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Aromatic Rings

Aromatic rings (also known as aromatic compounds or arenes) are hydrocarbons which contain benzene, or some other related ring structure.  Benzene, C6H6, is often drawn as a ring of six carbon atoms, with alternating double bonds and single bonds:

This simple picture has some complications, however.  (Everything in organic chemistry has complications!)  Carbon-carbon single bonds are longer than carbon-carbon double bonds, so if there were "real" single and double bonds in the molecule, the shape of the benzene molecule would be a distorted hexagon:

What is actually found is that all of the bond lengths in the benzene rings are 1.397 angstroms, which is roughly intermediate between the typical lengths of single bonds (~1.5 angstroms) and double bonds (~1.3 angstroms).  All of the carbon atoms in the benzene rings are sp2-hybridized:  the overlap of the sp2 orbitals around the ring produces a framework of six sigma bonds, while the unhybridized p-orbitals which are perpendicular to this plane overlap in a side-to-side fashion to form three pi-bonds.  These pi-bonds are delocalized around the ring, leading to an unusual stability for the benzene ring compared to other alkenes.

Instead of three "real" single bonds and three "real" double bonds in the ring, benzene may be thought of as consisting of "six one-and-a-half" bonds, represented by the resonance structures shown below:

All of the positions on the benzene ring are thus equivalent; benzene is sometimes represented as a hexagon with a circle inside it to emphasize this equivalence:

Since all of the atoms in the ring are sp2-hybridized, they are all trigonal planar, with bond angles of 120°, and the benzene ring is a flat molecule, shaped like a hexagon.

Aromatic hydrocarbons are nonpolar, and are insoluble in water.  However, when other atoms are substituted on the benzene ring, they may be very water-soluble.  For instance, phenol, which has an —OH group attached to the benzene ring, is very water-soluble.

Because of the stability imparted by the delocalized pi-electrons in the ring, aromatic molecules do not undergo many of the reactions which are typical for alkenes, but there are a number of interesting and useful substitution reactions that they undergo, in which hydrogen atoms on the ring are replaced with other functional groups.

Molecules which do not contain benzene rings are referred to aliphatic ("fat-like") compounds.

Aromatic molecules with simple alkyl groups as substituents are named as derivatives of benzene.  For instance, a benzene with an ethyl group attached to one of the carbons in the ring is simply called "ethylbenzene."

A number of aromatic molecules are known by common names; for instance, benzene with a —CH3 group attached is called "toluene"; benzene with an —NH2 group attached is called "aniline"; a benzene with a —CO2H group is called "benzoic acid," etc.

For more complicated substituents, the benzene ring is named as a substituent, in which case it is called "phenyl."  For instance, an eight-carbon chain with a benzene ring on the third carbon is called "3-phenyloctane."

When there are two substituents on the benzene ring, numbers can be used to identify the position of the substituents, but an older system of nomenclature is often used instead, in which the prefixes ortho-, meta-, and para- (often abbreviated as o-, m-, and p-) are used to indicate the relative placement of the substituents:

For three or more substituents, numbers must be used to indicate the placement of substituents.


Benzene 3D

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Benzene was first isolated by Michael Faraday in 1825, from the whale oil used in gaslights; he also determined that it had an empirical formula of CH.  Eilhardt Mitscherlich synthesized benzene in 1834, and showed it to have a molecular formula of C6H6.  Many other compounds with similar properties to benzene were discovered in the 1800s, all having a low ratio of hydrogen to carbon.  This usually indicates the presence of carbon-carbon double bonds, but these compounds did not undergo the reactions typical of alkenes under ordinary lab conditions.  Since many of these molecules had pleasant aromas, they were called "aromatic compounds."  The structure of benzene and its derivatives was a puzzle for many years, until Friedrich August Kekulé proposed the cyclic structure shown above in 1866 (supposedly after having a dream about a snake biting its own tail).  The term "aromatic" in organic chemistry now means that the molecule contains benzene, or its structural relatives.

Benzene itself is a clear, colorless, highly flammable liquid, which boils at 80.1°C, and melts at 5.5°C.  It was used extensively as a solvent for many organic reactions, but it is toxic by ingestion and inhalation, and may cause bone marrow problems or leukemia with prolonged exposure.

Toluene 3D

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Toluene (or methylbenzene) is a commonly used organic solvent; it is less carcinogenic than benzene because the methyl group is easily oxidized, and can be converted to products that can be eliminated from the body with relative ease.  Toluene was first isolated from the gum of the Toluifera balsamum tree of South America, which produces a balsam that has been used in cough syrups and perfumes.

