Aromatic hydrocarbon



An aromatic hydrocarbon (abbreviated as AH) or arene [1] is a benzene. Aromatic hydrocarbons can be monocyclic or polycyclic.

Some non-benzene-based compounds called heteroarenes, which follow pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.[2]

Benzene ring model

 

Main article: aromaticity

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 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 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 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:

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 (H8).

Common examples are corannulene.

See also

References

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