Pyridine
What Is Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group (=CH−) replaced by a nitrogen atom (=N−). It is a highly flammable, weakly alkaline, water-miscible liquid with a distinctive, unpleasant fish-like smell. Pyridine is colorless, but older or impure samples can appear yellow, due to the formation of extended, unsaturated polymeric chains, which show significant electrical conductivity. The pyridine ring occurs in many important compounds, including agrochemicals, pharmaceuticals, and vitamins. Historically, pyridine was produced from coal tar.
Advantages of Pyridine
Important in medicinal chemistry
Pyridines are important in medicinal chemistry because of their properties, which include weak basicity, water solubility, in vivo /chemical stability, hydrogen bond-forming ability, and small molecular size. Pyridine moieties are incorporated in many drugs and pesticides. Water solubility is one role of pyridines.
Basic solvent that is relatively unreactive
Pyridine is an effective, basic solvent that is relatively unreactive, which makes it a good acid scavenger. Pyridine is the solvent of choice for acylation and dehydrochlorination reactions. It is also used as a solvent for paint, rubber, pharmaceuticals, polycarbonate resins and textile water repellants.
Improves pharmacokinetic characteristics of lead molecules
Being a polar and ionisable aromatic compound, it improves pharmacokinetic characteristics of lead molecules and thus used as a remedy to optimize solubility and bioavailability parameters of proposed poorly soluble lead molecules. Pyridine derivatives have occupied a unique place in the field of medicinal chemistry.
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Properties of Pyridine
Pyridine and its derivatives are stable and unreactive liquids. It has a strong penetrating odour that is unpleasant. Pyridine is the hydrogen derivative of this ring. This ring is benzene in which one ch- is replaced by a nitrogen atom. The structure of pyridine is analogous to benzene. Though relatable by replacement of ch by n. The molecular weight or molar mass of pyridine is 79.1 gram per mole. The density of pyridine is 982 kilogram per meter cube. Also, the boiling point of pyridine is 115oc and the melting point of pyridine is −41.6oc.
Physical properties of pyridine
The molecular electric dipole moment is 2.2 debyes. Pyridine is diamagnetic. Pyridine has a diamagnetic susceptibility of −48.7×10−6cm3mol−1. The standard enthalpy of formation of pyridine is 100.2kjmol−1 in the liquid phase and 140.4 kj·mol−1 in the gas phase. At 25oc pyridine has a viscosity of 0.88 mpa/s. At 25oc thermal conductivity of pyridine is 0.166wm−1k−1. The enthalpy of vaporization of pyridine is 35.09kjmol−1 at the boiling point and normal pressure. Also, the enthalpy of fusion of pyridine is 8.28kjmol−1 at the melting point. Some critical parameters of pyridine are temperature 620 k, pressure 6.70 mpa, and volume 229cm3mol−1. Pyridine ring forms a c5n hexagon. The optical absorption spectrum of pyridine in hexane has three bands at the wavelengths of 195 nm, 251 nm and 270 nm.
Chemical properties of pyridine
Due to the presence of the electronegative nitrogen in the pyridine ring, the molecule is relatively electron deficient. That’s why it enters less readily into electrophilic aromatic substitution reactions than benzene derivatives. Pyridine is more prone to nucleophilic substitution as it is evidenced by the ease of metalation by strong organometallic bases.
With electrophiles, electrophilic substitution takes place where pyridine shows aromatic properties. With nucleophiles, pyridine reacts at 2 and 4 positions and behaves similar to imines and carbonyls. The reaction with lewi’s acids results in the addition to the nitrogen atom of pyridine. It is similar to the reactivity of tertiary amines. The nitrogen centre of pyridine is a basic lone pair of electrons. Protonation of pyridine gives pyridinium ion i.E. C5h5nh+. The pka of the conjugate acid is 5.25. The structures of pyridine and pyridinium are almost the same. The pyridinium cation is isoelectronic to benzene. Pyridinium p-toluenesulfonate is an illustrative pyridinium salt. This salt is produced by treating pyridine with p-toluenesulfonic acid. Pyridine undergoes n-centred alkylation, acylation, and n-oxidation.
As fascinating as chemistry can be, it becomes more intriguing when you delve deep into the uses of specific compounds. The uses of pyridine are extensive and profoundly impact our everyday life. Pyridine serves as a versatile solvent and reagent in laboratories and industries alike. You'll find its applications predominantly in the pharmaceutical industry. Furthermore, its utility extends to the manufacturing of dyes, rubber products, and adhesives. Pyridine also serves as a denaturant in antifreeze mixtures, providing an unpleasant taste to discourage accidental ingestion. In the agricultural sector, pyridine and its derivatives are used to manufacture herbicides, insecticides, and fungicides. In the realm of food processing, pyridine finds its place as a flavouring agent.
