Simple aromatic rings, also known as simple arenes or simple aromatics, are aromatic organic compound

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s that consist only of a conjugated

Conjugated system

In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...

 planar ring system with delocalized

Delocalized electron

In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond....

 pi electron clouds. Many simple aromatic rings have trivial names. They are usually found as substructures of more complex molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s ("substituted aromatics"). Typical simple aromatic compounds are benzene

Benzene

Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

, indole

Indole

Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is a popular component of fragrances and the precursor to many pharmaceuticals. Compounds that contain an...

, and cyclotetradecaheptaene

Cyclotetradecaheptaene

Cyclotetradecaheptaene is an aromatic annulene with molecular formula C14H14. It is somewhat unstable due to ring strain....

.

Simple aromatic rings can be heterocyclic if they contain non-carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 ring atoms, for example, oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, or sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

. They can be monocyclic as in benzene, bicyclic as in naphthalene

Naphthalene

Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...

, or polycyclic as in anthracene

Anthracene

Anthracene is a solid polycyclic aromatic hydrocarbon consisting of three fused benzene rings. It is a component of coal-tar. Anthracene is used in the production of the red dye alizarin and other dyes...

. Simple monocyclic aromatic rings are usually five-membered rings like pyrrole

Pyrrole

Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...

 or six-membered rings like pyridine

Pyridine

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

. Fused aromatic rings consist of monocyclic rings that share their connecting bonds.

EWLINE Table of simple aromatic rings Furan Furan Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans.... Benzofuran Benzofuran Benzofuran is the heterocyclic compound consisting of fused benzene and furan rings. This colourless liquid is a component of coal tar. Benzofuran is the "parent" of many related compounds with more complex structures. For example, psoralen is a benzofuran derivative that occurs in several... Isobenzofuran Isobenzofuran Isobenzofuran is a heterocyclic compound consisting of fused benzene and furan rings. It is isomeric with benzofuran.Isobenzofuran is highly reactive and rapidly polymerizes; however, it has been identified and prepared by thermolysis of suitable precursors and trapped at low temperature.Though... Pyrrole Pyrrole Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3... Indole Indole Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is a popular component of fragrances and the precursor to many pharmaceuticals. Compounds that contain an... Isoindole Isoindole Isoindole in heterocyclic chemistry is a benzo fused pyrrole. The compound is an isomer of indole and its reduced form is an isoindoline.In solution its tautomer without full aromaticity over the whole ring system is predominant:... Thiophene Thiophene Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a flat five-membered ring, it is aromatic as indicated by its extensive substitution reactions. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene... Benzothiophene Benzothiophene Benzothiophene is an aromatic organic compound with a molecular formula C8H6S and an odor similar to naphthalene . It occurs naturally as a constituent of petroleum-related deposits such as lignite tar. Benzothiophene has no household use... Benzo[c]thiophene Benzo(c)thiophene Benzo[c]thiophene is an organic compound with the chemical formula C8H6S.... Imidazole Imidazole Imidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is a diazole and is classified as an alkaloid. Imidazole refers to the parent compound, whereas imidazoles are a class of heterocycles with similar ring structure, but varying substituents... Benzimidazole Benzimidazole Benzimidazole is a heterocyclic aromatic organic compound. This bicyclic compound consists of the fusion of benzene and imidazole. The most prominent benzimidazole compound in nature is N-ribosyl-dimethylbenzimidazole, which serves as an axial ligand for cobalt in vitamin B12. Benzimidazole, in... Purine Purine A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature.... Pyrazole Pyrazole Pyrazole refers both to the class of simple aromatic ring organic compounds of the heterocyclic diazole series characterized by a 5-membered ring structure composed of three carbon atoms and two nitrogen atoms in adjacent positions, and to the unsubstituted parent compound... Indazole Indazole Indazole, also called benzpyrazole or isoindazone, is a heterocyclic aromatic organic compound.Indazole derivatives display a broad variety of biological activities....   Oxazole Oxazole Oxazole is the parent compound for a vast class of heterocyclic aromatic organic compounds. These are azoles with an oxygen and a nitrogen separated by one carbon. Oxazoles are aromatic compounds but less so than the thiazoles... Benzoxazole Benzoxazole Benzoxazole is an aromatic organic compound with a molecular formula C7H5NO, a benzene-fused oxazole ring structure, and an odor similar to pyridine. Benzoxazole is used primarily in industry and research, and has no household use....   Isoxazole Isoxazole Isoxazole is an azole with an oxygen atom next to the nitrogen. It is also the class of compounds containing this ring.Isoxazole rings are found in some natural products, such as ibotenic acid. Isoxazoles also form the basis for a number of drugs, including the COX-2 inhibitor valdecoxib... Benzisoxazole Benzisoxazole Benzisoxazole is an aromatic organic compound with a molecular formula C7H5NO containing a benzene-fused isoxazole ring structure. Benzisoxazole has no household use. It is used primarily in industry and research....   Thiazole Thiazole Thiazole, or 1,3-thiazole, is a heterocyclic compound that contains both sulfur and nitrogen; the term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular formula C3H3NS... Benzothiazole Benzothiazole Benzothiazole is an aromatic heterocyclic compound with the chemical formula . It is colorless, slightly viscous liquid. Although the parent compound, benzothiazole is not widely used, many of its derivatives are found in commercial products or in nature...     Six-membered rings: Fused six-membered rings:   Benzene Benzene Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6.... Naphthalene Naphthalene Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings... Anthracene Anthracene Anthracene is a solid polycyclic aromatic hydrocarbon consisting of three fused benzene rings. It is a component of coal-tar. Anthracene is used in the production of the red dye alizarin and other dyes... Pyridine Pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom... Quinoline Quinoline Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown... Isoquinoline Isoquinoline Isoquinoline is a heterocyclic aromatic organic compound. It is a structural isomer of quinoline. Isoquinoline and quinoline are benzopyridines, which are composed of a benzene ring fused to a pyridine ring. In a broader sense, the term isoquinoline is used to make reference to isoquinoline... Pyrazine Pyrazine Pyrazine is a heterocyclic aromatic organic compound with the chemical formula C4H4N2.Pyrazine is a symmetrical molecule with point group D2h. Derivatives like phenazine are well known for their antitumor, antibiotic and diuretic activity. Pyrazine is less basic in nature than pyridine, pyridazine... Quinoxaline Quinoxaline A quinoxaline, also called a benzopyrazine, in organic chemistry, is a heterocyclic compound containing a ring complex made up of a benzene ring and a pyrazine ring... Acridine Acridine Acridine, C13H9N, is an organic compound and a nitrogen heterocycle. Acridine is also used to describe compounds containing the C13N tricycle.... Pyrimidine Pyrimidine Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring... Quinazoline Quinazoline Quinazoline is a compound made up of two fused six-membered simple aromatic rings, a benzene ring and a pyrimidine ring. Its chemical formula is C8H6N2. Quinazoline is yellow and crystalline...   Pyridazine Pyridazine Pyridazine is a heteroaromatic organic compound with the molecular formula C4H4N2, sometimes called 1,2-diazine. It contains a six-membered ring with two adjacent nitrogen atoms. It is a colorless liquid with a boiling point of 208 °C.... Cinnoline Cinnoline Cinnoline is an aromatic heterocyclic compound with the formula C8H6N2. It is isomeric with quinoxaline, phthalazine and quinazoline.-Properties:...

