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Sci-Tech Dictionary:

aromatic hydrocarbon

(¦ar·ə¦mad·ik ¦hī·drə′kär·bən)

(organic chemistry) A member of the class of hydrocarbons, of which benzene is the first member, consisting of assemblages of cyclic conjugated carbon atoms and characterized by large resonance energies. Also known as arene.

 
 
Sci-Tech Encyclopedia: Aromatic hydrocarbon

A hydrocarbon with a chemistry similar to that of benzene. Aromatic hydrocarbons are either benzenoid or nonbenzenoid. Benzenoid aromatic hydrocarbons contain one or more benzene rings and are by far the more common and the more important commercially. Nonbenzenoid aromatic hydrocarbons have carbon rings that are either smaller or larger than the six-membered benzene ring. Their importance arises mainly from a theoretical interest in understanding those structural features that impart the property of aromaticity.

Benzenoid aromatic hydrocarbons are also called arenes. Benzene itself is the prototypical arene. The properties associated with aromaticity have little to do with aroma, although the aromatic hydrocarbons were first studied in connection with naturally occurring fragrances. Instead, these compounds possess special stability; take part in certain types of reactions; and exhibit persistence of the structural integrity of aromatic rings during chemical reactions, while groups attached to those rings are chemically altered or manipulated.

Benzene

With molecular formula (1), benzene is highly unsaturated; it has three double bonds, alternating with single bonds. The double bonds in the benzene structure can be arranged in two ways, (1′) and (1″). Benzene is a resonance

hybrid of these two structures, called Kekulé structures; the double-headed arrow is used to signify that the benzene structure is neither (1′) nor (1″), but a single structure that is a hybrid of the two. That is, the bonds between adjacent carbon atoms are neither double nor single, but of some intermediate or hybrid type.

Each carbon atom in benzene is connected to three atoms, two adjacent carbon atoms and a hydrogen atom. These three bonds lie in a single plane and use three of the carbon's four valence electrons. The fourth valence electron of each carbon is located in a p orbital, extending perpendicularly above and below the plane of the other three bonds. These electrons, one from each carbon atom and called π electrons, form three molecular orbitals located above and below, but parallel to, the plane of the ring.

The symbol of a hexagon with an inscribed circle (2) is

often used to express the delocalized nature of the π electrons in benzene and other arenes. There is physical evidence that the π electrons circulate around the ring carbons, as implied by this formula. For example, in the nuclear magnetic resonance (NMR) spectra of arenes, the chemical shifts of arene hydrogen atoms (protons) are characteristically at lower magnetic fields than those of protons attached to carbon-carbon double bonds. This difference is due to an induced magnetic field caused by circulation of the π electrons in the molecular orbitals above and below the arene ring plane. Indeed, this chemical shift difference, due to a diamagnetic ring current, is sometimes used as evidence for aromaticity in nonbenzenoid aromatic hydrocarbons. See also Delocalization; Molecular orbital theory.

Other arenes

Besides benzene itself, several alkylbenzenes are commercially important and produced on a large scale—millions of pounds annually. Production is commonly by the cyclodehydrogenation of alkanes at high temperatures over metallic catalysts such as platinum.

Benzene, toluene, and the xylenes are added to unleaded gasoline to raise the octane number. These arenes are also essential to the petrochemical industry. Products derived from them include polyesters, polyurethanes, polystyrene, and synthetic rubber; alkylbenzenesulfonate detergents; phenol and acetone; pharmaceuticals, flavors, and perfumes; plasticizers; and many others. See also Petrochemical.

Arenes with fused rings are also known as polynuclear aromatic hydrocarbons. Rings are said to be fused when they share two carbon atoms. The simplest example is naphthalene (3), a colorless crystalline compound found in coal tar, best known as a moth repellent.