Xylene 3D


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There are three structural isomers of the xylenes, which have two methyl groups on the benzene ring.  The methyl groups may be on adjacent carbons (ortho), they may be on carbons separated by one other carbon (meta), or they may be on opposite sides of the benzene ring (para).   Xylol is a common industrial solvent which contains a mixture of all three xylene isomers.  Toluene and the xylenes are found in high-performance BTX gasoline.

Aniline 3D

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Aniline is a colorless, oily liquid.  It is easily converted into a number of other useful organic compounds, and is also a component of many dyes and pharmaceutical compounds.

Phenol 3D

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Phenol is a mild acid in aqueous solution (where it is known as carbolic acid), and was originally used as a disinfectant in hospitals.  For more information on phenol and its derivatives, see the page on Phenols.

Benzoic Acid 3D

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Benzoic Acid is is a white, crystallize solid at room temperature.  Benzoic acid and its salts (sodium benzoate, potassium benzoate, etc.) are commonly used as preservatives in foods to prevent the growth of mold.  For more information of benzoic acid and its derivatives, see the page on Carboxylic Acids.



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Styrene 3D

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Styrene consists of a benzene ring connected to a carbon-carbon double bond.  Styrene is used in the manufacture of polystyrene and Styrofoam (see the section on Addition Polymers).

Benzaldehyde 3D

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Benzaldehyde consists of a benzene ring connected to an aldehyde group (a carbon-oxygen double bond bearing a hydrogen, —CHO).  Benzaldehyde is partially responsible for the odor of almonds and cherries, and is also found in peach and apricot pits.  The natural source of benzaldehyde is a molecule called amygdalin.

Amygdalin (Laetrile) 3D

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Amygdalin is a disaccharide consisting of two glucose molecules in which a carbon bearing a cyano (CN) group and a benzene ring is attached to one of the oxygen atoms on a glucose.  The combination of a cyano group with a OH or OR group on a single carbon is called a cyanohydrin; cyanohydrins easily break down to release the cyanide ion, CN-.

Amygdalin occurs in almonds, cherries, and peach and apricot pits.  When apricot or peach pits are ground up, the enzyme emulsin breaks down amygdalin, releasing benzaldehyde and hydrogen cyanide, HCN, which is a deadly poison.  This preparation was used by the ancient Egyptians and Romans in making various poisons.

Under the name laetrile, amygdalin was used as an anti-cancer drug.  It was never approved for sale in the United States, but was a popular "alternative medicine" for use in chemotherapy in other countries.  However, more research showed that not only is laetrile ineffective against cancer, it is extremely toxic, due to the release of hydrogen cyanide.

Hyacinthin 3D

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Hyacinthin is commonly used in perfumes; it is responsible for the floral scent found in hyacinth (oddly enough).

Cinnamaldehyde 3D

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Cinnamaldehyde is found in oil of cinnamon, which is found in the bark of the cinnamon tree.  Cinnamon is a carminative — it stimulates the release of gases such as hydrogen sulfide, methane, and hydrogen from the stomach and intestines.  (These gases can have some interesting social consequences depending on which direction they exit from the digestive tract, but I'll leave that to your imagination.)

Anethole 3D

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Anethole is found in oil of aniseed, fennel, and tarragon.

Benzyl acetate 3D

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Benzyl acetate is one of the compounds found in oil of jasmine, which is a common ingredient in perfumes and toiletries.

Phenylethanol 3D

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Phenylethanol is found in oil of citronella, geraniums, and (with geraniol) is partially responsible for the odor or roses.





Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons consist of two or more benzene rings fused together.  They are produced when organic compounds are heated to high temperatures, and are present in tobacco smoke, car exhaust, and sometimes in heavily browned foods.

Naphthalene 3D

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Naphthalene is a white crystalline solid, derived from coal tar, with a characteristic odor of mothballs — which is not a coincidence, since naphthalene is frequently used in mothballs.  Naphthalene consists of two benzene rings that are fused together; the resulting molecule is still aromatic, and undergoes the reactions that are typical of benzene itself.

Anthracene 3D

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Anthracene is a white crystalline solid which exhibits a blue fluorescence under ultraviolet light.

