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Non Benzenoid Aromatic Compounds

Hydrocarbons are organic compounds which are formed by bonding between carbon and hydrogen atoms. We know that carbon exhibits tetravalency means form four chemical bonds with similar or different atoms or groups.  Another important property of carbon atom is catenation which is property to form chemical bond with same element to form long chain compounds. 

These two properties make carbon atom able to form vast number of chemical compounds. On the basis of structures, hydrocarbons can be classified as aliphatic and aromatic hydrocarbons. Aliphatic hydrocarbons can be further classified as open chain and cyclic compounds. Open chain compounds have straight or branched or cyclic carbon chain in the molecule. Open chain compounds can be saturated or unsaturated. Saturated hydrocarbons have only single bonded carbon atoms like alkane. 

Unsaturated hydrocarbons have multiple covalent bonds between carbon atoms such as double covalent bond in alkenes and triple covalent bonds in alkynes. The cyclic aliphatic compounds have ring of carbon atoms with single or multiple bonds. They are commonly called as cycloalkane and cycloalkene. Another type of hydrocarbons is aromatic compounds. Aromatic compounds or arenes or aromatics are hydrocarbons with conjugated pi-electron system that makes them more stable compare to aliphatic hydrocarbons with same molecular formula. 

Conjugated pi-system contains alternate single and double bonds in the molecule. Some common example of aromatic compounds are benzene, toluene etc. Aromatic compounds must have 4n+2 pi-electrons. Here n can be from 1, 2, 3 …. This is known as Huckel’s rule of aromaticity.  Aromaticity is the property of aromatic compound. According to this, planer, cyclic compounds with conjugated pi-system exhibit extra stability and called as aromatic compounds. 

Benzene is simplest aromatic compound with 6 carbon atoms in hexagonal cyclic ring. There are three pi-bonds which are arranged in alternate manner in the cyclic ring of benzene. But it is not necessary that all aromatic compounds contain benzene ring. On that basis, aromatic compounds can be further classified as benzenoid and non-benzenoid compounds. Benzenoid compounds have at least one benzene ring in the molecule whereas non-benzenoid compounds are aromatic compounds with conjugated pi-system but do not benzene ring.

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Non Benzenoid Aromatic Compounds Definition

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Non benzenoid aromatic compound are chemical compounds with conjugated pi-electron system with ring of 5 to 7 carbon atoms.  They exhibit aromaticity due to alternate pi-bonds in the molecule. They do not have benzene ring therefore called as non-benzenoid compounds. The chemical reactions of these compounds are like benzenoid compounds only. 

Non Benzenoid Aromatic Compounds Examples

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Some common classes of non-benzenoid compounds are listed below.

Azulenes: They have unsaturated seven-membered and five-membered ring structures. So we can say that they are usually structural isomers of naphthalene. But unlike naphthalene, they are colored compounds such as Azulene is a dark blue color aromatic compound. They can be prepared with the help of other aromatic compounds such as reaction of pyrylium salts with cyclopentadienides, from tropone derivatives and reaction of oxaazulanones with enamines.
Oxaazulanones: These non-benzenoid compounds also have azulene rings with oxygen atoms such as 5-Isopropyl-3-(methoxycarbonyl)-2H-cycloheptafuran-2-one. 
Tropones & Tropolones: Tropone is 2, 4, 6-cycloheptatrien-1-one; a non-benzenoid compound. Similarly torpolone is 2-Hydroxy-2, 4, 6-cycloheptatrien-1-one. 
Pentafulvene: Fulvenes or Pentafulvenes have same formula as of benzene. But the ring contains 5 carbon atoms only. The presence of conjugated pi-electron system makes the aromatic compounds.
Tropylium Ion: It is an aromatic ion with molecular formula $[C_{7}H_{7}]^{+}$. Salts of tropylium ions are stable due to aromaticity. 

Examples of non-benzenoid compounds with their structures are listed below. 

SI.No  Class of non-benzenoid compounds  Name of compound  Structure 
 1.  Azulenes  Azulene
Azulene
     Azulene-1-carboxaldehyde 
Azulene-1-Carboxaldehyde
     1,3-Diiodoazulene 1,3-Diiodoazulene
     1,3-Dibromoazulene 1,3-Dibromoazulene
     Guaiazulene Guaiazulene
     7-Isopropyl-1,4-dimethylazulene-3-carboxaldehyde 7-Isopropyl-1,4-Dimethylazulene-3-Carboxaldehyde
     Sodium 7-Isopropyl-1,4-dimethylazulene-3-sulfonate 
Sodium-7-Isopropyl-1,4-Dimethylazulene-3-Sulfonate
 2.
 Oxaazulanones  5-Isopropyl-3-(methoxycarbonyl)-2H-cycloheptafuran-2-one  Cycloheptafuran
     3-(Methoxycarbonyl)-2H-cycloheptafuran-2-one Cycloheptafuran
 3.  Tropones & Tropolones  Colchicine Colchicine 
     2-Chlorotropone 2-Chlorotropone
     Dibenzosuberenone Dibenzosuberenone
     Hinokitiol Hinokitiol
     2-Methoxytropone 2-Methoxytropone
     Tropolone Tropolone
     Tropolone Tosylate Tropolone Tosylate
 4. Pentafulvene  6-(Dimethylamino)fulvene  6-(Dimethylamino)fulvene
     6,6-Dimethylfulvene 6,6-Dimethylfulvene
      6,6-Diphenylfulvene 6,6-Diphenylfulvene 
 5.  Tropylium Ion  Tropylium Tetrafluoroborate Tropylium Tetrafluoroborate

Examples of Non Benzenoid Aromatic Compounds

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Benzenoid and non-benzenoid compounds are aromatic compounds with conjugated pi-system.  The presence of alternate pi-bonds in the molecule delocalized the pi-electron and provides extra stability to the molecule. The aromatic compounds with no benzene ring are called as non-benzenoid aromatic compounds. 

The aromaticity of aromatic compounds can be explained with the help of Hückel rules which states that aromatic compounds must have 4n+2 pi electrons. Here n can be 0, 1, 2, 3 . . . confers that compounds with 2, 6, 10, 14 . . . Ï€ electrons are aromatic compound. For example; benzene has 6 Ï€ electrons; an aromatic compound, but cyclobutadiene with 4 pi-electrons and cyclooctatraene with 8 pi-electrons are non-aromatic compounds. Conjugated polyenes which are also called as annulenes follow Hückel rule and aromatic compounds. For example; [10]-annulene and [18]-Annulene follow Hückel rule so aromatic in nature. 

18-Annulene
 
Tropolone is another example of non-benzenoid compound which is aromatic in nature but does not have any benzene ring. The aromaticity is due to conjugated pi-electron system in the molecule. 

Tropolone

Difference Between Benzenoid and Non Benzenoid Compounds

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Let’s discuss the difference between benzenoid and non benzenoid compounds. No doubt both are aromatic compounds and exhibit extra stability due to conjugated pi-electron system. 

The major difference in benzenoid and non-benzenoid compound is the structure of molecules. Benzenoid compounds have at least one benzene ring whereas non-benzenoid compound do not have benzene ring.  For example, toluene, aniline phenol, nitrobenzene are examples of benzenoid compounds. Similarly azulene, tropolone are example of non-benzenoid compounds. The structural difference definitely affects the chemical reactivity of molecules.  

Benzenoid Compounds
Non Benzenoid Compounds
 
Overall both benzenoid and non-benzenoid are aromatic compounds with conjugated pi-system and different structural features. 

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