aryl group

7/28/2019 Aryl Group

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ARYL GROUPIn the context of organic molecules, aryl refers to any functional group or

substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl,indolyl, etc. "Aryl" is used for the sake of abbreviation or generalization, and"Ar" is used as a placeholder for the aryl group in chemical structure diagrams.

A simple aryl group is phenyl, C6H5; it is derived from benzene. The tolyl group,

CH3C6H4, is derived from toluene (methylbenzene). The xylyl group, (CH3)2C6H3,

is derived from xylene (dimethylbenzene), while the naphthyl group, C10H7, is

derived from naphthalene.

Ar----R

Definition:

An aryl group is a functional group derived from a simple aromatic ring

compound where one hydrogen atom is removed from the ring.Two commonly encountered substituent groups that incorporate a benzene ring are

phenyl, abbreviated Ph-, and benzyl, abbreviated Bn-. These are shown here with

examples of their use. Be careful not to confuse a phenyl (pronounced fenyl) group

with the compound phenol (pronounced feenol). A general and useful generic

notation that complements the use ofR- for an alkyl group is Ar- for an aryl group

(any aromatic ring).
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Aryl Halides

An aryl halide is a compound formed by the substitution of a halogen atom for

a hydrogen atom on benzene. Another name for an aryl halide is halobenzene.

Nomenclature

Aryl halides are named by prefixing the name of the halogen to benzene. For

example:

Preparation of aryl halide;

Halogenation of Benzene.

HETROCYCYCLIC COMPOUND

A heterocyclic compound is a cyclic compound that has atoms of at least

two different elements as members of its ring(s). The counterparts of

heterocyclic compounds are , the rings of which are

made of a single element.
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Although heterocyclic compounds may be inorganic, most contain at least

one carbon. Since in organic chemistry non-carbons usually are considered to

replace carbon atoms, they are called heteroatoms, meaning 'different from

carbon and hydrogen' (rings of heteroatoms of the same element are

homocyclic). The IUPAC recommends the Hantzsch-Widman nomenclature fornaming heterocyclic compounds.eg

Pyridine

Classification based on electronic structure

Heterocyclic compounds can be usefully classified based on their

electronic structure. The saturated heterocycles behave like the acyclic

derivatives. Thus, piperidine and tetrahydrofuran are conventional amines and

ethers, with modified steric profiles. Therefore, the study of heterocyclic

chemistry focuses especially on unsaturated derivatives, and the

preponderance of work and applications involves unstrained 5- and 6-

membered rings. Included are pyridine, thiophene, pyrrole, and furan. Another

large class of heterocycles is fused to benzene rings, which for pyridine,

thiophene, pyrrole, and furan are quinoline, benzothiophene, indole, andbenzofuran, respectively. Fusion of two benzene rings gives rise to a third large

family of compounds, respectively the acridine, dibenzothiophene, carbazole,

and dibenzofuran. The unsaturated rings can be classified according to the

participation of the heteroatom in the pi-system.

NomenclatureDevising a systematic nomenclature system for heterocyclic compounds

presented a formidable challenge, which has not been uniformly concluded.

Many heterocycles, especially amines, were identified early on, and received

trivial names which are still preferred. Some monocyclic compounds of this

kind are shown in the following chart, with the common (trivial) name in bold

and a systematic name based on the Hantzsch-Widman system given beneath

it in blu e. The rules for using this system will be given later. For most students,

learning these common names will provide an adequate nomenclature
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background

But here we will discuss only oxygen and nitrogen containing aromatic

compound.

FuranFuran is a heterocyclic organic compound, consisting of a five-membered

aromatic ring with four carbon atoms and one oxygen. The class of compounds

containing such rings are also referred to as furans.

Furan is a colorless, flammable, highly volatile liquid with a boiling point

close to room temperature. It is soluble in common organic solvents, including

alcohol, ether and acetone, but is insoluble in water. It is toxic and may be

carcinogenic. Furan is used as a starting point to other specialty chemicals.

History
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The namefuran comes from the Latin furfur, which means bran. The

first furan derivative to be described was 2-furoic acid, by Carl Wilhelm Scheele

in 1780. Another important derivative, furfural, was reported by Johann

Wolfgang Dbereiner in 1831 and characterised nine years later by John

Stenhouse. Furan itself was first prepared by Heinrich Limpricht in 1870,although he called it tetraphenol.

