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Aromatic hydrocarbon

Aromatic hydrocarbon

Overview
An aromatic hydrocarbon or arene (or sometimes aryl hydrocarbon) is a hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 with alternating double and single bonds between carbon atoms. The term 'aromatic' was assigned before the physical mechanism determining aromaticity
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

 was discovered, and was derived from the fact that many of the compounds have a sweet scent. The configuration of six carbon atoms in aromatic compounds is known as a benzene ring
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....

, after the simplest possible such hydrocarbon, 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....

.
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Unanswered Questions
Encyclopedia
An aromatic hydrocarbon or arene (or sometimes aryl hydrocarbon) is a hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 with alternating double and single bonds between carbon atoms. The term 'aromatic' was assigned before the physical mechanism determining aromaticity
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

 was discovered, and was derived from the fact that many of the compounds have a sweet scent. The configuration of six carbon atoms in aromatic compounds is known as a benzene ring
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....

, after the simplest possible such hydrocarbon, 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....

. Aromatic hydrocarbons can be monocyclic (MAH) or polycyclic (PAH).

Some non-benzene-based compounds called heteroarenes, which follow Hückel's rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...

, are also aromatic compounds. In these compounds, at least one carbon atom is replaced by one of the heteroatom
Heteroatom
In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure...

s 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...

. Examples of non-benzene compounds with aromatic properties are 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....

, a heterocyclic compound with a five-membered ring that includes an oxygen atom, and 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...

, a heterocyclic compound with a six-membered ring containing one nitrogen atom.

Benzene ring model



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....

, C6H6, is the simplest aromatic hydrocarbon and was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by Friedrich August Kekulé von Stradonitz
Friedrich August Kekulé von Stradonitz
Friedrich August Kekule von Stradonitz was a German organic chemist. From the 1850s until his death, Kekule was one of the most prominent chemists in Europe, especially in theoretical chemistry...

 in the 19th century.
Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one each to the two neighboring carbons. This leaves one to share with one of its two neighboring carbon atoms, which is why the benzene molecule is drawn with alternating single and double bonds around the hexagon.

The structure is also illustrated as a circle around the inside of the ring to show six electrons floating around in delocalized molecular orbitals the size of the ring itself. This also represents the equivalent nature of the six carbon-carbon bonds all of bond order
Bond order
Bond order is the number of chemical bonds between a pair of atoms. For example, in diatomic nitrogen N≡N the bond order is 3, while in acetylene H−C≡C−H the bond order between the two carbon atoms is also 3, and the C−H bond order is 1. Bond order gives an indication to the stability of a bond....

 ~1.5. This equivalency is well explained by resonance form
Resonance (chemistry)
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...

s. The electrons are visualized as floating above and below the ring with the electromagnetic fields they generate acting to keep the ring flat.

General properties:
  1. Display aromaticity
    Aromaticity
    In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

    .
  2. The carbon-hydrogen ratio is high.
  3. They burn with a sooty yellow flame because of the high carbon-hydrogen ratio.
  4. They undergo electrophilic substitution reactions and nucleophilic aromatic substitution
    Nucleophilic aromatic substitution
    right|300px|Aromatic nucleophilic substitutionA nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring...

    s.


The circle symbol for aromaticity was introduced by Sir Robert Robinson in 1925 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry. The proper use of the symbol is debated, it is used to describe any cyclic pi system in some publications, or only those pi systems that obey Hückel's rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...

 on others. Jensen argues that in line with Robinson's original proposal, the use of the circle symbol should be limited to monocyclic 6 pi-electron systems. In this way the circle symbol for a 6c–6e bond can be compared to the Y symbol for a 3c–2e bond.

