Chemistry Of Aromatic And Aliphatic Heterocyclic Compounds Pdf

chemistry of aromatic and aliphatic heterocyclic compounds pdf

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A short summary of this paper. IntroductionOrganic compounds can have a variety of structures. These structures can be acyclic or cyclic. The cyclic systems containing only carbon atoms are called carbocyclic and the cyclic systems containing carbons and at least one other element are called heterocyclic.

Though a number of heteroatoms are known to be part of the heterocyclic rings, the most common heteroatoms are nitrogen, oxygen or sulphur. A heterocyclic ring may contain one or more heteroatoms which may or may not be same. Also the rings may be saturated or unsaturated. Nearly half of the known organic compounds contain at least one heterocyclic ring. Many heterocyclic compounds occur naturally and are actively involved in biology e. The study of heterocyclic chemistry is a vast and expanding area of chemistry because of their applications in medicine, agriculture, photodiodes and other fields.

Heterocyclic compounds may be classified as aliphatic and aromatic heterocycles. The aliphatic heterocycles are the cyclic analogues of amines, ethers and thioethers and their properties are influenced by the ring strain.

The three and four membered aliphatic heterocyclic rings are more strained and reactive compared to five and six membered rings. The p-orbitals on each carbon atom perpendicular to the plane of the ring then interact with each other.

Energy Fig. However, if the number of electron pairs is even total number of electrons is 4n , the last pair will occupy the degenerate orbital pair so that one electron is present in each degenerate orbital with parallel spins Fig. Therefore cyclic conjugated polyenes are aromatic if they are thermodynamically more stable than the corresponding open-chain analogs and it is antiaromatic if it is less stable.

Molecules which show neither stabilization nor destabilization are known as non-aromatic. Planar unsaturated heterocycles containing five atoms can be considered aromatic system if they have an uninterrupted cycle of porbitals containing six electrons in all. Pyrrole, furan and thiophene are examples of this type. In all these heterocycles, four carbon atoms and the heteroatom are sp 2 hybridized. The third p-orbital, which is orthogonal to the plane of the atoms, contributes its lone pair of electrons.

It is evident from the resonance structures given above and the data in Fig. Similarly furan and thiophene are also aromatic in nature and can be represented by the similar delocalized structures as in pyrrole wherein one of the lone pairs on oxygen or sulphur contributes to the aromatic sextet.

However, unlike pyrrole, the delocalization is not as extensive in furan because of high electronegativity of oxygen and so the lone pairs are held more tightly by the oxygen. Thiophene with the least electronegative heteroatom is regarded as aromatic and has the highest resonance energy whereas furan has the least resonance energy.

All these heterocycles have lower resonance energies than benzene. These are more reactive than benzene towards electrophilic substitution because of the increased electron density on each carbon in the ring. Therefore, the attack of the electrophile which is the rate determining step is more facile with these heterocycles.

Unlike pyrrole, furan and thiophene do not undergo protonation. Therefore, if pyrrole is protonated, it loses its aromaticity as the non bonding electrons are no more available for delocalization to form the aromatic sextet. The number of five membered aromatic heterocycles is much larger than that of the six membered aromatic heterocycles. This is because one of the heteroatoms in the ring can be divalent and so more heteroatoms can be incorporated into the five-membered rings than six-membered rings.

High boiling point of pyrrole is due to hydrogen bonding between pyrrole molecules. PyridineAll six membered fully unsaturated heterocycles are related to benzene as these can be represented by replacement of one or more CH groups by a trivalent heteroatom.

Pyridine ring consists of five carbon atoms and a nitrogen atom. Pyridine is a planar molecule. The ring is a slightly distorted hexagon as the C-N bonds are shorter than C-C bonds. N N Fig. The lone pair of electrons on sp 2 hybridized nitrogen is responsible for the basicity of pyridine and it behaves like a tertiary amine. Its aromaticity is not affected by protonation unlike pyrrole which loses aromaticity upon protonation.

The lone pair of nitrogen in pyridine is sp 2 hybridized which is more electronegative than the sp 3 hybridized nitrogen. Therefore pyridine is less basic than amines and anilines.

Heterocycles in which other benzenoid CH groups are replaced with nitrogen, also behave similarly. Because of the involvement of non-bonding lone pair of heteroatom in aromatization, it makes the 5-membered rings more prone to attack by electrophilic reagents. Because of the involvement of lone pair of nitrogen, charge separated cannonical forms can be written for indole as shown in Fig.

