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 that includes an oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.^[2]

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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:

• Alkyne trimerization, [2+2+2] cyclization of three alkynes
• Dötz reaction
• Diels-Alder reactions of alkynes with pyrone or cyclopentadienone with expulsion of carbon dioxide or carbon monoxide.
• Aromatization of cyclohexanes and other aliphatic rings: reagents are, catalysts used in hydrogenation such as platinum, palladium and nickel (reverse hydrogenation), quinones and the elements sulfur and selenium ^[3].
• Bergman cyclization, enyne plus hydrogen donor

Arene reactions

The main arene reactions are

• Electrophilic aromatic substitution
• Nucleophilic aromatic substitution
• Many coupling reactions to biraryls
• Hydrogenation to saturated rings

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):

• Ethylbenzene C₆H₅-CH₂-CH₃
• Toluene C₆H₅-CH₃
• Xylene, m-Xylene, p-Xylene C₆H₄(-CH₃)₂
• Mesitylene, Pseudocumene, Hemimellitene C₆H₃(-CH₃)₃
• Prehnitene, Isodurene, Durene C₆H₂(-CH₃)₄
• Pentamethylbenzene C₆H(-CH₃)₅
• Mellitene C₆(-CH₃)₆

Examples of other aromatic compounds:

• Aniline C₆H₅-NH₂
• Acetylsalicylic acid C₆H₄(-O-C(=O)-CH₃)(-COOH)
• Benzoic acid C₆H₅-COOH
• Biphenyl (C₆H₅)₂
• Chlorobenzene C₆H₅-Cl
• Nitrobenzene C₆H₅-NO₂
• Paracetamol C₆H₄(-NH-C(=O)-CH₃)(-OH)
• Phenacetin C₆H₄(-NH-C(=O)-CH₃)(-O-CH₂-CH₃)
• Phenol C₆H₅-OH
• Picric acid C₆H₂(-OH)(-NO₂)₃
• Salicylic acid C₆H₄(-OH)(-COOH)
• Trinitrotoluene C₆H₂(-CH₃)(-NO₂)₃

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

• Asphaltene
• Simple aromatic rings

References

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