Arene synthesis


A reaction that forms an arene compound from an unsaturated or partially unsaturated cyclic precursor is simply called an aromatization. Many laboratory methods exist for the organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

 of arenes from non-arene precursors. Many methods rely on cycloaddition
Cycloaddition
A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction.Cycloadditions are usually described by the...

 reactions. Alkyne trimerization describes the [2+2+2] cyclization of three alkynes, in the Dötz reaction
Dötz reaction
The Wulff-Dötz reaction is the chemical reaction of an aromatic or vinylic alkoxy pentacarbonyl chromium carbene complex with an alkyne and carbon monoxide to give a Cr3-coordinated substituted phenol. Several reviews have been published...

 an alkyne, carbon monoxide and a chromium carbene complex are the reactants.Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...

s of alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...

s with pyrone
Pyrone
Pyrones or pyranones are a class of cyclic chemical compounds. They contain an unsaturated six membered ring containing one oxygen atom and a ketone functional group. There are two isomers denoted as 2-pyrone and 4-pyrone. The 2-pyrone structure is found in nature as part of the coumarin ring...

 or cyclopentadienone with expulsion of carbon dioxide or carbon monoxide also form arene compounds. In Bergman cyclization
Bergman cyclization
The Bergman cyclization or Bergman reaction or Bergman cycloaromatization is an organic reaction and more specifically a rearrangement reaction taking place when an enediyne is heated in presence of a suitable hydrogen donor . It is named for the American chemist Robert George Bergman...

 the reactants are an enyne
Enyne
An enyne is a functional group in organic chemistry consisting of a conjugated alkyne and alkene group....

 plus a hydrogen donor.

Another set of methods is the aromatization of cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...

s and other aliphatic rings: reagents are catalysts used in hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...

 such as platinum, palladium and nickel (reverse hydrogenation), quinone
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...

s and the elements 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...

 and selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

.

Aromatic substitution


In aromatic substitution one substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

 on the arene ring, usually hydrogen, is replaced by another substituent. The two main types are electrophilic aromatic substitution
Electrophilic aromatic substitution
Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...

 when the active reagent is an electrophile and nucleophilic aromatic substitution
Nucleophilic aromatic substitution
right|300px|Aromatic nucleophilic substitutionA nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring...

 when the reagent is a nucleophile. In radical-nucleophilic aromatic substitution
Radical-nucleophilic aromatic substitution
Radical-nucleophilic aromatic substitution or SRN1 in organic chemistry is a type of substitution reaction in which a certain substituent on an aromatic compound is replaced by a nucleophile through an intermediary free radical species:...

 the active reagent is a radical. An example is the nitration of salicylic acid
Salicylic acid
Salicylic acid is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin...

 :

Coupling reactions


In coupling reaction
Coupling reaction
A coupling reaction in organic chemistry is a catch-all term for a variety of reactions where two hydrocarbon fragments are coupled with the aid of a metal catalyst...

s a metal catalyses a coupling between two formal radical fragments. Common coupling reactions with arenes result in the formation of new carbon-carbon bond
Carbon-carbon bond
A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is said to be formed between one hybridized orbital from each...

s e.g., alkylarenes, vinyl arenes, biraryls, new carbon-nitrogen bond
Carbon-nitrogen bond
A carbon–nitrogen bond is a covalent bond between carbon and nitrogen and is one of the most abundant bonds in organic chemistry and biochemistry....

s (anilines) or new carbon-oxygen bond
Carbon-oxygen bond
A carbon–oxygen bond is a covalent bond between carbon and oxygen and one of the most abundant in organic chemistry and biochemistry. Oxygen has 6 valence electrons and prefers to share two electrons in bonding with carbon, leaving the remaining 4 nonbonding electrons in 2 lone pairs...

s (aryloxy compounds). An example is the direct arylation of perfluorobenzenes 

Hydrogenation


Hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...

 of arenes create saturated rings. The compound 1-naphthol
Naphthol
Naphthol may refer to:* 1-Naphthol* 2-Naphthol...

 is completely reduced to a mixture of decalin-ol isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s.

The compound resorcinol
Resorcinol
Resorcinol is a dihydroxy benzene. It is the 1,3-isomer of benzenediol with the formula C6H42.-Nomenclature:Benzene-1,3-diol is the name recommended by the International Union of Pure and Applied Chemistry in its 1993 Recommendations for the Nomenclature of Organic Chemistry.-Production:It is...