Health Effects, Prevalence in the Diet

Furan at a dose of 2 mg/kg body weight (bw) and 4 mg/kg bw has beenshown to cause bile duct cancer in rats and liver cancer in mice (National

Toxicology Program, 1993; Moser et al. 2009). No long term trials on the

adverse effects of furan on human health have been conducted. Furan is found

in coffee, and, canned and jarred food, including baby food(http://voices.yahoo.com/the-carcinogen-furan-found-coffee-baby-food-

11230244.html?cat=70).

Production

Industrially, furan is manufactured by the palladium-catalyzed decarboxylationoffurfural, or by the copper-catalyzed oxidation of1, 3-butadiene:

In the laboratory, furan can be obtained from furfuralby oxidation to furan-2-

carboxylic acid, followed by decarboxylation. It can also be prepared directly

by thermal decomposition ofpentose-containing materials, cellulosic solidsespecially pine-wood.

Synthesis of furans

The Feist-Benary synthesis is a classic way to synthesize furans, although many

syntheses have been developed. One of the simplest synthesis methods for

furans is the reaction of1,4-diketones with phosphorus pentoxide (P2O5) in thePaal-Knorr Synthesis. The thiophene formation reaction of 1,4-diketones with

Lawesson's reagent also forms furans as side products. 2,4-Disubstituted furanscan be synthesized by sulfone-mediated cyclization of 1,3-diketones

Chemistry
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Furan is aromaticbecause one of the lone pairs ofelectrons on the oxygen atom

is delocalized into the ring, creating a 4n+2 aromatic system (see Hckel's rule)

similar to benzene. Because of the aromaticity, the molecule is flat and lacks

discrete double bonds. The other lone pair of electrons of the oxygen atom

extends in the plane of the flat ring system. The sp2

hybridization is to allow oneof the lone pairs of oxygen to reside in a p orbital and thus allow it to interactwithin the pi-system.

Due to its aromaticity, furan's behavior is quite dissimilar to that of the moretypical heterocyclic ethers such as tetrahydrofuran.

It is considerably more reactive than benzene in electrophilic

substitution reactions, due to the electron-donating effects of the

oxygen heteroatom. Examination of the resonance contributors shows

the increased electron density of the ring, leading to increased rates of

electrophilic substitution.[10]

Furan serves as a diene in Diels-Alder reactions with electron-deficient

dienophiles such as ethyl (E)-3-nitroacrylate.[11]The reaction product is a

mixture of isomers with preference for the endo isomer:

Pyridine

pyridine is a basic heterocyclic organic compound with the chemical

formula C5H5N. It is structurally related to benzene, with one

methylidyne (CH) group replaced by a nitrogen atom. The pyridine ring

occurs in many important compounds, including azines and the vitaminsniacin and pyridoxal.

Pyridine was discovered in 1849 by the Scottish chemist Thomas

Anderson as one of the constituents ofbone oil. Two years later,

Anderson isolated pure pyridine through fractional distillation of the oil.

It is a colorless, highly flammable, weakly alkaline, water-soluble liquid

with a distinctive, unpleasant fish-like odor.

Pyridine is used as a precursorto agrochemicals and pharmaceuticals and

is also an important solvent and reagent. Pyridine is added to ethanol to

make it unsuitable for drinking ( Denatured alcohol). It is used in the invitro synthesis ofDNA, in the synthesis ofsulfapyridine (a drug against
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bacterial and viral infections), antihistaminic drugs tripelennamine and

mepyramine, as well as water repellents, bactericides and herbicides.

Some chemical compounds, although not synthesized from pyridine,

contain its ring structure. They include B vitamins niacin and pyridoxal,

an anti-tuberculosis drug isoniazid, nicotine and other nitrogen-containingplant products. Historically, pyridine was produced from coal tarand as aby-product of the coal gasification. However, increased demand for

pyridine resulted in the development of more economical methods of

synthesis from acetaldehyde and ammonia, and more than 20,000 tonnesper year are manufactured worldwide

Physical properties

Crystal structure of pyridine

Pyridine is a colorless liquid that boils at 115.2 C and freezes at 41.6 C.