Quinoline and isoquinoline are weak bases but slightly more basic than pyridine but less basic than anilines since the nitrogen in quinoline and isoquinoline is more electronegative being sp 2 hybridized compared to sp 3 hybridized nitrogen of anilines. Isoquinoline has higher dipole moment 2. Two moles of ethyl acetoacetate are used eq. Synthesis of PyridinePyridine and picolines methylpyridine are found in coal-tar.

Picolines can be oxidised to corresponding pyridine carboxylic acids and converted to other pyridine derivatives. There are several modification of the Hantzsch synthesis. Synthesis of Indole i Fischer-Indole Synthesis:The most important synthesis of indole is the Fischerindole Synthesis which has been investigated very widely.

The Fischer-indole synthesis is carried out by heating phenyl hydrazone or substituted phenyl hydrazone of an aldehyde or ketone. The reaction is catalyzed by zinc chloride, polyphosphoric acid, sulphuric acid or boron trifluoride and proceeds with elimination of a molecule of ammonia eqs. Unsymmetrical phenyl hydrazones give a mixture of two differently substituted indoles, the ratio of which depends on steric factors and reaction conditions eq.

Dehydrogenation of 3,4-dihydroquinoline over Pd, S or Se yields 1substituted isoquinoline Scheme 3. There are various modifications of the method. The two modifications are Pictat-Gams synthesis eq. Reactions of Heterocycles Reactions of Pyrrole, Furan and Thiophene i Electrophilic Substitution Reactions:Pyrrole, furan and thiophene undergo electrophilic substitution reactions like nitration, sulphonation, halogenation etc. It has already been described in Sec. The electrophilic substitution takes place preferentially at 2-position C Furan is not as reactive as pyrrole in electrophilic substitution reactions because the oxygen in furan is more electronegative than nitrogen in pyrrole and therefore does not enhance the electron density of carbons as much as pyrrole.

Thiophene is less reactive than furan towards electrophilic substitution because the p-electrons of sulphur are in 3p orbital which overlaps less effectively than the 2p orbital of nitrogen or oxygen with 2p orbitals of carbon. The electrophilic substitution takes place at C-3 in pyridine. This orientation can be explained by comparing the relative stabilities of the intermediates arising from attack at C-3, C-4 or C-2 Fig.

The electrophilic attack at C-3 gives a carbocation which is hybrid of three resonance structures in which the positive charge is on the carbon atoms only. The electrophilic attacks at C-4 or C-2 also give an ion which is hybrid of three resonances structures but one of the resonance structures in each contains positively charged nitrogen which is a sextet and so unstable and hence does not contribute significantly to the resonance hybrid.

Thus the resonance hybrid resulting from electrophilic attack at C-3 is more stable and the preferred site of electrophilic attack. Some typical examples of electrophilic substitutions in pyridine are given below Fig. All these reactions involve substitution of a pyridinium cation. Friedel-crafts alkylation and acylation does not proceed with pyridine. Alkyl halides on heating with pyridine give pyridinium salts.

Nitration also gives poor yields. The high reactivity of pyridine towards nucleophilic substitution can cause displacement of even powerful hydride H - ion, e.

It is obvious from the above structures that nucleophilic attack at C-2 and C-4 gives hybrid of three resonance structures in which one of the contributory structures contains negative charge on nitrogen which is more electronegative atom and therefore contributes significantly towards its stabilization unlike nucleophilic attack at C-3 in which the negative charge is present on only carbon atoms.

Since the intermediate resulting from nucleophilic attack at C-2 and C-4 is more stable, nucleophilic substitution at these positions is preferred. Other nucleophilic substitution reactions of pyridine are given below eqs. If the leaving group at C-2 and C-4 are different, the incoming nucleophile will preferentially substitute for the weaker base the better leaving group eqs.

However, in dilute solutions it undergoes protonation of nitrogen to give indolium cation in which the aromaticity of benzene ring is retained eq. Indole does not undergo easy alkylation of nitrogen unlike amines for the same reason. Therefore, the heterocyclic ring of indole is more reactive towards electrophiles compared to its benzene ring. The electrophilic substitution in indole takes place at C-3 and not at C-2 as in pyrrole. This can be explained from the following observations. Electrophilic attack at C-2 and C-3 gives different intermediates as shown below Scheme 6 :The carbocation resulting from electrophilic attack at C-2 is less favourable than the carbocation resulting from electrophilic attack at C-3 because though the former has more resonance structures, the aromaticity is completely lost whereas in the latter intermediate, the positive charge resides on heterocyclic ring carbon or the nitrogen atom without affecting the benzene ring.

