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Heterocyclic Aromatic Compounds - Introduction, Important structures, Bicyclic compounds, Practice Problems & FAQs

Heterocyclic Aromatic Compounds - Introduction, Important structures, Bicyclic compounds, Practice Problems & FAQs

We come across many helpful organic compounds in our daily lives whose generic names we all recognise, but we are not familiar with their chemical make-ups or chemical nomenclature.

The important genetic components DNA and RNA, which are found in human bodies, are made up of heterocyclic substances. All of the dyes used to color clothing, including jeans, include heterocyclic compounds. Vitamins we intake are heterocyclic compounds.

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Although heterocyclic chemistry is difficult to explain, it is important to realize that the chemicals produced can be employed for a variety of useful and advantageous purposes in applications ranging from pharmaceuticals to TV displays.

They are mostly used as veterinary goods, agrochemicals, and medications. Additionally, they are used as sanitisers, developers, antioxidants, corrosion inhibitors, copolymers, and dye ingredients. They serve as the means by which other organic compounds are synthesized.

The majority of medications, pesticides, dyes, and polymers are made of heterocyclic compounds.

So let's get into more depth about them and learn more about a special category of heterocyclic compounds known as heterocyclic aromatic compounds.

TABLE OF CONTENT

  • Heterocyclic Compounds -Introduction
  • Heterocyclic Aromatic Compounds
  • Six-Membered Heteroaromatic Systems
  • Five-Membered Heteroaromatic Systems
  • Bicyclic Heteroaromatic Compounds
  • Practice Problems
  • Frequently Asked Questions – FAQs

Heterocyclic Compounds - Introduction

  • Heterocyclic compounds are cyclic compounds having an extra element, such as oxygen, nitrogen, phosphorus, or sulfur, attached to the carbon ring.
  • A heterocyclic compound has at least two different components in its ring. Unstrained 5- and 6-membered rings are the major ones. Most research and applications in the field of heterocyclic chemistry are focused on in this section, which places a focus on unsaturated derivatives in a particular area.
  • Among them are pyridine, thiophene, pyrrole, and furan. A substantial subclass of heterocycles is those fused to benzene rings. For instance, the fused benzene derivatives of pyridine, thiophene, pyrrole, and furan are quinoline, benzothiophene, indole, and benzofuran, respectively.
  • A sizable third class of compounds when two benzene rings are combined. This third class of chemicals includes the molecules dibenzothiophene, acridine, carbazole, and dibenzofuran respectively as equivalents to the previously listed heterocycles.

Heterocyclic Aromatic Compounds:

As their title indicates, aromatic heterocyclic compounds are cyclic aromatic compounds. Huckel’s Rule states that a compound must abide by the following conditions in order to have an aromatic nature:

  1. It must be planar in form.
  1. The system shouldn't contain any sp3 hybridized atoms.
  1. There must be a total of (4n+2) electrons in it.
  1. We can remove the second criterion if only one lone pair of sp3 hybridized atoms take part in the conjugation.

Heterocyclic aromatic compounds include Furan, Indole, Pyrrole, Thiophene, Pyridazine, Oxazole, Pyrazole, Imidazole, Pyridine, Pyridazine, Purine, Quinoline, Isoquinoline, Carbazole, and others.

Six-Membered Heteroaromatic Systems:

Structure of Pyridine:

By substituting N for CH, pyridine's structure is completely similar to that of benzene. The three main differences are

  1. The heteroatom's presence causes the hexagonal geometry to deviate from being perfectly regular, particularly because of the shorter carbon-nitrogen bonds;
  1. An unshared electron pair is used to substitute the hydrogen in the ring's plane. This unshared electron pair is positioned in a sp2 hybrid orbital and is not at all connected to the aromatic π electron sextet.
  1. A strong permanent dipole, traceable to the greater electronegativity of nitrogen compared with carbon.

Here new aromatic heterocycles, pyridazine, pyrimidine, and pyrazine, would be discovered if we continued the procedure of replacing further CH groups with nitrogen atoms on paper.

Oxygen-containing six-membered heterocyclic aromatic compound:

Although the six-membered aromatic heterocyclic pyridine is by far the most significant, there are oxygen heterocycles called pyrones that resemble pyridones. Although the -Pyrone is somewhat unstable, the pyrones are aromatic.

The stable aromatic cations known as pyrylium salts are metal complexes that give various floral colors.

Five-Membered Heteroaromatic Systems

Structure of Pyrrole:

Since pyrrole is similarly aromatic, it undergoes the typical aromatic substitution processes (Friedel-Crafts, nitration, and halogenation) rather than addition reactions, making it more electrophilic reactive than benzene or pyridine. It is crucial to understand that the pyrrole's nitrogen lone pair is a component of the aromatic six-π electron system.

Structures of Thiophene and Furan:

With the exception of the NH being substituted by S and O, respectively, the structures of thiophene and furan are quite similar to those covered in detail for pyrrole above. As a result, each heteroatom has two lone pairs—one of which is part of the aromatic sextet, like in pyrrole, and the other of which is not—both of which are positioned in sp2 hybrid orbitals in the plane of the ring.