, hydrogenated with Raney nickel
Raney nickel
Raney nickel is a solid catalyst composed of fine grains of a nickel-aluminium alloy, used in many industrial processes. It was developed in 1926 by American]] engineer Murray Raney as an alternative catalyst for the hydrogenation of vegetable oils in industrial processes...

 in presence of aqueous sodium hydroxide forms an enolate which is alkylated with methyl iodide to 2-methyl-1,3-cyclohexandione:

Cycloadditions


Cycloaddition
Cycloaddition
A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction.Cycloadditions are usually described by the...

 reaction are not common. Unusual thermal Diels-Alder reactivity of arenes can be found in the Wagner-Jauregg reaction
Wagner-Jauregg reaction
The Wagner-Jauregg reaction is a classic organic reaction in organic chemistry, named after Theodor Wagner-Jauregg, describing the double Diels–Alder reaction of 2 equivalents of maleic anhydride with a 1,1-diarylethylene...

. Other photochemical cycloaddition reactions with alkenes occur through excimer
Excimer
An excimer is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which is in an electronic excited state. Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is...

s.

Benzene and derivatives of benzene


Benzene derivatives have from one to six substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

s attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...

, which carries a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 group and toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

 with a methyl group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the arene substitution patterns
Arene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...

 ortho, meta, and para are devised. For example, three isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s exist for cresol
Cresol
Cresols are organic compounds which are methylphenols. They are a widely occurring natural and manufactured group of aromatic organic compounds which are categorized as phenols . Depending on the temperature, cresols can be solid or liquid because they have melting points not far from room...

 because the methyl group and the hydroxyl group can be placed next to each other (ortho), one position removed from each other (meta), or two positions removed from each other (para). Xylenol
Xylenol
Xylenol or dimethylphenol is an arene compound with two methyl groups and a hydroxyl group. 6 isomers exist of xylenol of which 2,6-xylenol with both methyl group in an ortho position with respect to the hydroxyl group is the most important...

 has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist.
The arene ring has an ability to stabilize charges. This is seen in, for example, phenol (C6H5-OH), which is acidic at the hydroxyl (OH), since a charge on this oxygen (alkoxide -O) is partially delocalized into the benzene ring.

Polyaromatic hydrocarbons



Poly-aromatic hydrocarbons (PAHs), also known as polycyclic aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings
Simple aromatic ring
Simple aromatic rings, also known as simple arenes or simple aromatics, are aromatic organic compounds that consist only of a conjugated planar ring system with delocalized pi electron clouds. Many simple aromatic rings have trivial names. They are usually found as substructures of more complex...

 and do not contain heteroatom
Heteroatom
In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure...

s or carry substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

s. 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...

 is the simplest example of a PAH. PAHs occur in oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

, coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

, and tar
Tar
Tar is modified pitch produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden vessels against rot. The largest...

 deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As a pollutant, they are of concern because some compounds have been identified as carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

ic, mutagen
Mutagen
In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. As many mutations cause cancer, mutagens are therefore also likely to be carcinogens...

ic, and teratogenic. PAHs are also found in cooked foods. Studies have shown that high levels of PAHs are found, for example, in meat cooked at high temperatures such as grilling or barbecuing, and in smoked fish.

They are also found in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

, in comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, and in meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s and are a candidate molecule to act as a basis for the earliest forms of life
PAH world hypothesis
The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbons , assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules, leading into the RNA world...

. In graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...

 the PAH motif is extended to large 2D sheets.

Occurrence and pollution


Polycyclic aromatic hydrocarbons are lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...

, meaning they mix more easily with oil than water. The larger compounds are less water-soluble and less volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...

 (i.e., less prone to evaporate). Because of these properties, PAHs in the environment are found primarily in soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, sediment and oily substances, as opposed to in water or air. However, they are also a component of concern in particulate matter suspended in air.

Natural crude oil and coal deposits contain significant amounts of PAHs, arising from chemical conversion of natural product molecules, such as steroids, to aromatic hydrocarbons. They are also found in processed fossil fuels, tar
Tar
Tar is modified pitch produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden vessels against rot. The largest...

 and various edible oils.

PAHs are one of the most widespread organic pollutants. In addition to their presence in fossil fuels they are also formed by incomplete combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 of 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...

-containing fuels such as wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

, coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

, diesel, fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

, tobacco
Tobacco
Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...

, and incense
Incense
Incense is composed of aromatic biotic materials, which release fragrant smoke when burned. The term "incense" refers to the substance itself, rather than to the odor that it produces. It is used in religious ceremonies, ritual purification, aromatherapy, meditation, for creating a mood, and for...