Phenanthrene 3D

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Phenanthrene is a structural isomer of anthracene, in which the three benzene rings are fused to make an angle.

ortho-Phenanthroline 3D

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ortho-Phenanthroline is a phenanthrene molecule with two of the CH groups replaced with nitrogen atoms.  The lone pairs of electrons on the nitrogen atoms, combined with the rigidity of the aromatic ring system, makes ortho-phenanthroline useful in forming metal complexes.

Dibenz[a,h]anthracene 3D

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Dibenz[a,h]anthracene is a carcinogenic compound found in tobacco smoke and automobile exhaust.



Pyrene 3D

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Pyrene is also a suspected carcinogen.

Benzo[a]pyrene 3D

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Benzo[a]pyrene is a carcinogenic compound found in tobacco smoke and automobile exhaust.






4n+2 rule

Molecules whose Lewis structures can be drawn as a ring with alternating single and double bonds are known as annulenes.  Annulenes are named by prefixing the number of carbon atoms in the rings in brackets before the word annulene:  hence, benzene can be named as [6] annulene.

Hückel's Rule (Erich Hückel, 1931) states that annulenes with have (4n + 2) p electrons (where n is an integer from 0, 1, 2, 3, ...) are resonance-delocalized in the same way that benzene is, and should be aromatic.  Thus, molecules with 2, 6, 10, 14, 18, etc., p electrons will be aromatic, while annulenes with 4, 8, 12, 16, 20, etc. p electrons will not be aromatic, and their chemistry will be more similar to that of alkenes than to benzene.




Heterocyclic Aromatic Compounds

Heterocyclic aromatic compounds contain atoms other than carbon (hence "hetero," different) in their ring systems, but are nevertheless aromatic, because they follow Hückel's Rule.  These compounds have chemical properties that are very similar to those of benzene, and many are found in important biological systems.

Pyridine 3D

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In the pyridine molecule, one of the CH groups of the benzene ring is replaced with a nitrogen atom.  Pyridine is colorless liquid with a very strong, unpleasant odor.  (In my experience, while there are things that smell much worse than pyridine, while you're using this stuff, it's hard to think of them.)

Pyridine is a common solvent in research and industry because not only is it good a dissolving organic compounds, but it is also a weak base — something that is necessary for many organic reactions.

Methyl 2-pyridyl ketone 3D

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Methyl 2-pyridyl ketone is one of the main components in the odor of popcorn.  One of the ironies of organic chemistry is that while pyridine itself smells like something that died a long time ago, hanging a ketone group off the pyridine ring produces the pleasant, mouth-watering odor of popcorn.

Pyrazine 3D

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In the pyrazine molecule, two CH groups on opposite sides of the benzene ring are replaced by nitrogen atoms.  Pyrazine itself has a strong, unpleasant odor, similar to that of pyridine, but substituted pyrazines contribute to many pleasant odors, such as the smell of bread crusts, rum, whisky, chocolate, and some vegetables and peppers.



2-Methoxy-5-methylpyrazine 3D

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2-Methoxy-5-methylpyrazine is a substituted pyrazine found in the odor of peanuts

Pyrimidine 3D

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In the pyrimidine molecule, two CH groups "meta" to each other in the benzene ring are replaced by nitrogen atoms.  Pyrimidine derivatives, such as cytosine, thymine, and uracil, are found in DNA and RNA.



Purine 3D

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In the purine molecule, four nitrogen atoms are found in two fused rings.  Purine derivatives, such as adenine and guanine, are found in DNA and RNA, as well as in many other important molecules, such as caffeine.



Imidazole 3D

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Imidazole is important in the catalytic activity of many enzymes.  


Furan 3D

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Furan, which has an oxygen atom in a ring with four carbon atoms, is also an aromatic molecule; a lone pair of electrons on the oxygen contributes two electrons in addition to the four electrons from the carbon-carbon double bonds, making a total of six p electrons in the ring system.  If the double bonds are reduced to single bonds by the addition of four hydrogen atoms, the molecule tetrahydrofuran is produced; this is a very commonly used organic solvent.  
Thiophene 3D

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Thiophene has a sulfur atom in a ring with four carbon atoms, and, like furan, is aromatic because there are six p electrons in the ring (four from the double bonds and two from a lone pair on sulfur).  
Pyrrole 3D

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Pyrrole is a another aromatic molecule, once again having six p electrons in the ring system.  Molecules containing pyrrole occur in a number of important biological systems, ranging from photosynthesis to oxygen transport in the blood.  (See Porphine, Heme, and Chlorophyll below.)  


