Aromatic compounds, also known as "mono- and polycyclic aromatic
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s", are
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s containing one or more
aromatic rings. The parent member of aromatic compounds is
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
. The word "aromatic" originates from the past grouping of molecules based on smell, before their general chemical properties are understood. The current definition of aromatic compounds does not have any relation with their smell.
Heteroarenes are closely related, since at least one carbon atom of CH group is replaced by one of the
heteroatoms
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, or
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
. Examples of non-benzene compounds with aromatic properties are
furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen atom. Chemical compounds containing such rings are also referred to as furans.
Furan is a colorless, flammable, highl ...
, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...
, a heterocyclic compound with a six-membered ring containing one nitrogen atom. Hydrocarbons without an aromatic ring are called
aliphatic
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane ...
.
Benzene ring model
Benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
,
C6H6, is the least complex aromatic hydrocarbon, and it was the first one named as such. The nature of its bonding was first recognized by
August Kekulé in the 19th century. Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one to each of the two neighboring carbons. This leaves one electron to share with one of the two neighboring carbon atoms, thus creating a double bond with one carbon and leaving a single bond with the other, which is why some representations of the benzene molecule portray it as a hexagon with alternating single and double bonds.
Other depictions of the structure portray the hexagon with a circle inside it, to indicate that the six electrons are floating around in delocalized molecular orbitals the size of the ring itself. This represents the equivalent nature of the six carbon-carbon bonds all of
bond order
In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds.
The bond order itself is the number of electron pairs ( covalent bonds) between two atoms. For example, in diat ...
1.5; the equivalency is explained by
resonance form
In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or ' ...
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 of aromatic hydrocarbons:
# They display
aromaticity
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
# The carbon-hydrogen ratio is high
# They burn with a strong sooty yellow flame because of the high carbon–hydrogen ratio
# They undergo
electrophilic substitution reactions and
nucleophilic aromatic substitutions
The circle symbol for aromaticity was introduced by
Sir Robert Robinson and his student James Armit in 1925 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry. The proper use of the symbol is debated: some publications use it to ''any'' cyclic π system, while others use it only for those π systems that obey
Hückel's rule. Jensen argues that, in line with Robinson's original proposal, the use of the circle symbol should be limited to monocyclic 6 π-electron systems. In this way the circle symbol for a six-center six-electron bond can be compared to the Y symbol for a
three-center two-electron bond.
Reactions
Aromatic ring systems participate in many organic reactions.
Aromatic substitution
In aromatic
substitution
Substitution may refer to:
Arts and media
*Chord substitution, in music, swapping one chord for a related one within a chord progression
*Substitution (poetry), a variation in poetic scansion
* "Substitution" (song), a 2009 song by Silversun Pic ...
one
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...
on the arene ring, usually hydrogen, is replaced by another substituent. The two main types are
electrophilic aromatic substitution, when the active reagent is an electrophile, and
nucleophilic aromatic substitution, when the reagent is a nucleophile. In
radical-nucleophilic aromatic substitution, the active reagent is a
radical. An example of electrophilic aromatic substitution is the nitration of
salicylic acid
Salicylic acid is an organic compound with the formula HOC6H4CO2H. A colorless, bitter-tasting solid, it is a precursor to and a metabolite of aspirin (acetylsalicylic acid). It is a plant hormone, and has been listed by the EPA Toxic Substa ...
:
:
Coupling reactions
In
coupling reactions a metal catalyzes a coupling between two formal radical fragments. Common coupling reactions with arenes result in the formation of new
carbon–carbon bonds e.g., alkylarenes, vinyl arenes, biraryls, new
carbon–nitrogen bonds (anilines) or new
carbon–oxygen bonds (aryloxy compounds). An example is a direct arylation of
perfluorobenzenes
:
Hydrogenation
Hydrogenation
Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic ...
of arenes create saturated rings. The compound
1-naphthol
1-Naphthol, or α-naphthol, is a fluorescent organic compound with the formula . It is a white solid. It is an isomer of 2-naphthol differing by the location of the hydroxyl group on the naphthalene ring. The naphthols are naphthalene homolog ...
is completely reduced to a mixture of
decalin-ol
isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Is ...
s.
:
The compound
resorcinol
Resorcinol (or resorcin) is an organic compound with the formula C6H4(OH)2. It is one of three isomeric benzenediols, the 1,3-isomer (or '' meta''-isomer). Resorcinol crystallizes from benzene as colorless needles that are readily soluble in ...