. Different types of combustion yield different distributions of PAHs in both relative amounts of individual PAHs and in which isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s are produced. Thus, coal burning produces a different mixture than motor-fuel combustion or a forest fire, making the compounds potentially useful as indicators of the burning history. Hydrocarbon emissions from fossil fuel-burning engines are regulated in developed countries.

List of PAHs


Although the health effects of individual PAHs are not exactly alike, the following 17 PAHs are considered as a group in this profile issued by the Agency for Toxic Substances and Disease Registry
Agency for Toxic Substances and Disease Registry
The Agency for Toxic Substances and Disease Registry is a federal public health agency within the United States Department of Health and Human Services. The agency focuses on minimizing human health risks associated with exposure to hazardous substances...

 (ATSDR):
  • acenaphthene
    Acenaphthene
    Acenaphthene is a polycyclic aromatic hydrocarbon consisting of naphthalene with an ethylene bridge connecting positions 1 and 8. An alternative name, 1,2-dihydroacenaphthylene, emphasizes that it is a hydrogenated form of acenaphthylene...

  • acenaphthylene
    Acenaphthylene
    Acenaphthylene is a polycyclic aromatic hydrocarbon consisting of naphthalene with an ethylene bridge connecting positions 1 and 8. It is a constituent of coal tar. Reduction of the ethylene group gives the related compound acenaphthene. Unlike most polycyclic aromatic hydrocarbons, it has no...

  • 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...

  • benz[a]anthracene
  • benzo[a]pyrene
  • benzo[e]pyrene
    Benzo(e)pyrene
    Benzo[e]pyrene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12. It is listed as a Group 3 carcinogen by the IARC.-See also:* Benzopyrene* Benzo[a]pyrene* Benzene* Pyrene, a four-ring analogue...

  • benzo[b]fluoranthene
  • benzo[ghi]perylene
    Benzo(ghi)perylene
    Benzo[ghi]perylene is a polycyclic aromatic hydrocarbon with the chemical formula C22H12....

  • benzo[j]fluoranthene
    Benzo(j)fluoranthene
    Benzo[j]fluoranthene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12....

  • benzo[k]fluoranthene
    Benzo(k)fluoranthene
    Benzo[k]fluoranthene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12....

  • chrysene
    Chrysene
    Chrysene is a polycyclic aromatic hydrocarbon with the molecular formula that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized...

  • dibenz(a,h)anthracene
    Dibenz(a,h)anthracene
    Dibenzanthracene is an organic compound with the chemical formula C22H14....

  • fluoranthene
    Fluoranthene
    Fluoranthene is a polycyclic aromatic hydrocarbon consisting of a naphthalene and a benzene unit connected by a five-membered ring. It is a member of the class of PAHs known as non-alternant PAHs because it has rings other than those with six carbon atoms. It is a structural isomer of the...

  • fluorene
    Fluorene
    Fluorene, or 9H-fluorene, is a polycyclic aromatic hydrocarbon. It forms white crystals that exhibit a characteristic, aromatic odor similar to that of naphthalene. It is combustible. It has a violet fluorescence, hence its name. For commercial purposes it is obtained from coal tar...

  • indeno(1,2,3-cd)pyrene
  • phenanthrene
    Phenanthrene
    Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name phenanthrene is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light...

  • pyrene
    Pyrene
    Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This colourless solid is the smallest peri-fused PAH...


Human health


PAHs toxicity is very structurally dependent, with isomers (PAHs with the same formula and number of rings) varying from being nontoxic to being extremely toxic. Thus, highly carcinogenic PAHs may be small or large. One PAH compound, benzo[a]pyrene, is notable for being the first chemical carcinogen to be discovered (and is one of many carcinogens found in cigarette smoke
Chemicals in a cigarette
According to the U.S. Department of Health and Human Services, the following are known human carcinogens found in cigarette smoke:...

). The EPA has classified seven PAH compounds as probable human carcinogens: benz[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene
Benzo(k)fluoranthene
Benzo[k]fluoranthene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12....

, chrysene
Chrysene
Chrysene is a polycyclic aromatic hydrocarbon with the molecular formula that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized...