OUT on pyrrole.

(I know you're out there, I can hear you breathing.)

Porphine 3D

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Porphine contains four pyrrole molecules which have been joined to form a larger ring system.  This molecule has 18 p electrons in the ring system, and is therefore aromatic.  Like other aromatic molecules, this ring system is especially stable.

The ring system is flat, since all of the carbon and nitrogen atoms in the ring are sp2-hybridized, and therefore trigonal planar in shape.  This leaves the molecule with a large open cavity in the center, with four nitrogen atoms pointing towards the center of the cavity.  Porphine and its derivatives thus are capable of forming very stable complexes with many small metal cations, such as Mg2+, Fe2+, etc.

Porphine is the parent compound of a class of molecules called the porphyrins, in which various substituents replace the hydrogen atoms on the outside of the porphine ring.  The name "porphyrin" is derived from the fact that many of these substances form purple crystalline solids (the Greek word for "purple" is "porphyros").  Porphyrins occur ubiquitously in nature, especially in systems involving respiration (see Heme and Chlorophyll below).

Heme / Iron protoporphyrin IX 3D

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Heme is a porphyrin derivative in which an iron(II) ion is held rigidly by the nitrogen atoms in the center of the macrocycle.  The protein hemoglobin contains four subunits (two "alpha" and two "beta" subunits), each of which complex a heme molecule; this protein is responsible for oxygen transport in the body.  The iron atom in heme forms complexes with oxygen molecules, which are carried throughout the body by the hemoglobin in the red blood cells.  
Chlorophyll 3D
Chlorophyll (Greek, chloros "green" and phyllon "leaf") is a porphyrin derivative found in green plants and cyanobacteria which allows these organisms to perform photosynthesis.

Shown below are the structures of chlorophyll a and chlorophyll b; the magnesium-free forms of these molecules are called pheophytin a and pheophytin b.  (The three-dimensional structure shown on the right is that of pheophytin a.)  A double bond in one of the pyrrole rings in the porphyrin macrocycle is reduced to a single bond (in the structure below, this is the carbon-carbon bond to which the long side-chain, sometimes called the phytyl group, is attached); these "dihydroporphyrins" are known as chlorins.

Chlorophyll absorbs visible light in the violet (~420 nm) and red (~680 nm) regions of the electromagnetic spectrum; this renders the light that is reflected off of this pigment to be green.  The energy from the sunlight that is absorbed by chlorophyll in the chloroplasts of green plants is used to drive the synthesis of carbohydrates such as glucose (C6H12O6) from carbon dioxide and water:

(This equation summarizes a lot of complex chemistry, which it is beyond the scope of this page to try to summarize.)

In the fall, the green color of many plants disappears as the chlorophyll starts to break down; this allows other pigments, such as carotenes, in the leaves to "show their colors," producing the vivid yellows and oranges associated with fall colors.

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Chlorophyll a

Chlorophyll b





Buckminsterfullerene 3D

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Buckminsterfullerene, or C60, is a soccer-ball shaped molecule consisting of 60 carbon atoms.  The molecule was discovered by H. W. Kroto, R. E. Smalley, and R. F. Curl in the 1980s in experiments involving graphite vaporized with lasers (they were awarded the Nobel Prize in Chemistry for their discovery in 1996).  The structure, a truncated icosahedron, having 12 pentagons and 20 hexagons, reminded them of the shape of the geodesic dome designed by the architect R. Buckminster Fuller, and they named the molecule in his honor.  Similar spherical-shaped carbon-only molecules, such as C70, are often referred to as fullerenes or "buckyballs."

All of the carbon atoms in in these molecules are sp2 hybridized, and the entire molecule has some aromatic character.

The fullerenes are considered another allotrope (stable structural form) of carbon, in addition to graphite and diamond.

In the 1990s it was discovered that C60 could be made in larger quantities by heating graphite in an inert atmosphere.  Since then, these molecules have been intensely investigated.

Nanotubes are cylindrical versions of the fullerenes; they look something like a chain link fence rolled into a cylinder, with a dome-shaped cap on the end (half of a buckyball).  Nanotubes (also known as "buckytubes") are extremely strong, as well as being very lightweight (since they are made of nothing but carbon atoms).  These materials are being tested for potential use in many materials; some nanotubes also conduct electricity, leading to some potential applications in circuit design and electronics.

C70 3D

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C70 is another example of a fullerene.  It has a slightly elongated, oval shape.






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