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is a chemical compound
with the formula C5H5N
. It is a liquid with a distinctively putrid, fish-like odour. Pyridine is a simple and fundamentally important heterocyclic aromatic organic compound
. It 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 sometimes used as a ligand
in coordination chemistry
. As a ligand, it is usually abbreviated "py".
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 cation. The bond lengths and bond angles in pyridine and the pyridinium ion are almost identical because protonation does not disrupt the aromatic pi system. In addition, the pyridinium cation is isoelectronic with benzene.
Pyridine as a solvent
Pyridine is a widely used and versatile solvent
: it is polar
. It is miscible with a broad range of solvents including hexane
pyridine, called pyridine-d5
, is a common solvent for1
H NMR spectroscopy
Role in chemical synthesis
Pyridine is important in industrial chemistry, both as a fundamental building block and as a solvent and reagent in organic synthesis
. It is used as a solvent in Knoevenagel condensations
It is also a starting material in the synthesis of compounds used as an intermediate in making insecticides, herbicides, pharmaceuticals, food flavorings, dyes, rubber chemicals, adhesives, paints, explosives and disinfectants. Pyridine is also used as a denaturant for antifreeze mixtures, for ethyl alcohol, for fungicides, and as a dyeing aid for textiles.
Role in chemical analysis
Pyridine, along with barbituric acid
, is commonly used in colorimetric
determinations of cyanide
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 together, 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 than pyridine, allowing for a more stable equivalence point and a faster reaction rate. Imidazole also has the advantage of being odorless.
Preparation and occurrence
Many methods exist in industry and in the laboratory (some of them named reactions
) for the synthesis of pyridine and its derivatives: Pyridine was originally isolated industrially from crude coal tar
. It is currently synthesized
, a process that involves the intermediacy of acrolein
- CH2O + NH3 + 2 CH3CHO → C5H5N + 3 H2O
By substituting other aldehydes for acetaldehyde, one obtains alkyl and aryl substituted pyridines. 26,000 tons were produced worldwide in 1989.
In organic reactions
pyridine behaves both as a tertiary amine
, undergoing protonation
, and N-oxidation
at nitrogen, and as an aromatic compound
, undergoing Nucleophilic substitutions
Safety and environmental
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.
Structurally or chemically related compounds are
- DMAP is short for 4-dimethylaminopyridine
- Bipyridine and viologen are simple polypyridine compounds consisting of two pyridine molecules joined by a single bond
- The herbicides paraquat and diquat are quarterny N-alkylammonium salts of 4,4'- and 2,2'-bipyridine
- Terpyridine, a molecule of three pyridine rings connected together by two single bonds.
- Quinoline and Isoquinoline have pyridine and a benzene ring fused together.
- Aniline is a benzene derivative with an attached NH2 group and NOT a pyridine
- Diazines are compounds with one more carbon replaced by nitrogen such as Pyrazine and Pyrimidine
- Triazines are compounds with two more carbons replaced by nitrogen and a tetrazine has four nitrogen atoms
- 2,6-Lutidine is a trivial name for 2,6-dimethylpyridine.
- Collidine is the trivial name for 2,4,6-trimethylpyridine.
- Pyridinium p-toluenesulfonate (PPTS) is a salt formed by proton exchange between pyridine and p-toluenesulfonic acid
- 2-Chloropyridine is a toxic environmentally significant component of the breakdown of the pesticide imidacloprid.