, hydrogenated with
Raney nickel in presence of aqueous
sodium hydroxide forms an
enolate which is alkylated with
methyl iodide
Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one ...
to 2-methyl-1,3-cyclohexandione:
:
Cycloadditions
Cycloaddition
In organic chemistry, a cycloaddition is a chemical reaction in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity" ...
reactions are not common. Unusual thermal Diels–Alder reactivity of arenes can be found in the
Wagner-Jauregg reaction The Wagner-Jauregg reaction is a classic organic reaction in organic chemistry, named after (son of Julius Wagner-Jauregg), describing the double Diels–Alder reaction of 2 equivalents of maleic anhydride with a 1,1-diarylethylene. After aromatiza ...
. Other photochemical cycloaddition reactions with alkenes occur through
excimers.
Dearomatization
In
dearomatization reactions the aromaticity of the reactant is permanently lost.
Benzene and derivatives of benzene
Benzene derivatives have from one to six
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...
s attached to the central benzene core. Examples of benzene compounds with just one substituent are
phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
, which carries a
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...
group, and
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...
with a
methyl
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ...
group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the
arene substitution patterns ''ortho'', ''meta'', and ''para'' are devised. For example, three
isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Is ...
s exist for
cresol
Cresols (also hydroxytoluene or cresylic acid) are a group of aromatic organic compounds. They are widely-occurring phenols (sometimes called ''phenolics'') which may be either natural or manufactured. They are also categorized as methylphenol ...
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 has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist.
File:toluene.svg, Toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...
File:Ethylbenzene-2D-skeletal.png, Ethylbenzene
Ethylbenzene is an organic compound with the formula . It is a highly flammable, colorless liquid with an odor similar to that of gasoline. This monocyclic aromatic hydrocarbon is important in the petrochemical industry as an reaction interme ...
File:Para-Xylol - para-xylene.svg, ''p''-Xylene
File:Meta-Xylol_-_meta-xylene_2.svg, ''m''-Xylene
File:1,3,5-Trimethylbenzene.svg, Mesitylene
File:Durene.png, Durene
File:2-phenyl-hexane.png, 2-Phenylhexane
File:Bifenyl.svg, Biphenyl
File:Phenol-2D-skeletal.png, Phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
File:Aniline.svg, Aniline
Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starti ...
File:Nitrobenzol.svg, Nitrobenzene
File:Benzoic acid.svg, Benzoic acid
Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin ...
File:Aspirin-skeletal.svg, Aspirin
Aspirin, also known as acetylsalicylic acid (ASA), is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and/or inflammation, and as an antithrombotic. Specific inflammatory conditions which aspirin is used to treat inc ...
File:Paracetamol-skeletal.svg, Paracetamol
Paracetamol, also known as acetaminophen, is a medication used to treat fever and mild to moderate pain. Common brand names include Tylenol and Panadol.
At a standard dose, paracetamol only slightly decreases body temperature; it is inferio ...
File:Pikrinsäure.svg, Picric acid
The arene ring has the ability to stabilize charges. This is seen in, for example, phenol (C
6H
5–OH), which is
acidic at the hydroxyl (OH), since a charge on this oxygen (alkoxide –O
−) is partially delocalized into the benzene ring.
Other monocyclic aromatic hydrocarbon
Other monocyclic aromatic hydrocarbon include
Cyclotetradecaheptaene
Cyclotetradecaheptaene, often referred to as 4nnulene, is a hydrocarbon with molecular formula C14H14, which played an important role in the development of criteria (Hückel's rule) for aromaticity, a stabilizing property of central importance in ...
or
Cyclooctadecanonaene.
Polycyclic aromatic hydrocarbons
Polynuclear aromatic hydrocarbons (PAHs) are aromatic hydrocarbons that consist of fused
aromatic
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
rings and do not contain
heteroatoms or carry
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...
s.
Naphthalene
Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromat ...
is the simplest example of a PAH. PAHs occur in
oil,
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when ...
, and
tar
Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black bi ...
deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As pollutants, they are of concern because some compounds have been identified as
carcinogen
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...
ic,
mutagenic, 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, in
comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...
s, and in
meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...
s and are a
candidate molecule to act as a basis for the earliest forms of life. In
graphene
Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure. the PAH motif is extended to large 2D sheets.
See also
* Aromatic substituents:
Aryl
In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...
,
Aryloxy and
Arenediyl
*
Asphaltene
*
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, ...
*
Simple aromatic rings
*
Rhodium-platinum oxide
Rhodium-platinum oxide (Rh–Pt oxide), or Nishimura's catalyst, is an inorganic compound used as a hydrogenation catalyst.
Uses
Rh–Pt oxide is used to reduce various aromatic compounds to their respective cycloalkanes or saturated heterocycle ...
, a catalyst used to hydrogenate aromatic compounds.
References
External links
*
{{DEFAULTSORT:Aromatic Hydrocarbon