, dibenz(a,h)anthracene
Dibenz(a,h)anthracene
Dibenzanthracene is an organic compound with the chemical formula C22H14....

, and indeno(1,2,3-cd)pyrene.

PAHs known for their carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

ic, mutagen
Mutagen
In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. As many mutations cause cancer, mutagens are therefore also likely to be carcinogens...

ic and teratogenic properties are benz[a]anthracene and chrysene
Chrysene
Chrysene is a polycyclic aromatic hydrocarbon with the molecular formula that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized...

, benzo[b]fluoranthene, benzo[j]fluoranthene
Benzo(j)fluoranthene
Benzo[j]fluoranthene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12....

, benzo[k]fluoranthene
Benzo(k)fluoranthene
Benzo[k]fluoranthene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12....

, benzo[a]pyrene, benzo[ghi]perylene
Benzo(ghi)perylene
Benzo[ghi]perylene is a polycyclic aromatic hydrocarbon with the chemical formula C22H12....

, coronene
Coronene
Coronene is a polycyclic aromatic hydrocarbon comprising six peri-fused benzene rings. Its chemical formula is . It is a yellow material that dissolves in such solvents as benzene, toluene, and dichloromethane. Its solutions emit blue light fluorescence under UV light...

, dibenz(a,h)anthracene
Dibenz(a,h)anthracene
Dibenzanthracene is an organic compound with the chemical formula C22H14....

 (C20H14), indeno(1,2,3-cd)pyrene (C22H12) and ovalene
Ovalene
Ovalene is a polycyclic aromatic hydrocarbon with the formula C32H14, which consists of ten peri-fused six-membered rings. It is very similar to coronene....

.

High prenatal exposure to PAH is associated with lower IQ and childhood asthma. The Center for Children's Environmental Health reports studies that demonstrate that exposure to PAH pollution during pregnancy is related to adverse birth outcomes including low birth weight, premature delivery, and heart malformations. Cord blood of exposed babies shows DNA damage that has been linked to cancer. Follow-up studies show a higher level of developmental delays at age three, lower scores on IQ tests and increased behaviorial problems at ages six and eight.

Chemistry


The simplest PAHs, as defined by the International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...

 (IUPAC) (G.P Moss, IUPAC nomenclature for fused-ring systems), are phenanthrene
Phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name phenanthrene is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light...

 and 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...

, which both contain three fused aromatic rings. Smaller molecules, such as 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....

, are not PAHs.

PAHs may contain four-, five-, six- or seven-member rings, but those with five or six are most common. PAHs composed only of six-membered rings are called alternant PAHs. Certain alternant PAHs are called benzenoid PAHs. The name comes from 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....

, an aromatic hydrocarbon with a single, six-membered ring. These can be benzene rings interconnected with each other by single carbon-carbon bonds and with no rings remaining that do not contain a complete benzene ring.

The set of alternant PAHs is closely related to a set of mathematical entities called polyhex
Polyhex (mathematics)
In recreational mathematics, a polyhex is a polyform with a regular hexagon as the base form.As with polyominoes, polyhexes may be enumerated as free polyhexes , fixed polyhexes and one-sided polyhexes In recreational mathematics, a polyhex is a polyform with a regular hexagon (or 'hex' for...

es, which are planar figures composed by conjoining regular hexagons of identical size.

PAHs containing up to six fused aromatic rings are often known as "small" PAHs, and those containing more than six aromatic rings are called "large" PAHs. Due to the availability of samples of the various small PAHs, the bulk of research on PAHs has been of those of up to six rings. The biological activity and occurrence of the large PAHs does appear to be a continuation of the small PAHs. They are found as combustion products, but at lower levels than the small PAHs due to the kinetic limitation of their production through addition of successive rings. In addition, with many more isomers possible for larger PAHs, the occurrence of specific structures is much smaller.

PAHs possess very characteristic UV absorbance spectra
Ultraviolet-visible spectroscopy
Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent ranges...

. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s, each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). The extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain large PAHs also exhibiting semi-conducting
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 and other behaviors.

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...

