Major Classes of Heterocyclic Compounds

A heterocyclic or ring structure may be a cyclic compound that has atoms of a minimum of two different elements as members of its ring. Heterocyclic chemistry is the branch of chemistry that helps in handling the synthesis, properties, and applications of these heterocycles.

Examples of heterocyclic compounds include the bulk of medicine, all of the nucleic acids, most biomass such as cellulose and related materials, and lots of natural and artificial dyes quite half-known compounds are heterocycles. 59% of folks FDA-approved drugs contain nitrogen heterocycles.

The history of heterocyclic chemistry was found to begin within the 1800s, in step with the event of chemistry, that includes some noteworthy developments they are:

• 1818: Brugnatelli isolates alloxan from acid.

• 1832: By treating starch with vitriol Dobereiner produces furfural which is a furan. 

• 1834: Runge obtains pyrrole also known as "fiery oil", by the process of dry distillation of bones.

• 1906: Friedlander, allows synthetic chemistry to displace an outsized agricultural industry by synthesizing the indigo dye.

• 1936: Treibs isolates chlorophyll derivatives from petroleum, explaining the biological origin of petroleum.

• 1951: Chargaff's rules are described that are used to highlight the role of heterocyclic compounds that include purines and pyrimidines within the ordering.

Five and Six Membered Rings

The study of heterocyclic chemistry especially focuses on unsaturated derivatives, and thus the applications involve the compounds of unstrained five- and six-membered rings. Included are pyridine, thiophene, pyrrole, and furan. To those that are fused to the benzene rings another large class of heterocycles is referred to. For instance, the fused benzene derivatives of pyrrole, thiophene, pyridine, and furan are indole, benzothiophene, quinoline, and benzofuran, respectively. A third large family of compounds is formed by the fusion of the two benzene rings. The heterocycles that are mentioned previously have analogs for this third family of compounds such as carbazole, dibenzothiophene, acridine, and dibenzofuran, respectively.

Heterocyclic compounds are often usefully classified as supported by their electronic structure. The saturated heterocycles behave like acyclic derivatives. Thus, piperidine and tetrahydrofuran are conventional amines and ethers, with modified steric profiles. Hence the study of this heterocyclic chemistry mainly focuses on the unsaturated rings.

Examples of some of the heterocycles that contain no carbon atom are borazine (B3N3 ring), hexachlorophosphazene (P3N3 rings), and S4N4. For naming heterocyclic compounds IUPAC recommends the Hantzsch-Widman nomenclature.

General Features of Five and Six-Membered Rings

The five- or six-membered rings are the most common heterocycles and these heterocycles contain the heteroatoms of nitrogen (N), oxygen (O), or sulfur (S). Pyridine, pyrrole, furan, and thiophene are the best-known heterocycles. A molecule of pyridine contains a hoop of six atoms that are arranged as five carbon atoms and one nitrogen atom. Pyrrole, furan, and thiophene molecules each contain five-membered rings, composed of four atoms of carbon and one atom of nitrogen, oxygen, or sulfur, respectively.

Pyridine and Pyrrole - Physical and Chemical Properties 

Pyridine and pyrrole are both nitrogen heterocycles-their molecules contain nitrogen atoms alongside carbon atoms within the rings. The molecules of most of the materials that are biological in nature partially consist of pyridine and pyrrole rings, and such types of materials yield small amounts of pyridine and pyrrole upon strong heating. In fact, both of these substances such as pyridine and pyrrole were discovered within the 1850s that are formed by the strong heating of bones in an oily mixture. At the present day, the synthesis of pyridine and pyrrole is done by synthetic reactions. Their main commercial interest lies in the conversion of these substances to other substances, such as dyestuffs and medicines. Pyridine is used in the applications of a rubber additive, a waterproofing agent, a solvent, an alcohol denaturant, and a dyeing adjunct.

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Physical Properties of Pyridine

• It is a colorless liquid.

• The boiling point is 388 K and the melting point is 232 K.

• It has an unpleasant odor.

• It can be easily detected with the help of gas chromatography and mass spectrometry methods.

Chemical Properties of Pyridine

• The miscible of pyridine is found with the water and all organic solvents.

• Pyridine behaves as a tertiary amine.

• The pyridine reaction characteristics are classified into three groups:

1. In the participation of heteroatoms.

2. Substitution of the hydrogen atom in the pyridine ring.

3. Reduction and oxidation reactions.

Physical Properties of Pyrrole

• It is a colorless and volatile liquid.

• Its boiling point lies between 402 to 404 K and the melting point is 250 K.

• It is readily soluble in alcohol and ether but sparingly soluble in water.

Chemical Properties of Pyrrole

The pyrrole act as both weak base and weak acid and undergoes the following reactions:

• Reduction and oxidation reaction.

• Ring expansion reaction.

• Reimer-Tiemann reaction.

• Electrophilic aromatic substitution reaction.

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Furan - Physical and Chemical Properties

Furan is an oxygen-containing heterocycle that is primarily employed for the conversion to other substances that include pyrrole as well. Furfural, an in-depth chemical relative of furan, is obtained from oat hulls and corn cobs and is employed within the production of intermediates for nylon. A sulfur heterocycle, Thiophene resembles the benzene both in its physical and chemical properties. It's a frequent contaminant of benzene that is obtained from natural sources and during the purification of benzene thiophene was first discovered. Just like the other compounds, it's used primarily for conversion to other substances. Furan and thiophene were both discovered within the latter part of the 19th century. 

Physical Properties of Furan

• It is a colorless and volatile liquid.

• Its boiling point is 304.4 K and its melting point is 187.5 K.

• It is insoluble in water and soluble in benzene and ether.

Chemical Properties of Furan

• Furan behaves as a resonance hybrid.

• It undergoes sulphonation, halogenation, nitration, acylation, alkylation, Vilsmeier Haack formylation reactions, etc.

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Heterocyclic compounds have a good range of applications: They're predominant among the sort of compounds used as pharmaceuticals, as agrochemicals, and as veterinary products. They also find applications as sensitizers, developers, antioxidants, corrosion inhibitors, copolymers, dyestuff. They are used as vehicles within the synthesis of other organic compounds. Some of the natural products e.g. antibiotics like penicillin, cephalosporin; alkaloids like vinblastine, morphine, reserpine, etc. have heterocyclic moiety.

One of the reasons for the widespread use of heterocyclic compounds is that their structures are often subtly manipulated to realize a required modification in function. Many heterocycles can be fitted into one among a couple of broad groups of structures that have overall similarities in their properties but significant variations within the group. These types of variations can include differences in acidity or basicity and different polarity. The possible structural variations include the change of one heteroatom for an additional ring and different positioning of equivalent heteroatoms within the ring.

Conclusion

The rate at which major classes of heterocyclic compounds are still being invented testifies to the strength and vitality of this area of chemistry. The challenges of discovering new heterocyclic systems and of understanding their properties also still stimulate research within the area. Heterocyclic chemistry deals with about sixty-five percent of chemistry literature that is found in the heterocyclic compounds. Heterocyclic compounds are cosmopolitan in nature and essential to life; they play an important role within the metabolism of all living cells. Genetic material DNA is also composed of heterocyclic bases-pyrimidines and purines. A large number of these heterocyclic compounds, both synthetic and natural compounds, are pharmacologically active and are in clinical use.