Additional arene rings can be fused. For example, anthracene, tetracene, and pentacene are linearly fused, while phenanthrene, triphenylene, and pyrene are angularly fused (see illustration). In general, angular fusion results in more stable systems than linear fusion. Phenanthrene, for example, is about 6 kcal/mol more stable than its linear isomer anthracene. Stability falls off sharply in the linearly fused series, and compounds with more than seven such rings are unknown.

Structures of some fused-ring (polynuclear) aromatic hydrocarbons.
Structures of some fused-ring (polynuclear) aromatic hydrocarbons.

Hückel rule

From molecular orbital theory, E. Hückel derived the rule that planar, cyclic conjugated (alternate single and double bonds) systems with 4n + 2π electrons (n is an integer, 0, 1, 2, …) will be aromatic and have substantial resonance energy, whereas those with 4n such electrons will not; indeed, it was later shown that 4n systems are often destabilized, hence antiaromatic. Benzene is a 4n + 2 system (n = 1) and aromatic. As striking confirmation of these ideas, pentalene (4), a planar

analog of cyclooctatetraene, is exceptionally reactive, unstable, and antiaromatic (a 4n system, n = 2), whereas the purple hydrocarbon azulene (5; a 4n = 2 system, n = 2, and an isomer of naphthalene) is stable and undergoes substitution reactions analogous to those of benzenoid arenes.

 
Wikipedia: aromatic hydrocarbon

An aromatic hydrocarbon (abbreviated as AH) or arene [1] is a hydrocarbon, the molecular structure of which incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent. This sweet scent actually came from impurities in the compounds (which are not actually aromatic in the sense initially described). The configuration of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible aromatic hydrocarbon, benzene. Aromatic hydrocarbons can be monocyclic or polycyclic.

Some non-benzene-based compounds called heteroarenes which follow Hückel's rule are also aromatic compounds. In these compounds at least one carbon atom is replaced by one of the heteroatoms oxygen, nitrogen or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring which includes an oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.[2]

Benzene ring model


Main article: aromaticity

Benzene, C6H6, is the simplest AH and was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by Friedrich August Kekulé von Stradonitz 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.

Many chemists draw 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 accurately represents the equivalent nature of the six bonds all of bond order ~1.5. This equivalency is well explained by resonance forms. The electrons float above and below the ring, and the electromagnetic fields they generate keep the ring flat. General properties:

  1. Display aromaticity.
  2. The Carbon-Hydrogen ratio is very large.
  3. They burn with a sooty yellow flame because of the high carbon-hydrogen ratio.
  4. They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions.

Arene synthesis

Many laboratory methods exist for the organic synthesis of arenes from non-arene precursors:

Arene reactions

The main arene reactions are

Lesser known reactions:

  • Unusual thermal Diels-Alder reactivity of arenes can be found in the Wagner-Jauregg reaction
  • Other photochemical cycloaddition reactions with alkenes through excimers.

Benzene and derivatives of benzene

Benzene derivatives have from one to six substituents attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol which carries a hydroxyl group and toluene 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 ortho, meta and para are devised. For example three isomers exist for cresol 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.

Examples of benzene derivatives with alkyl substituents (alkylbenzenes):

Examples of other aromatic compounds:

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

Main article: Polycyclic aromatic hydrocarbon

Some important arenes are the polyaromatic hydrocarbons (PAH); they are also called polycyclic aromatic hydrocarbons and polynuclear aromatic hydrocarbons. They are composed of more than one aromatic ring. The simplest PAHs are benzocyclopropene (C7H6), benzocyclopropane (C7H8), benzocyclobutadiene (C8H6), and benzocyclobutene (C8H8).

Common examples are naphthalene with two fused rings, anthracene with three, tetracene with four, and pentacene with five linearly fused rings. Phenanthrene and triphenylene are examples of non-linear connections. More exotic examples are helicenes and corannulene.

See also

External links

References

  1. ^ Definition IUPAC Gold Book Link
  2. ^ HighBeam Encyclopedia: aromatic compound
  3. ^ Jerry March Advanced Organic Chemistry 3Ed., ISBN 0-471-85472-7

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

 
 

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