pyrrole
American Heritage Dictionary:
Top
pyr·role
Top
One of a group of organic compounds containing a doubly unsaturated five-membered ring in which nitrogen occupies one of the ring positions. Pyrrole (1) is a representative compound. The pyrrole system is found in the green leaf pigment, chlorophyll, in the red blood pigment, hemoglobin, and in the blue dye, indigo. Interest in these colored bodies has been largely responsible for the intensive study of pyrroles. Tetrahydropyrrole, or pyrrolidine (2), is part of the structures of two protein amino acids, proline and hydroxyproline. 1
Pyrrole is a liquid that darkens and resinifies on standing in air, and that polymerizes quickly when treated with mineral acid. Familiar substitution processes, such as halogenation, nitration, sulfonation, and acylation, can be realized. Pyrrole, by virtue of its heterocyclic nitrogen, is very weakly basic. The hydrogen at the 1 position is removable as a proton, and accordingly, pyrrole is also an acid, although a weak one.
Pyrrolidine can be prepared by catalytic hydrogenation of pyrrole or by ring-closure reactions. Another derivative of pyrrole, 2-ketopyrrolidine, or pyrrolidone, is of considerable interest in connection with the preparation of polyvinylpyrrolidone. Pyrrolidone is combined with acetylene to form vinylpyrrolidone; polymerization of this material furnishes polyvinylpyrrolidone, which is suitable for maintaining osmotic pressure in blood and so acting as an extender for plasma or whole blood. See also Pyridine.
Wiley Dictionary of Flavors:
Top
Pyrrole
Five-membered ring with two double bonds, one nitrogen, and four carbons.
Saunders Veterinary Dictionary:
Top
pyrrole
A basic, cyclic substance, obtained by destructive distillation of various animal substances. Critical components in the synthesis of porphyrins.
Wikipedia on Answers.com:
Top
Pyrrole
| Pyrrole | |
|---|---|
 |
 |
 |
 |
|
1H-Pyrrole |
|
| Identifiers | |
| CAS number | 109-97-7  |
| PubChem | 8027 |
| ChemSpider | 7736 |
| UNII | 86S1ZD6L2C |
| EC number | 203-724-7 |
| UN number | 1992, 1993 |
| ChEBI | CHEBI:19203 |
| ChEMBL | CHEMBL16225 |
| RTECS number | UX9275000 |
| Beilstein Reference | 1159 |
| Gmelin Reference | 1705 |
| Jmol-3D images | Image 1 Image 2 |
|
|
|
|
| Properties | |
| Molecular formula | C4H5N |
| Molar mass | 67.09 g mol−1 |
| Density | 0.967 g cm-3 |
| Melting point |
−23 °C, 250 K, -9 °F |
| Boiling point |
129-131 °C, 402-404 K, 264-268 °F |
| Vapor pressure | 7 mmHg at 23 °C |
| Viscosity | 0.001225 Pa s |
| Thermochemistry | |
| Std enthalpy of formation ΔfH |
108.2 kJ mol-1 (gas) |
| Std enthalpy of combustion ΔcH |
2242 kJ mol-1 |
| Specific heat capacity, C | 1.903 J k-1 mol k-1 |
| Hazards | |
| NFPA 704 |
 2
2
0
|
| Flash point | 33.33 °C |
| Autoignition temperature |
550 °C |
| Explosive limits | 3.1-14.8% |
 (verify) (what is:  / ?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
|
| Infobox references | |
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH.[1] It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3. Porphobilinogen, a trisubstituted pyrrole, is the biosynthetic precursor to many natural products such as heme.[2]
Pyrroles are components of more complex macrocycles, including the porphyrins of heme, the chlorins, bacteriochlorins, chlorophyll, porphyrinogens.[3]
|
Contents
|
Properties
Pyrrole has very low basicity compared to conventional amines and some other aromatic compounds like pyridine. This decreased basicity is attributed to the delocalization of the lone pair of electrons of the nitrogen atom in the aromatic ring. Pyrrole is a very weak base with a pKaH of about −1 to -2. Protonation results in loss of aromaticity, and is, therefore, unfavorable.
Like many amines, pyrrole slowly decomposes on exposure to air and light. It turns brown over time due to accumulation of impurities such as polypyrrole and various amine oxides. It is usually purified by distillation immediately before use.[4]
-
Impure pyrrole being distilled to separate it from colored impurities
-
Pure pyrrole collected from the still is now colorless and transparent to all wavelengths of visible light due to the removal of impurities
Synthesis
Pyrrole is prepared industrially by treatment of furan with ammonia in the presence of solid acid catalysts.[5]
One synthetic route to pyrrole involves the decarboxylation of ammonium mucate, the ammonium salt of mucic acid. The salt is typically heated in a distillation setup with glycerol as a solvent.[6]
Substituted pyrroles
Many methods exist for the organic synthesis of pyrrole derivatives. Classic "named reactions" are the Knorr pyrrole synthesis, the Hantzsch pyrrole synthesis, and the Paal-Knorr synthesis. More specialized methods are listed here.
The starting materials in the Piloty-Robinson pyrrole synthesis are 2 equivalents of an aldehyde and hydrazine.[7][8] The product is a pyrrole with specific substituents in the 3 and 4 positions. The aldehyde reacts with the diamine to an intermediate di-imine (R–C=N−N=C–R), which, with added hydrochloric acid, gives ring-closure and loss of ammonia to the pyrrole.
In one modification, propionaldehyde is treated first with hydrazine and then with benzoyl chloride at high temperatures and assisted by microwave irradiation:[9]
In the second step, a [3,3]sigmatropic reaction takes place between two intermediates.
Pyrrole can be polymerized to form polypyrrole.