Its density, 0.9819 g/cm3, is close to that of water, and its refractive index is

1.5093 at a wavelength of 589 nm and a temperature of 20 C. Addition of up to

40 mol% of water to pyridine gradually lowers its melting point from 41.6 C

to 65.0 C. The molecularelectric dipole moment is 2.2 Debye. Pyridine is

diamagnetic and has a diamagnetic susceptibility of 48.7106 cm3mol1. The
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standard enthalpy of formation is 100.2 kJmol1 in the liquid phase and 140.4

kJmol1 in the gas phase. At 25 C pyridine has a viscosity of 0.88 mPa/s and

thermal conductivity of 0.166 Wm1K1. The enthalpy of vaporization is 35.09

kJmol1

at the boiling point and normal pressure. The enthalpy of fusion is 8.28

kJmol1

at the melting point.

Pyridine crystallizes in an orthorhombic crystal system ;but the number of

molecules per cell is only 4. This difference is partly related to the lowersymmetry of the individual pyridine molecule (C2v vs. D6h for benzene

The critical parameters of pyridine are pressure 6.70 MPa, temperature 620 K

and volume 229 cm3mol

1. In the temperature range 340426 C its vapor

pressurep can be described with the Antoine equation

where Tis temperature,A = 4.16272,B = 1371.358 K and C= -58.496 K.[18]

Chemical properties

Pyridine is miscible with water and virtually all organic solvents. It is weakly

basic, and with hydrochloric acid it forms a crystalline hydrochloride salt which

melts at 145147 C. Most chemical properties of pyridine are typical of aheteroaromatic compound. In organic reactions, pyridine behaves both as a

tertiary amine, undergoing protonation, alkylation, acylation, and N-oxidation at

the nitrogen atom, and as an aromatic compound, undergoing nucleophilicsubstitutions.

Because of the electronegative nitrogen in the pyridine ring, the molecule is

relatively electron deficient. It therefore enters less readily electrophilic

aromatic substitution reactions, which are characteristic of benzene derivatives.

However, unlike benzene and its derivatives, pyridine is more prone tonucleophilic substitution and metalation of the ring by strong organometallic

bases. The reactivity of pyridine can be distinguished for three chemical groups.

With electrophiles, electrophilic substitution takes place where pyridine

expresses aromatic properties. With nucleophiles, pyridine reacts via its 2nd and

4th carbon atoms and thus behaves similar to imines and carbonyls. Thereaction with many Lewis acids results in the addition to the nitrogen atom of

pyridine, which is similar to the reactivity of tertiary amines. The ability of

pyridine and its derivatives to oxidize, forming amine oxides (N-oxides), is also

a feature of tertiary amines.
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1,10-phenanthroline

The nitrogen center of pyridine features a basic lone pairofelectrons. Because

this lone pair is not part of the aromatic ring, pyridine is a base, having chemical

properties similar to those oftertiary amines. The pKa of the conjugate acid is

5.25. Pyridine is protonatedby reaction with acids and forms a positively

charged aromatic polyatomic ion called pyridinium. The bond lengths and bond

angles in pyridine and pyridinium are almost identical. The pyridinium cation is

isoelectronic with benzene. Pyridiniump-toluenesulfonate (PPTS) is anillustrative pyridinium salt; it is produced by treating pyridine withp-toluenesulfonic acid.

Pyridine can act as Lewis base, donating its pair of electron to a Lewis acid asin the sulfur trioxide pyridine complex.

Impure pyridine was undoubtedly prepared by early alchemists by heating

animal bones and other organic matter, but the earliest documented reference is

attributed to the Scottish scientist Thomas Anderson. In 1849, Anderson

examined the contents of the oil obtained through high-temperature heating ofanimal bones. Among other substances, he separated from the oil a colorlessliquid with unpleasant odor, from which he isolated pure pyridine two years

later. He described it as highly soluble in water, readily soluble in concentrated

acids and salts upon heating, and only slightly soluble in oils. Owing to its

flammability, Anderson named the new substancepyridine, afterGreek:

() (pyr) meaningfire. The suffix -idine was added in compliance with the

chemical nomenclature, as in toluidine, to indicate a carbon cycle containing anitrogen atom.

The chemical structure of pyridine was determined decades after its discovery.