 (C10H8 constituent of mothball
Mothball
Mothballs are small balls of chemical pesticide and deodorant used when storing clothing and other articles susceptible to damage from mold or moth larvae ....

s), consisting of two coplanar six-membered rings sharing an edge, is another aromatic hydrocarbon. By formal convention, it is not a true PAH, though is referred to as a bicyclic aromatic hydrocarbon.

Aqueous solubility decreases approximately one order of magnitude for each additional ring.

PAH compounds


Chemical compound Chemical compound
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...

 
Benzo[a]pyrene 
Chrysene
Chrysene
Chrysene is a polycyclic aromatic hydrocarbon with the molecular formula that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized...

 
Coronene
Coronene
Coronene is a polycyclic aromatic hydrocarbon comprising six peri-fused benzene rings. Its chemical formula is . It is a yellow material that dissolves in such solvents as benzene, toluene, and dichloromethane. Its solutions emit blue light fluorescence under UV light...

 
Corannulene
Corannulene
Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is [5]circulene. It is of scientific interest because it is a geodesic polyarene and can be considered a fragment of...

 

Tetracene
Tetracene
Tetracene, also called naphthacene, is a polycyclic aromatic hydrocarbon. It has the appearance of a pale orange powder. Tetracene is the four-ringed member of the series of acenes, the previous one being anthracene and the next one being pentacene.Tetracene is a molecular organic semiconductor,...

 
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...

 

Pentacene
Pentacene
Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings. This highly conjugated compound is an organic semiconductor. The compound generates excitons upon absorption of ultra-violet or visible light; this makes it very sensitive to oxidation...

 
Phenanthrene
Phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name phenanthrene is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light...

 
Pyrene
Pyrene
Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This colourless solid is the smallest peri-fused PAH...

 
Triphenylene
Triphenylene
In chemistry, the organic compound triphenylene is a flat polycyclic aromatic hydrocarbon consisting of four fused benzene rings. Triphenylene can be isolated from coal tar, but it is also made synthetically using benzyn chemistry. One molecule of triphenylene has delocalized 18-π-electron systems...

 

Ovalene
Ovalene
Ovalene is a polycyclic aromatic hydrocarbon with the formula C32H14, which consists of ten peri-fused six-membered rings. It is very similar to coronene....

 


The United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 (EPA) has designated 32 PAH compounds as priority pollutants. The original 16 are listed. They are 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...

, acenaphthylene
Acenaphthylene
Acenaphthylene is a polycyclic aromatic hydrocarbon consisting of naphthalene with an ethylene bridge connecting positions 1 and 8. It is a constituent of coal tar. Reduction of the ethylene group gives the related compound acenaphthene. Unlike most polycyclic aromatic hydrocarbons, it has no...

, acenaphthene
Acenaphthene
Acenaphthene is a polycyclic aromatic hydrocarbon consisting of naphthalene with an ethylene bridge connecting positions 1 and 8. An alternative name, 1,2-dihydroacenaphthylene, emphasizes that it is a hydrogenated form of acenaphthylene...

, fluorene
Fluorene
Fluorene, or 9H-fluorene, is a polycyclic aromatic hydrocarbon. It forms white crystals that exhibit a characteristic, aromatic odor similar to that of naphthalene. It is combustible. It has a violet fluorescence, hence its name. For commercial purposes it is obtained from coal tar...

, phenanthrene
Phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name phenanthrene is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light...

, 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...

, fluoranthene
Fluoranthene
Fluoranthene is a polycyclic aromatic hydrocarbon consisting of a naphthalene and a benzene unit connected by a five-membered ring. It is a member of the class of PAHs known as non-alternant PAHs because it has rings other than those with six carbon atoms. It is a structural isomer of the...

, pyrene
Pyrene
Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This colourless solid is the smallest peri-fused PAH...

, benzo[a]anthracene
Benzo(a)anthracene
Benz[a]anthracene is a polycyclic aromatic hydrocarbon with the chemical formula C18H12....

, chrysene
Chrysene
Chrysene is a polycyclic aromatic hydrocarbon with the molecular formula that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized...

, benzo[b]fluoranthene, benzo[k]flouranthene
Benzo(k)fluoranthene
Benzo[k]fluoranthene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12....