Heterocyclic aromatic rings

The nitrogen (N)-containing aromatic rings can be separated into basic aromatic ring

Basic aromatic ring

Basic aromatic rings are aromatic rings in which the lone pair of electrons of a ring-nitrogen atom is not part of the aromatic system and extends in the plane of the ring. This lone pair is responsible for the basicity of these nitrogenous bases, similar to the nitrogen atom in amines. In these...

s that are easily protonated

Protonation

In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...

, and form aromatic cations and salts (e.g., pyridinium

Pyridinium

Pyridinium refers to the cationic form of pyridine. This can either be due to protonation of the ring nitrogen or because of addition of a substituent to the ring nitrogen, typically via alkylation. The lone pair of electrons on the nitrogen atom of pyridine is not delocalized, and thus pyridine...

), and non-basic aromatic rings.

• In the basic aromatic rings, the lone pair
  Lone pair
  In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
  
   of electron
  Electron
  The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
  
  s is not part of the aromatic system and extends in the plane of the ring. This lone pair is responsible for the basicity of these nitrogenous base
  Nitrogenous base
  A nitrogenous base is a nitrogen-containing molecule having the chemical properties of a base. It is an organic compound that owes its property as a base to the lone pair of electrons of a nitrogen atom. In biological sciences, nitrogenous bases are typically classified as the derivatives of two...
  
  s, similar to the nitrogen atom in amine
  Amine
  Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
  
  s. In these compounds the nitrogen atom is not connected to a hydrogen atom. Examples of basic aromatic rings are pyridine
  Pyridine
  Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
  
   or quinoline
  Quinoline
  Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown...
  
  . Several rings contain basic as well as non-basic nitrogen atoms, e.g., imidazole
  Imidazole
  Imidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is a diazole and is classified as an alkaloid. Imidazole refers to the parent compound, whereas imidazoles are a class of heterocycles with similar ring structure, but varying substituents...
  
   and purine
  Purine
  A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
  
  .

• In the non-basic rings, the lone pair of electrons of the nitrogen atom is delocalized and contributes to the aromatic pi electron system. In these compounds, the nitrogen atom is connected to a hydrogen atom. Examples of non-basic nitrogen-containing aromatic rings are pyrrole
  Pyrrole
  Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...
  
   and indole
  Indole
  Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is a popular component of fragrances and the precursor to many pharmaceuticals. Compounds that contain an...
  
  .

In the oxygen- and sulfur-containing aromatic rings, one of the electron pairs of the heteroatoms contributes to the aromatic system (similar to the non-basic nitrogen-containing rings), whereas the second lone pair extends in the plane of the ring (similar to the basic nitrogen-containing rings).

Criteria for aromaticity

• Molecule must be cyclic.
• Every atom in the ring must have a p orbital, which overlaps with p orbitals on either side (completely conjugated
  Conjugated system
  In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
  
  ).
• Molecule must be planar.
• It must contain an odd number of pairs of pi electrons; must satisfy Huckel's rule: (4n+2) pi electrons, where n is an integer starting at zero.

In contrast, molecules with 4n pi electrons are antiaromatic.