Wilhelm Krner(1869) and James Dewar(1871) independently suggested that,

in analogy between quinoline and naphthalene, the structure of pyridine is

derived from benzeneby substituting one C-H unit with a nitrogen atom. The

suggestion by Krner and Dewar was later confirmed in an experiment where

pyridine was reduced to piperidine with sodium alcohol. In 1876, William

Ramsay combined acetylene and hydrogen cyanide into pyridine in a red-hotiron-tube furnace. This was the first synthesis of a hetero-aromatic compound.
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The contemporary methods of pyridine production had a low yield, and the

increasing demand for the new compound urged to search for more efficient

routes. A breakthrough came in 1924 when the Russian chemist Aleksei

Chichibabin invented a pyridine synthesis reaction which was based on

inexpensive reagents. This method is still used for the industrial production ofpyridine

Occurrence

Pyridine is not abundant in nature, except for the leaves and roots of belladonna

(Atropa belladonna) and in marshmallow (Althaea officinalis). Pyridine

derivatives, however, are often part of biomolecules such as the eponymous

pyridine nucleotides and alkaloids.

In daily life, trace amounts of pyridine are components of the volatile organic

compounds that are produced in roasting and canningprocesses, e.g. in fried

chicken, sukiyaki, roasted coffee, potato chips, and fried bacon. Traces ofpyridine can be found in Beaufort cheese, vaginal secretions, black tea, saliva of

those suffering from gingevitis, and sunflower honey. The smoke oftobaccoand marijuana also contain small amounts of pyridine.

Nomenclature

The systematic name of pyridine, within the HantzschWidman nomenclature

recommended by the IUPAC, is azine. However, systematic names for simplecompounds are used very rarely, instead heterocyclic nomenclature follows

historically established common names. IUPAC discourages the use ofazine in

favor ofpyridine. The numbering of the ring atoms in pyridine starts at the

nitrogen (see infobox). An allocation of positions by letter of the Greek alphabet

(-) and thesubstitution pattern nomenclature common for homoaromatic

systems (ortho, meta,para) are used sometimes. Here (ortho), (meta) and

(para) refer to the 2, 3 and 4 position, respectively. The systematic name for the

pyridine derivatives ispyridinyl, wherein the position of the substituted atom ispreceded by a number. However, here again the historical namepyridylis

encouraged by the IUPAC and used instead of the systematic name. The

cationic derivative formed by the addition of an electrophile to the nitrogenatom is calledpyridinium.
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4-bromopyridine

2,2-bipyridine

pyridine-2,6-dicarboxylic acid (dipicolinic acid)

General form of the pyridinium cation

Production

Historically, pyridine was extracted from coal tar or obtained as a by-product of

coal gasification. The process was labor consuming and inefficient: coal tar

contains only about 0.1% pyridine, and therefore a multi-stage purification was

required, which further reduced the output. Nowadays, most pyridine isproduced synthetically using various name reactions, and the major ones arediscussed below.

Chichibabin synthesis

The Chichibabin pyridine synthesis was reported in 1924 and is still in use

industrially. In its general form, the reaction can be described as a condensationreaction ofaldehydes, ketones,,-Unsaturated carbonyl compounds, or any

combination of the above, in ammonia orammonia derivatives. In particular,unsubstituted pyridine is produced from formaldehyde and acetaldehyde, which
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are inexpensive and widely available. First, acrolein is formed in a Knoevenagel

condensation from the acetaldehyde and formaldehyde. It is then condensed

with acetaldehyde and ammonia into dihydropyridine, and then oxidized with a

solid-state catalyst to pyridine. This process is carried out in a gas phase at 400

450 C. The product consists of a mixture of pyridine, simple methylatedpyridines (picoline) and lutidine; its composition depends on the catalyst usedand can be adapted to the needs of the manufacturer. The catalyst is usually a

transition metal salt such as cadmium(II) fluoride ormanganese(II) fluoride, but

cobalt and thallium compounds can also be used. The recovered pyridine isseparated from by-products in a multistage process.

Formation of acrolein from acetaldehyde and formaldehyde

Condensation of pyridine from acrolein and acetaldehyde

Practical application of the traditional Chichibabin pyridine synthesis are

limited by its consistently low yield, typically about 20%. This low yield,

together with the high prevalence of byproducts, render unmodified forms ofChichibabin's method unpopular.[61]

Dealkylation of alkylpyridines

Pyridine can be prepared by dealkylation of alkylated pyridines, which areobtained as by-products in the syntheses of other pyridines. The oxidative

dealkylation is carried out either using air overvanadium(V) oxide catalyst,[62]

by vapor-dealkylation on nickel-based catalyst, or hydrodealkylation with a

silverorplatinumbased catalyst. Yields of pyridine up to be 93% can beachieved with the nickel-based catalyst.

