|UN number||1992, 1993|
|Jmol-3D images||Image 1
|Molar mass||67.09 g mol−1|
|Density||0.967 g cm–3|
|Melting point||−23 °C (−9 °F; 250 K)|
|Boiling point||129 to 131 °C (264 to 268 °F; 402 to 404 K)|
|Vapor pressure||7 mmHg at 23 °C|
|Viscosity||0.001225 Pa s|
heat capacity C
|1.903 J k–1 mol k–1|
|Std enthalpy of
|108.2 kJ mol–1 (gas)|
|Std enthalpy of
|2242 kJ mol–1|
|Flash point||33.33 °C (91.99 °F; 306.48 K)|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. 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.
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.
Pure pyrrole collected from the still is now colorless and transparent to all wavelengths of visible light due to the removal of impurities
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. 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 the second step, a [3,3]sigmatropic reaction takes place between two intermediates.
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.
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), both of which are compatible with a variety of pyrrole substrates.
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 °C and in darkness to control the polymerization.
Pyrrole is essential to the production of many different chemicals. N-methylpyrrole is a precursor to N-methylpyrrolecarboxylic acid, a building-block in pharmaceutical chemistry. Although there is a claim that pyrrole is used as an additive to cigarettes, it is typically listed as a constituent of tobacco smoke and not as an ingredient.
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
- 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
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- Jonas Jusélius and Dage Sundholm (2000). "The aromatic pathways of porphins, chlorins and bacteriochlorins". Phys. Chem. Chem. Phys. (Open access) 2 (10): 2145–2151. doi:10.1039/b000260g.
- Armarego, Wilfred, L.F.; Chai, Christina, L.L. (2003). Purification of Laboratory Chemicals (5th ed.). Elsevier. p. 346.
- 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.
- 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
- Fowles, Jefferson; Michael Bates and Dominique Noiton (March 2000). "The Chemical Constituents in Cigarettes and Cigarette Smoke: Priorities for Harm Reduction". Porirua, New Zealand: New Zealand Ministry of Health. pp. 20, 49–65. Retrieved 2012-09-23.
- 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.