You'll be amazed to discover the extent to which pyridine touches your everyday life. For instance, pyridine is used in the production of povidone-iodine, a broad spectrum antiseptic for topical application, and niacin, a form of vitamin b that is vital for human health. The pharmaceutical industry sees pyridine employed in the production of drugs like sulfa drugs, which are among the very first systematically used antibiotic medicines. In the realm of cosmetics, pyridine-based compounds are used in hair dyes. Taking a look at your household items, pyridine is used in making water repellents, adhesives, and sealants that protect your objects and surfaces from water damage.
Exploring pyridine reactions can bridge the gap between theoretical knowledge and practical applications. As an aromatic compound, pyridine undergoes several different types of reactions. One such reaction is the quaternisation reaction. When reacted with an alkyl or benzyl halide, pyridine undergoes a nucleophilic substitution reaction to form pyridinium salts: Pyridine + rx → pyridinium salt, (py + r-x \rightarrow pyr^{+}x^{-}. Another common reactions of pyridine are the n-oxidation and schiff base formation. In the n-oxidation reaction, pyridine is transformed into the corresponding n-oxide when treated with oxidising agents. In schiff base formation, pyridine reacts with carbonyl compounds to yield imines, known as schiff bases, in presence of a suitable acid catalyst.
Chemistry is a marvellous field, marked by endless discoveries and abundant knowledge. Pyridine, with its versatile applicability and rich collection of reactions, is undoubtedly one of the many chemical treasures our world has. To get more insights and in-depth understanding of pyridine and its properties, studying the functionality, uses and implications of pyridine can prove to be a remarkable journey.
Reactions of Pyridine
Electrophilic substitutions
Owing to the decreased electron density in the aromatic system, electrophilic substitutions are suppressed in pyridine and its derivatives. Friedel–Crafts alkylation or acylation, usually fail for pyridine because they lead only to the addition at the nitrogen atom. Substitutions usually occur at the 3-position, which is the most electron-rich carbon atom in the ring and is, therefore, more susceptible to an electrophilic addition.Direct nitration of pyridine is sluggish. Pyridine derivatives wherein the nitrogen atom is screened sterically and/or electronically can be obtained by nitration with nitronium tetrafluoroborate (NO2BF4). In this way, 3-nitropyridine can be obtained via the synthesis of 2,6-dibromopyridine followed by nitration and debromination. Sulfonation of pyridine is even more difficult than nitration. However, pyridine-3-sulfonic acid can be obtained. Reaction with the SO3 group also facilitates addition of sulfur to the nitrogen atom, especially in the presence of a mercury(II) sulfate catalyst. In contrast to the sluggish nitrations and sulfonations, the bromination and chlorination of pyridine proceed well.
Pyridine N-oxide
Oxidation of pyridine occurs at nitrogen to give pyridine N-oxide. The oxidation can be achieved with peracids: C5H5N + RCO3H → C5H5NO + RCO2H。 Some electrophilic substitutions on the pyridine are usefully effected using pyridine N-oxide followed by deoxygenation. Addition of oxygen suppresses further reactions at nitrogen atom and promotes substitution at the 2- and 4-carbons. The oxygen atom can then be removed, e.g., using zinc dust.
Nucleophilic substitutions
In contrast to benzene ring, pyridine efficiently supports several nucleophilic substitutions. The reason for this is relatively lower electron density of the carbon atoms of the ring. These reactions include substitutions with elimination of a hydride ion and elimination-additions with formation of an intermediate aryne configuration, and usually proceed at the 2- or 4-position. Many nucleophilic substitutions occur more easily not with bare pyridine but with pyridine modified with bromine, chlorine, fluorine, or sulfonic acid fragments that then become a leaving group. So fluorine is the best leaving group for the substitution with organolithium compounds. The nucleophilic attack compounds may be alkoxides, thiolates, amines, and ammonia (at elevated pressures). In general, the hydride ion is a poor leaving group and occurs only in a few heterocyclic reactions. They include the Chichibabin reaction, which yields pyridine derivatives aminated at the 2-position. Here, sodium amide is used as the nucleophile yielding 2-aminopyridine. The hydride ion released in this reaction combines with a proton of an available amino group, forming a hydrogen molecule. Analogous to benzene, nucleophilic substitutions to pyridine can result in the formation of pyridyne intermediates as heteroaryne. For this purpose, pyridine derivatives can be eliminated with good leaving groups using strong bases such as sodium and potassium tert-butoxide. The subsequent addition of a nucleophile to the triple bond has low selectivity, and the result is a mixture of the two possible adducts.
