufacture of Narcotic Drugs and Psychotropic Substances Under International Control, International Narcotics Control Board

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