, benzo[a]pyrene, dibenz(ah)anthracene, benzo[ghi]perylene
Benzo(ghi)perylene
Benzo[ghi]perylene is a polycyclic aromatic hydrocarbon with the chemical formula C22H12....

, and indeno(1,2,3-cd)pyrene. This list of the 16 EPA priority PAHs is often targeted for measurement in environmental samples.

Aromaticity


Although PAHs clearly are aromatic compounds, the degree of aromaticity
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

 can be different for each ring segment. According to Clar's rule (formulated by Erich Clar in 1964) for PAHs the resonance structure with the most disjoint aromatic п-sextets—i.e., benzene-like moieties—is the most important for the characterization of the properties.
Phenanthrene (1) Anthracene (2) Chrysene (3) Clar rule


For example, in phenanthrene
Phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name phenanthrene is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light...

 (1) one Clar structure has two sextets at the extremities, while the other resonance structure has just one central sextet. Therefore in this molecule the outer rings are firmly aromatic while its central ring is less aromatic and therefore more reactive. In contrast, 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...

 (2) the number of sextets is just one and aromaticity spreads out. This difference in number of sextets is reflected in the UV absorbance spectra of these two isomers. Phenanthrene
Phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name phenanthrene is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light...

 has a highest wavelength absorbance around 290 nm, while 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...

 has highest wavelength bands around 380 nm. Three Clar structures with two sextets are present in chrysene
Chrysene
Chrysene is a polycyclic aromatic hydrocarbon with the molecular formula that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized...

 (3) and by superposition the aromaticity in the outer ring is larger than in the inner rings. Another relevant Clar hydrocarbon is zethrene
Zethrene
Dibenzo[de,mn]naphthacene or zethrene is an aromatic hydrocarbon consisting of two phenalene units fused together. According to Clar's rule the two exterior naphtalene units are truly aromatic and the two central double bonds are not aromatic at all. For this reason the compound is of some interest...

.

Origins of life



In January 2004 (at the 203rd Meeting of the American Astronomical Society
American Astronomical Society
The American Astronomical Society is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC...

), it was reported that a team led by A. Witt of the University of Toledo, Ohio
University of Toledo
The University of Toledo is a public university in Toledo, Ohio, United States. The Carnegie Foundation classified the university as "Doctoral/Research Extensive."-National recognition:...

 studied ultraviolet light emitted by the Red Rectangle nebula
Red Rectangle nebula
The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated with the AFCRL Infrared Sky Survey called Hi Star...

 and found the spectral signatures of 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...

 and pyrene
Pyrene
Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This colourless solid is the smallest peri-fused PAH...

 (no other such complex molecules had ever before been found in space). This discovery was considered as a controversial confirmation of a hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's core to get caught in stellar winds, and radiate outward. As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms. Witt and his team inferred that since they discovered PAHs—which may have been vital in the formation of early life on Earth—in a nebula, by necessity they must originate in nebulae. More recently, fullerenes (or "buckyballs"), have been detected in other nebulae. Fullerenes are also implicated in the origin of life; according to astronomer Letizia Stanghellini, "It’s possible that buckyballs from outer space provided seeds for life on Earth.”

Detection


Detection of PAHs in materials is often done using gas chromatography-mass spectrometry
Gas chromatography-mass spectrometry
Gas chromatography–mass spectrometry is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation,...

 or liquid chromatography with ultraviolet-visible
Ultraviolet-visible spectroscopy
Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent ranges...

 or fluorescence
Fluorescence spectroscopy
Fluorescence spectroscopy aka fluorometry or spectrofluorometry, is a type of electromagnetic spectroscopy which analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of certain compounds and causes them to emit...

 spectroscopic methods or by using rapid test PAH indicator strips.

See also

  • Asphaltene
  • Hydrodealkylation
    Hydrodealkylation
    Hydrodealkylation is a chemical reaction that often involves reacting an aromatic hydrocarbon, such as toluene, in the presence of hydrogen gas to form a simpler aromatic hydrocarbon devoid of functional groups. An example is the conversion of 1,2,4-trimethylbenzene to xylene. This chemical process...

  • Simple aromatic rings
  • Aromatic substituents: Aryl
    Aryl
    In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....

    , Aryloxy and Arenediyl

External links