[edit] Bnnemann cyclization
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Bnnemann cyclization

The trimerization of a part of a nitrile molecule and two parts ofacetylene into

pyridine is called Bnnemann cyclization. This modification of the Reppesynthesis can be activated either by heat orlight. While the thermal activation

requires high pressures and temperatures, the photoinduced cycloaddition

proceeds at ambient conditions with CoCp2(cod) (Cp = cyclopentadienyl, cod =

1,5-cyclooctadiene) as a catalyst, and can be performed even in water. A series

of pyridine derivatives can be produced in this way. When using acetonitrile asthe nitrile, 2-methylpyridine is obtained, which can be dealkylated to pyridine.

Other methods

The Krhnke 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 ofpyrrole with dichlorocarbene to 3-chloropyridine.

In the Gattermann-Skita synthesis,[74]

a malonate estersalt reacts withdichloromethylamine.[75]
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Biosynthesis

Several pyridine derivatives play important roles in biological systems. While

its biosynthesis is not fully understood, nicotinic acid (vitamin B3) occurs in

some bacteria, fungi and mammals. Mammals synthesize nicotinic acid throughoxidation of the amino acid tryptophan, where an intermediate product, aniline,

creates a pyridine derivative, kynurenine. On the contrary, the bacteria

Mycobacterium tuberculosisandEscherichia coliproduce nicotinic acid bycondensation ofglyceraldehyde 3-phosphate and aspartic acid.

Reactions

Many reactions that are characteristic of benzene proceed with pyridine either at

more complicated conditions or/and with low yield. Owing to the decreasedelectron density in the aromatic system, electrophilic substitutions are

suppressed in pyridine and its derivatives in favor ofaddition of nucleophiles at

the electron-rich nitrogen atom. The nucleophilic addition at the nitrogen atomleads to a further deactivation of the aromatic properties and hindering of the

electrophilic substitution. On the other hand, free-radical and nucleophilicsubstitutions occur more readily in pyridine than in benzene.

Electrophilic substitutions

Many electrophilic substitutions on pyridine either do not proceed or proceed

only partially; however, the heteroaromatic character can be activated by

electron-donating functionalization. Common alkylations and acylations, suchas FriedelCrafts alkylation or acylation, usually fail for pyridine because they

only lead 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 istherefore more susceptible to an electrophilic addition.
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Substitutions to pyridine at the 2- or 4-position result in an energetically

unfavorable complex. They can be promoted, however, using clever

experimental techniques, such as conducting electrophilic substitution on thepyridine-N-oxide followed by deoxygenation of the nitrogen atom. Addition of

oxygen reduces electron density on the nitrogen atom and promotes substitution

at the 2- and 4-carbons. The oxygen atom can then be removed via several

routes, most commonly with compounds of trivalent phosphorus or divalent

sulfurwhich are easily oxidized. Triphenylphosphine is a frequently usedreagent, which is oxidized in this reaction to triphenylphosphine oxide. Thefollowing paragraphs describe representative electrophilic substitution reactions

of pyridine.[23]

Direct nitration of pyridine requires harsh conditions and has very low yields.

The 3-nitropyridine can be obtained instead by reacting pyridine with dinitrogen

pentoxide in presence ofsodium.[77][78][79]

Pyridine derivatives where the

nitrogen atom is screened sterically and/or electronically can be obtained by

nitridation with nitronium tetrafluoroborate (NO2BF4). In this way, 3-

nitropyridine can be obtained via the synthesis of 2,6-dibromopyridine followedby removal of the bromine atoms.[80][81]Direct sulfonation of pyridine is even

more difficult than direct nitridation. However, pyridine-3-sulfonic acid can be

obtained at acceptable yield by boiling pyridine in an excess ofsulfuric acid at

320 C.[82]

Reaction with the SO3 group also facilitates addition of sulfur to thenitrogen atom, especially in the presence of a mercury(II) sulfate catalyst.