Some typical examples of electrophilic substitution of indole are given below Fig. If C-3 of indole is blocked, then electrophilic substitution takes place at C-2 and if both C-2 and C-3 are blocked, then electrophilic substitution takes place preferably at C-6 in aromatic ring.

The C 2 -C 3 double bond of indole is labile and can be easily cleaved by ozonolysis, peracids and sodium hypoiodate etc. Indole gives a resinous material whereas 3substituted or 2,3-disubstituted indoles undergo C 2 -C 3 cleavage as in case of ozonolysis eqs. The five membered ring is reduced by a number of reagents in acidic media e. Reactions of Quinoline and IsoquinolineQuinolines and isoquinolines behave similar to pyridine.

Heterocyclic Chemistry

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In chemistry , aromaticity is a property of cyclic ring-shaped , planar flat structures with pi bonds in resonance those containing delocalized electrons that gives increased stability compared to other geometric or connective arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds —they might be cyclic , but only aromatic rings have enhanced stability. Since the most common aromatic compounds are derivatives of benzene an aromatic hydrocarbon common in petroleum and its distillates , the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined.

This appendix provides information about the composition and chemistry of PHC fuels and their vapors. This information may be used to identify certain petroleum chemicals that pose the greatest potential risks through PVI. The chemicals that make up PHCs can be divided into two general groups: aliphatic and aromatic compounds.

Aromaticity

Heterocyclic compound , also called heterocycle , any of a major class of organic chemical compounds characterized by the fact that some or all of the atoms in their molecules are joined in rings containing at least one atom of an element other than carbon C. In their general structure, heterocyclic compounds resemble cyclic organic compounds that incorporate only carbon atoms in the rings—for example, cyclopropane with a three-carbon-atom ring or benzene with a six-carbon-atom ring —but the presence of the heteroatoms gives heterocyclic compounds physical and chemical properties that are often quite distinct from those of their all-carbon-ring analogs. Heterocyclic compounds include many of the biochemical material essential to life. For example, nucleic acids , the chemical substances that carry the genetic information controlling inheritance, consist of long chains of heterocyclic units held together by other types of materials. Many naturally occurring pigments , vitamins , and antibiotics are heterocyclic compounds, as are most hallucinogens.

Heterocyclic Chemistry. Compounds classified as heterocyclic probably constitute the largest and most varied family of organic compounds. After all, every carbocyclic compound, regardless of structure and functionality, may in principle be converted into a collection of heterocyclic analogs by replacing one or more of the ring carbon atoms with a different element. Even if we restrict our consideration to oxygen, nitrogen and sulfur the most common heterocyclic elements , the permutations and combinations of such a replacement are numerous.

Heterocyclic compound , also called heterocycle , any of a major class of organic chemical compounds characterized by the fact that some or all of the atoms in their molecules are joined in rings containing at least one atom of an element other than carbon C. In their general structure, heterocyclic compounds resemble cyclic organic compounds that incorporate only carbon atoms in the rings—for example, cyclopropane with a three-carbon-atom ring or benzene with a six-carbon-atom ring —but the presence of the heteroatoms gives heterocyclic compounds physical and chemical properties that are often quite distinct from those of their all-carbon-ring analogs. Heterocyclic compounds include many of the biochemical material essential to life. For example, nucleic acids , the chemical substances that carry the genetic information controlling inheritance, consist of long chains of heterocyclic units held together by other types of materials. Many naturally occurring pigments , vitamins , and antibiotics are heterocyclic compounds, as are most hallucinogens. Modern society is dependent on synthetic heterocycles for use as drugs , pesticides , dyes , and plastics.

Volume I: Principles, Three- and Four-Membered Heterocycles

However, this reaction proceeds slowly, which is inconvenient dangerous since hot, conc. See full list on byjus. In contrast to compounds that exhibit aromaticity, aliphatic compounds lack this delocalization. Phenol, which is a benzine ring with a hydroxyl group attached to it. Start learning today! The term "aromatic" in organic chemistry now means that the molecule contains benzene, or its structural relatives.

Pyridine is a basic heterocyclic organic compound with the chemical formula C 5 H 5 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. The pyridine ring occurs in many important compounds, including agrochemicals , pharmaceuticals , and vitamins. Historically, pyridine was produced from coal tar. Today it is synthesized on the scale of about 20, tonnes per year worldwide.

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Heterocyclic compound

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Heterocyclic Chemistry

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