Radical reactions
Pyridine supports a series of radical reactions, which is used in its dimerization to bipyridines. Radical dimerization of pyridine with elemental sodium or Raney nickel selectively yields 4,4'-bipyridine,[90] or 2,2'-bipyridine, which are important precursor reagents in the chemical industry. One of the name reactions involving free radicals is the Minisci reaction. It can produce 2-tert-butylpyridine upon reacting pyridine with pivalic acid, silver nitrate and ammonium in sulfuric acid with a yield of 97%.
Reactions on the nitrogen atom
Lewis acids easily add to the nitrogen atom of pyridine, forming pyridinium salts. The reaction with alkyl halides leads to alkylation of the nitrogen atom. This creates a positive charge in the ring that increases the reactivity of pyridine to both oxidation and reduction. The Zincke reaction is used for the selective introduction of radicals in pyridinium compounds (it has no relation to the chemical element zinc).
Reduction of pyridine to piperidine with Raney nickel
Piperidine is produced by hydrogenation of pyridine with a nickel-, cobalt-, or ruthenium-based catalyst at elevated temperatures. The hydrogenation of pyridine to piperidine releases 193.8 kJ/mol, which is slightly less than the energy of the hydrogenation of benzene (205.3 kJ/mol). Partially hydrogenated derivatives are obtained under milder conditions. For example, reduction with lithium aluminium hydride yields a mixture of 1,4-dihydropyridine, 1,2-dihydropyridine, and 2,5-dihydropyridine. Selective synthesis of 1,4-dihydropyridine is achieved in the presence of organometallic complexes of magnesium and zinc.
Pyridine and Its Metal Complexes
The spectrochemical series of ligands in crystal field theory (CFT), portray ligand arrangement pertaining to their metal d-orbital splitting capability, depicts pyridine as moderately strong ligand. This interprets to strong electrostatic interactions of pyridine lone pair to metal d- orbitals. Despite being neutral, pyridine causes moderately large d-orbital splitting implying to strong bonding interaction to metal centers. Beside the CFT, the valence bond theory (VBT), considers metal pyridine bonding to overlap of sp2 lone pair orbital of pyridine to hybridized metal orbitals. The extent of overlaps is happened to be highest in first transition metals in comparison to the second and third transition elements owing to the difference of shape, size, and energy of the combining orbitals. Apart from nitrogen lone pair orbitals, the ring π-electron is also capable of bonding interaction to metal ions.
Moreover delocalized π* anti-bonding orbitals can act as acceptor of metal electron density. The pyridine can also indulge in hydrogen bonding and π-π stacking-like weak interactions. Thus pyridine is enriched with multiple orbitals for bonding interactions with metal ions.
The pyridine transition metal complexes have a rich literature. Pyridine found to coordinate all the transition metals producing the variety of metal complexes in their different oxidation states. Efforts were made to incorporate the increasing number of pyridines in metal coordination sphere but exclusive pyridine complexes such as [M(py)4]n+ or [M(py)6]n+ are rare (where M = transition metal). The metal- pyridine chemistry incorporates pyridine and its derivatives with the capability of bidentate or tridentates ligands in the formation of metal complexes. The discussion here to be restricted to the domain of pyridine and it coordination to transition metals. A brief overview of pyridine transition metals complexes is presented here.
Scandium and yttrium preferably bind to three pyridine units in a four-coordinated geometry. The coordination number might vary with characteristics of binding ligands. Five and six coordinated complexes are also synthesized in combination of pyridine and thiocyanate (SCN) ligands. The pyridine derivative capable of acting as a bidentate ligand, such as picolinic acid, prefers to produce higher coordination number complexes. A good number of complexes are known with variously substituted pyridines. These complexes are known in +1 and + 3 states of Sc and Y. These complexes could be derived directly from their metal salt and pyridine at room temperature.
The Ti(II), Ti(III), and Zr(III) pyridine complexes required preparation under inert and moisture free environment. The presence of aerial oxygen facilitates decomposition and formation of +4 states of metal centers. This is evident from preparative methods of Ti(IV) and Zr(IV) pyridine complexes under oxygen. In general, simple pyridine complexes are sensitive to moisture and air. The electron donating substituents at two and four positions help “N” in forming the stronger coordinate bond and enhance the stability of resultant complexes. Beside the chelate pyridine ligands provide appreciably higher stability compare to the monodentate pyridine moiety.
The vanadium, niobium, and tantalum possess rich chemistry of pyridine complexes. The richness arises due to numerous valence states (0-V) of vanadium, which are found to exist with different pyridine complexes. Though pyridine complexes with V (0) and V (I) are very rare. Example of V (0) is [V2 (2-Me-py)4(CO). A handful of complexes with other oxidation states are reported. V (II) pyridine complexes were prepared with a range of monodentate and bidentate anionic ligands. Along with basic pyridine moiety, various substituted derivatives were found in the coordination sphere of V (II) centers.