In contrast to the nitration and sulfonation, the direct bromination and

chlorination of pyridine proceed well. The reaction of pyridine with molecular

bromine in sulfuric acid at 130 C readily produced 3-bromopyridine. The yieldis lower for 3-chloropyridine upon chlorination with molecularchlorine in the

presence ofaluminium chloride at 100 C. Both 2-bromopyridine and 2-

chloropyridine can be produced by direct reaction with halogen with a

palladium(II) chloride catalyst.Nucleophilic substitutions

In contrast to benzene, pyridine efficiently supports several nucleophilic

substitutions, and is regarded as a good nucleophile (donor number33.1). Thereason for this is relatively lower electron density of the carbon atoms of the
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ring. These reactions include substitutions with elimination of a hydride ion and

elimination-additions with formation of an intermediate arine configuration, andusually proceed at 2- or 4-position.

Many nucleophilic substitutions occur easier not with bare pyridine, but with

pyridine modified with bromine, chlorine, fluorine or sulfonic acid fragmentswhich 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)

The hydride ion is generally 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 asthe nucleophile yielding 2-aminopyridine. The hydride ion released in this

reaction combines with a proton of an available amino group forming ahydrogen molecule.

Analogous to benzene, nucleophilic substitutions to pyridine can result in the

formation of heteroarine intermediates. 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 possibleadducts.
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Pyridine is an important raw material in of the chemical industry, with the 1989

production of 26,000 tonnes in world-wide. Among major 25 production sites

for pyridine eleven are located in Europe (as of 1999). The major producers of

pyridine include Evonik Industries, Rtgers Chemicals, Imperial Chemical

Industries and Koei Chemical. The pyridine production has significantlyincreased in the early 2000s, with an annual production capacity of 30,000tonnes in mainland China alone. The US-Chinese joint venture Vertellus iscurrently the world leader in pyridine production.

APPLICATION

Pyridine is used as polar, basic, low-reactive solvent, for example inKnoevenagel condensations.[21]It is especially suitable for the dehalogenation,

where it acts as the base of the elimination reaction and bonds the resulting

hydrogen halide to form a pyridinium salt. In esterifications and acylations

pyridine activates the carboxylic acid halides or anhydrides. Even more active

in these reactions are the pyridine derivatives 4-dimethylaminopyridine(DMAP) and 4-(1-pyrrolidinyl) pyridine. Pyridine is also used as a base incondensation reactions.

Elimination reaction with pyridine to form pyridinium

Pyridinium chlorochromate was developed by Elias James Corey and William

Suggs in 1975 and is used to oxidizeprimary alcohols to aldehydes and

secondary alcohols to ketones.[97]

It is obtained by adding pyridine to a solution

ofchromic acid and concentrated hydrochloric acid:

C5H5N + HCl + CrO3 [C5H5NH][CrO3Cl]

The carcinogenicity of the side-product chromyl chloride (CrO2Cl2) urged to

look for alternative routes, such as treating chromium(VI) oxide withpyridinium chloride:

[C5H5NH+]Cl

+ CrO3 [C5H5NH][CrO3Cl]
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The Cornforth reagent (pyridinium dichromate, PDC), pyridinium

chlorochromate (PCC), the Collins reagent (complex ofchromium(VI) oxide

with pyridine in dichloromethane) and the Sarret reagent (complex of

chromium(VI) oxide with pyridine in pyridine) are similar chromium-based

pyridine compounds, which are also used for oxidation, namely conversion ofprimary and secondary alcohols to ketones. The Collins and Sarret reagents areboth difficult and dangerous to prepare, they are hygroscopic and can inflame

during preparation. For this reason, the use of PCC and PDC was preferred.

Those reagents were rather popular in the 1970s1980s, but because of theirtoxicity and confirmed carcinogenic status, they are rarely used nowadays

Hazards

Pyridine has a flash point (the lowest temperature at which it can vaporize toform an ignitable mixture in air) of only 17 C and is therefore highly

flammable. Its ignition temperature is 550 C, and mixtures of 1.710.6 vol% of

pyridine with airare explosive. The thermal modification of pyridine startsabove 490 C, resulting in bipyridine (mainly 2,2'-bipyridine and to a lesser

extent 2,3'-bipyridine and 2,4'-bipyridine), nitrogen oxides and carbon

monoxide.[13]Pyridine easily dissolves in water and harms both animals and

plants in aquatic systems.[111]

The permitted maximum allowable concentration

of pyridine was 1530 parts per million (ppm, or 1530 mgm3

in air) in mostcountries in the 1990s,[21]but was reduced to 5 ppm in the 2000s.[112]For

comparison, indoor air contaminated with tobacco smoke may contain up to 16gm3, and one cigarette contains 2132 g of pyridine.
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aryl group

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