Reactivity
The NH proton in pyrroles is moderately acidic with a pKa of 16.5. Pyrrole can be deprotonated with strong bases such as butyllithium and sodium hydride. The resulting alkali pyrrolide is nucleophilic. Treating this conjugate base with an electrophile such as methyl iodide gives N-methylpyrrole.
-
Resonance Contributors of Pyrrole
The resonance contributors of pyrrole provide insight to the reactivity of the compound. Like furan and thiophene, pyrrole is more reactive than benzene towards electrophilic aromatic substitution because it is able to stabilize the positive charge of the intermediate carbocation.
Pyrrole undergoes electrophilic aromatic substitution predominantly at the 2 and 5 positions. Two such reactions that are especially significant for producing functionalized pyrroles are the Mannich reaction and the Vilsmeier-Haack reaction (depicted below)[10][11], both of which compatible with a variety of pyrrole substrates.
-
Formylation of a pyrrole derivative (Garabatos-Perera 2007[10])
Pyrroles react with aldehydes to form porphyrins. For example, benzaldehyde condenses with pyrrole to give tetraphenylporphyrin. Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as under Lewis acid catalysis, heating, or high pressure.
Pyrrole polymerizes in light. An oxidizing agent, such as ammonium persulfate, can also be used, typically at 0 ºC and in darkness to control the polymerization.
Commercial uses
Pyrrole has no significant commercial application other than being used as an additive to cigarettes. N-methylpyrrole is a precursor to N-methylpyrrolecarboxylic acid, a building-block in pharmaceutical chemistry.[5]
Analogs and derivatives
Structural analogs of pyrrole include:
- Pyrroline, a partially saturated analog with one double bond
- Pyrrolidine, the saturated hydrogenated analog
- Kryptopyrrole, a pyrrole derivative once thought to be associated with schizophrenia
Heteroatom structural analogs of pyrrole include:
- Arsole, a moderately-aromatic arsenic analog
- Bismole, a bismuth analog
- Borole, a boron analog
- Furan, an aromatic oxygen analog
- Gallole, a gallium analog
- Germole, a germanium analog
- Phosphole, a non-aromatic phosphorus analog
- Pyrazole and imidazole, analogs with two nitrogen atoms
- Silole, a silicon analog
- Stannole, a tin analog
- Stibole, an antimony analog
- Thiophene, a sulfur analog
Derivatives of pyrrole include indole, a derivative with a fused benzene ring.
See also
References
- ^ Loudon, Marc G. (2002). "Chemistry of Naphthalene and the Aromatic Heterocycles.". Organic Chemistry (Fourth ed.). New York: Oxford University Press. pp. 1135–1136. ISBN 0-19-511999-1.
- ^ Cox, Michael; Lehninger, Albert L; Nelson, David R. (2000). Lehninger principles of biochemistry. New York: Worth Publishers. ISBN 1-57259-153-6.
- ^ Jonas Jusélius and Dage Sundholm (2000). "The aromatic pathways of porphins, chlorins and bacteriochlorins" (Open access). Phys. Chem. Chem. Phys. 2 (10): 2145–2151. doi:10.1039/b000260g.
- ^ Armarego, Wilfred, L.F.; Chai, Christina, L.L. (2003). Purification of Laboratory Chemicals (5th ed.). Elsevier. pp. 346.
- ^ a b Albrecht Ludwig Harreus "Pyrrole" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a22_453
- ^ Practical Organic Chemistry, Vogel, 1956, Page 837, Link (12 MB)
- ^ Piloty, O. (1910). "Synthese von Pyrrolderivaten: Pyrrole aus Succinylobernsteinsäureester, Pyrrole aus Azinen". Chem. Ber. 43: 489. doi:10.1002/cber.19100430182.
- ^ Robinson, Gertrude Maud; Robinson, Robert (1918). "LIV.—A new synthesis of tetraphenylpyrrole". J. Chem. Soc. 113: 639. doi:10.1039/CT9181300639.
- ^ Benjamin C. Milgram, Katrine Eskildsen, Steven M. Richter, W. Robert Scheidt, and Karl A. Scheidt (2007). "Microwave-Assisted Piloty-Robinson Synthesis of 3,4-Disubstituted Pyrroles" (Note). J. Org. Chem. 72 (10): 3941–3944. doi:10.1021/jo070389. PMC 1939979. PMID 17432915. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1939979.
- ^ a b Jose R. Garabatos-Perera, Benjamin H. Rotstein, and Alison Thompson (2007). "Comparison of Benzene, Nitrobenzene, and Dinitrobenzene 2-Arylsulfenylpyrroles". J. Org. Chem. 72 (19): 7382–7385. doi:10.1021/jo070493r. PMID 17705533.
- ^ The 2-sulfenyl group in the pyrrole substrate serves as an activating group and as a protective group that can be removed with Raney nickel
External links
- General Synthesis and Reactivity of Pyrrole
- Synthesis of pyrroles (overview of recent methods)
- Substitution reaction mechanisms of nitrogen-containing heteroaromatics
- Jolicoeur, Benoit; Chapman, Erin E.; Thompson, Alison; Lubell, William D. (2006). "Pyrrole protection". Tetrahedron 62 (50): 11531. doi:10.1016/j.tet.2006.08.071.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Post a question - any question - to the WikiAnswers community:
Copyrights:
American Heritage Dictionary
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
Read more
McGraw-Hill Science & Technology Encyclopedia
McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
Read more
Oxford Dictionary of Sports Science & Medicine
The Oxford Dictionary of Sports Science & Medicine. Copyright © Michael Kent 1998, 2006, 2007. All rights reserved.
Read more
Wiley Dictionary of Flavors
Copyright © 2008 by Wiley-Blackwell. Wiley and the Wiley logo are registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries. Used here by license. Read more
Saunders Veterinary Dictionary
Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.
Read more