Bicyclic Heteroaromatic Compounds:

  • It is simple to extrapolate to those systems that combine two (or more) of these types once the concepts of the structures of benzene, naphthalene, pyridine, and pyrrole have been internalized.
  • For example, quinoline is similar to naphthalene but only has one pyridine ring, and indole is similar to pyrrole but with a benzene ring attached. These are considered as bicyclic Heteroaromatic Compounds.
  • In Indole, if you'd rather think of it that way, it has 10 π electrons and is an aromatic system with eight coming from double bonds, four from single bonds, and one from the nitrogen atom.

Practice Problems:

Q1. Which of the following compounds is not a heterocyclic aromatic compound?

Answer: (D)

Solution: As their name suggests, aromatic heterocyclic compounds are cyclic aromatic compounds. It must be planar and have a total of (4n+2) electrons according to Huckel's Rule. It shouldn't contain any sp3 hybridized atoms. We can remove this requirement if only one lone pair of sp3 hybridized atoms participate in the conjugation.

In option (A), Its structure is planar and devoid of sp3 hybridized atoms. It has two distinct C and N atoms. Due to the presence of 6e-, lone pairs of nitrogens will not participate in the conjugation. As a result, the compound satisfies all of the aforementioned requirements. It is an aromatic heterocyclic chemical.

In option (B), It has a planar structure and the atoms C, O, and N are all different types. Nitrogens won't participate in the conjugation, but one lone pair of sp3 hybridized atoms from the element O will, resulting in the formation of the compound 6e-. As a result, it is an aromatic heterocyclic compound.

In option (C), It is planar in shape and has two separate types of atoms, C and N. The conjugation of one lone pair of the sp3 hybridized atom N results in the presence of 6e-. As a result, it is an aromatic heterocyclic compound.

In option (D), the sp3 hybridized carbon atom has a non-planar structure and will not take part in the conjugation. As a result, it is not an aromatic heterocycle.

Hence, the correct answer is an option (D).

Q2. Which statement is correct regarding thiophene and furan?

  1. Thiophene has more aromatic character than furan
  2. Furan has more aromatic character than thiophene
  3. Thiophene and Furan have the same aromatic character.
  4. Both Thiophene and Furan are anti-aromatic in nature

Answer: A

Solution: Relative aromaticity is determined by heteroatom electronegativity. Higher electronegativity atoms cling to their lone electron pair more strongly, which makes delocalization more difficult. Since oxygen is the more electronegative heteroatom among S and O, furan aromaticity declines relative to thiophene.

Q3. Which among the following is considered as bicyclic Heteroaromatic Compounds?

  1. Indole
  2. Quinoline
  3. A and B
  4. None of the above

Answer: C

Solution: For example, quinoline is similar to naphthalene but only has one pyridine ring, and indole is similar to pyrrole but with a benzene ring attached. These are considered as bicyclic Heteroaromatic Compounds.

Q4. Which of the following compounds is a heterocyclic aromatic compound?

Answer: B

Solution: The cyclic aromatic compounds known as aromatic heterocyclic compounds must be planar, devoid of sp3 hybridized atoms, and contain a total of (4n+2) electrons. The second condition can be removed if only one lone pair of sp3 hybridized atoms participates in the conjugation (no sp3 hybridized atoms ).

Option (A) comprises two distinct atoms, C and O, and has a planar structure. The conjugation of the single pair of sp3 hybridized atoms O will result in the presence of 4e-. As a result, it is not an aromatic heterocycle.

In option (B), It has a planar structure, no atoms that have undergone sp3 hybridization, and two distinct atoms, C and N. Since nitrogens won't participate in the conjugation, 10e- is the result. As a result, it is an aromatic heterocyclic compound.

The non-planar structure in options (C) and (D) contains sp3 hybridized atoms that will not take part in the conjugation. As a result, it is not an aromatic heterocycle.

Frequently Asked Questions – FAQs:

1. What do you understand about the term “azole”?
Solution:
The systematic suffix "-ole" designates a five-membered heterocyclic ring. The term "the azoles" is sometimes used to refer to all five-membered aromatic heterocycles having nitrogen in the ring. The oxygen and sulfur counterparts of imidazole are called oxazole and thiazole, respectively.

2. What is the most common use of the heterocyclic aromatic compound “Pyrimidine”?
Solution:
The medicinally advantageous framework pyrimidine is used in both bioactive natural items and manufactured medicines. Three functionalized pyrimidines, such as uracil, are present in both DNA and RNA structures. Many antiviral medications, particularly those used to treat HIV, are also made of modified DNA and contain pyrimidines.

3. Is Pyridine considered as a drug or not?
Solution:
Drugs containing pyridine and dihydropyridine are typically employed as antibacterial, antiviral, anticancer, antioxidant, antihypertensive, antidiabetic, antimalarial, and anti-inflammatory agents, as well as psychopharmacological antagonists and antiamebic compounds.

4. What are the bases present in DNA and RNA? Are they heterocyclic in nature?
Solution:
There are five types of heterocyclic bases present in DNA and RNA. Five different bases, two sugars, and the phosphate group make up a nucleic acid. All of the bases are aromatic and either monocyclic pyrimidines or bicyclic purines. Nucleic acids only include the two purine bases adenine (A), which we have already encountered, and guanine (G)

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