Non-Benzenoids

NON-BENZENOIDS

Non-benzenoids are aromatic compounds that do not contain a benzene ring. These compounds may contain other types of highly unsaturated rings.

Examples of non-benzenoids include:

1.     Cyclooctatetraene (COT): This is an eight-membered ring with four double bonds. It is not stable in its neutral form, but can form stable complexes with metals.

2.     Azulene: This is a bicyclic compound containing a five-membered ring fused to a seven-membered ring. It is known for its blue color and is used in cosmetics and dyes.

3.     Naphthalene: This is a bicyclic compound containing two fused six-membered rings. It is used as a starting material for the synthesis of many organic compounds.

4.     Anthracene: This is a tricyclic compound containing three fused six-membered rings. It is used in the production of dyes and plastics.

5.     Pyridine: This is a six-membered ring containing one nitrogen atom. It is a basic compound and is used in the production of many organic compounds.

Nucleophilic and electrophilic reaction of heterocycles

Heterocycles are organic compounds that contain a ring structure with at least one heteroatom, such as nitrogen, oxygen, or sulfur, in addition to carbon atoms. The reactivity of heterocycles is governed by the presence of heteroatoms and their electron density.

Nucleophilic reactions of heterocycles:

1.     Nucleophilic substitution: Heterocycles with nitrogen, oxygen, or sulfur atoms can undergo nucleophilic substitution reactions with nucleophiles such as alkoxides, amines, or thiols. For example, pyridine can be reacted with sodium ethoxide to give ethoxypyridine.

2.     Nucleophilic addition: Heterocycles with double bonds, such as pyridine or furan, can undergo nucleophilic addition reactions with nucleophiles such as ammonia or hydrazine. For example, pyridine can be reacted with ammonia to give pyridine-4-amine.

3.     Nucleophilic ring opening: Heterocycles with strained ring systems, such as aziridines or epoxides, can undergo ring opening reactions with nucleophiles such as water or alcohols. For example, aziridine can be reacted with water to give aminoethanol.

Electrophilic reactions of heterocycles:

1.     Electrophilic substitution: Heterocycles can undergo electrophilic substitution reactions with electrophiles such as nitric acid, sulfuric acid, or acylating agents. For example, pyridine can be reacted with nitric acid to give 3-nitropyridine.

2.     Electrophilic addition: Heterocycles with double bonds, such as pyridine or furan, can undergo electrophilic addition reactions with electrophiles such as hydrogen halides or halogens. For example, furan can be reacted with hydrogen chloride to give tetrahydrofuran-2-yl chloride.

3.     Electrophilic ring closure: Heterocycles with open-chain precursors can undergo electrophilic ring closure reactions with electrophiles such as Lewis acids or carbonyl compounds. For example, 2-aminobenzaldehyde can be reacted with acetic anhydride and sulfuric acid to give 2-amino-1,3-benzoxazole.

Aromaticity of heterocyclic compounds compounds

Aromaticity is a property of cyclic compounds that are highly stable due to the delocalization of electrons in their pi-systems. A compound is considered aromatic if it meets the following criteria:

1.     The compound is cyclic.

2.     The compound is planar.

3.     The compound is fully conjugated, meaning that all atoms in the ring must have a p-orbital or participate in the conjugation.

4.     The compound must follow Hückel's rule, which states that the number of pi-electrons in the ring must equal 4n+2, where n is a non-negative integer.

Heterocyclic compounds are cyclic compounds containing at least one heteroatom (such as nitrogen, oxygen, or sulfur) in the ring. Many heterocyclic compounds are aromatic, including pyridine, pyrrole, imidazole, pyrazine, and pyrimidine. These heterocycles can be considered aromatic if they satisfy the above-mentioned criteria of aromaticity.

For example, pyridine is a six-membered ring containing five carbon atoms and one nitrogen atom. The nitrogen atom has a lone pair of electrons that participates in the pi-system of the ring. Pyridine is planar and fully conjugated, with six pi-electrons in the ring. It follows Hückel's rule (4n+2), where n=1, and thus is considered aromatic.

Similarly, pyrrole is a five-membered ring containing four carbon atoms and one nitrogen atom. The nitrogen atom has a lone pair of electrons that participates in the pi-system of the ring. Pyrrole is planar and fully conjugated, with six pi-electrons in the ring. It follows Hückel's rule (4n+2), where n=1, and thus is considered aromatic.

In contrast, some heterocyclic compounds may not be aromatic, even if they contain a fully conjugated ring. For example, furan is a five-membered ring containing four carbon atoms and one oxygen atom. Furan is planar and fully conjugated, with four pi-electrons in the ring. It follows Hückel's rule (4n+2), where n=0. Since the number of pi-electrons does not satisfy Hückel's rule, furan is not considered aromatic.