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Pyridine
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Pyridine is a simple and important heterocyclic aromatic organic compound with the formula C5H5N. This colorless liquid with a distinctive fish-like odor is structurally related to benzene, wherein one CH group in the six-membered ring is replaced by a nitrogen atom. The pyridine ring occurs in many important compounds, including the nicotinamides. Pyridine is mainly useful as a precursor to agrochemicals and pharmaceuticals as well as a solvent.
dine was originally isolated industrially from crude coal tar.
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Pyridine is a simple and important heterocyclic aromatic organic compound with the formula C5H5N. This colorless liquid with a distinctive fish-like odor is structurally related to benzene, wherein one CH group in the six-membered ring is replaced by a nitrogen atom. The pyridine ring occurs in many important compounds, including the nicotinamides. Pyridine is mainly useful as a precursor to agrochemicals and pharmaceuticals as well as a solvent.
Preparation and occurrence
Pyridine was originally isolated industrially from crude coal tar. It is currently synthesized from acetaldehyde, formaldehyde and ammonia, the process being called the Chichibabin pyridine synthesis:
- CH2O + NH3 + 2 CH3CHO ? C5H5N + 3 H2O + H2
This process involves the intermediacy of acrolein. An estimated 26,000 tons were produced worldwide in 1989. Condensations of ammonia sources and related unsaturated carbon sources affords alkyl- and aryl-substituted pyridines, e.g. monomethyl compounds (picolines), dimethylated compounds (lutidines), and trimethyl derivatives (collidines).
For specialized applications, the synthesis of the pyridine skeleton is well developed. The Hantzsch pyridine synthesis, for example, is a multicomponent reaction involving formaldehyde, a keto-ester and a nitrogen donor. The Kröhnke pyridine synthesis involves the condensation of 1,5-diketones with ammonium acetate in acetic acid followed by oxidation. The Ciamician-Dennstedt Rearrangement entails the ring-expansion of pyrrole with dichlorocarbene to 3-chloropyridine. In the Gattermann-Skita synthesis, a malonate ester salt reacts with dichloromethylamine.
Reactions
As a base
In organic reactions pyridine behaves both as a tertiary amine, undergoing protonation, alkylation, acylation, and N-oxidation at nitrogen, and as an aromatic compound, undergoing Nucleophilic substitutions. The nitrogen atom on pyridine features a basic lone pair of electrons. Because this lone pair is not delocalized into the aromatic pi-system, pyridine is basic with chemical properties similar to tertiary amines. The pKa of the conjugate acid is 5.21. Pyridine is protonated by reaction with acids and forms a positively charged aromatic polyatomic ion called pyridinium. The bond lengths and bond angles in pyridine and the pyridinium ion are almost identical. In addition, the pyridinium cation is isoelectronic with benzene. Pyridinium p-toluenesulfonate (PPTS) is an illustrative pyridinium salt; it is formed by treating pyridine with p-toluenesulfonic acid.
As an N-nucleophile and ligand
Pyridine is a good nucleophile with a donor number of 33.1. It is easily attacked by alkylating agents to give N-alkylpyridinium salts. One example is cetylpyridinium chloride, a cationic surfactant that is a widely used disinfection and antiseptic agent. Pyridinium salts can be obtained in the Zincke reaction. Useful adducts of pyridine include Pyridine-borane, C5H5NBH3 (m.p. 10–11 °C), a mild reducing agent with improved stability relative to NaBH4 in protic solvents and improved solubility in aprotic organic solvents. Pyridine-sulfur trioxide, C5H5NSO3 (mp 175 °C) is a sulfonation agent used to convert alcohols to sulfonates, which in turn undergo C-O bond scission upon reduction with hydride agents.
Pyidine is widely used as a ligand in coordination chemistry. Also important are its chelating derivatives 2,2'-bipyridine, consisting of two pyridine molecules joined by a single bond, and terpyridine, a molecule of three pyridine rings linked together.
Nucleophilic reactions at the ring
Nucleophilic aromatic substitution occurs at C-2 and at C-4. For example in the Chichibabin reaction, pyridine reacts with sodium amide to give 2-aminopyridine. In the Emmert reaction, named for Bruno Emmert), pyridine reacts with a ketone in presence of aluminium or magnesium and mercuric chloride to give the carbinol also at C2.
Applications
Role in chemical synthesis
Pyridine is an important solvent and reagent in organic synthesis. It is used as a solvent in Knoevenagel condensations. It is the precursor to myriad insecticides, herbicides, pharmaceuticals, food flavorings, dyes, rubber chemicals, adhesives, paints, explosives and disinfectants. Examples include paraquat
Pyridine as a solvent
Pyridine is a widely used polar but aprotic solvent. It is miscible with a broad range of solvents including hexane and water. Deuterated pyridine, called pyridine-d5, is a common solvent for1H NMR spectroscopy.
Specialised uses
As a denaturant
Pyridine is also used as a denaturant for antifreeze mixtures, for ethyl alcohol, for fungicides, and as a dyeing aid for textiles.
Role in analytical chemistry
Pyridine, along with barbituric acid, is commonly used in colorimetric determinations of cyanide in aqueous matrices. Pyridine reacts with cyanogen chloride (formed in an earlier step by reaction of the cyanide anion with chloramine-T) to form a conjugated species that couples two molecules of barbituric acid, forming a red-colored dye. Color intensity is directly proportional to cyanide concentration. Pyridine was originally used as the base in the Karl Fischer titration, but has since been largely replaced by imidazole, which is more basic, allowing for a more stable equivalence point and a faster reaction rate. Imidazole also has the advantage of being odorless.
Safety and environmental aspects
The LD50 in rats (oral) is 891 mg kg–1. It is volatile and can be absorbed through skin. Available data indicate that "exposure to pyridine in drinking-water led to reduction of sperm motility at all dose levels in mice and increased estrous cycle length at the highest dose level in rats". Currently its evaluations as a possible carcinogenic agent showed there is inadequate evidence in humans for the carcinogenicity of pyridine, albeit there is limited evidence of carcinogenic effects on animals. Effects of an acute pyridine intoxication include dizziness, headache, nausea and anorexia. Further symptoms include abdominal pain and pulmonary congestion. Although resistant to oxidation, pyridine is readily degraded by bacteria to ammonia and carbon dioxide.
Other 6-membered aromatic rings
With one carbon replaced by another group, these molecules include borabenzene, silabenzene, germanabenzene, stannabenzene, phosphorine, and pyrylium salt.
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