Aromatic hydrocarbon

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An aromatic hydrocarbon (abbreviated as AH), or arene 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. After the simplest possible aromatic hydrocarbon, benzene, such a configuration of six carbon atoms is known as a benzene ring.

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Benzene ring model

Image:Toluene.pngTemplate:Main 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 just draw a circle around the inside of the ring to show that there are 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.

In modern terminology, benzene rings can be described as compounds in which a continuous, closed system of rings contains separate sets of sigma and pi electrons. The atomic orbitals forming the sigma system are sp2 hybridized, and those forming the pi system are pure p orbitals.

  • Properties:
  1. They have close conjugation.
  2. The Carbon atoms are sp2 hybridized, and therefore have a trigonal planar structure.
  3. The Carbon-Hydrogen ratio is very large.
  4. Hückel's rules apply
  5. They burn with a sooty yellow flame because of the high carbon-hydrogen ratio.
  6. They undergo electrophilic substitution reactions
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Benzene and derivatives of benzene

Image:Benzene-structure.pngTemplate:Main Aromatic hydrocarbons can be monocyclic or polycyclic. 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.

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PAHs

Template:Main Image:Napthalene.pngSome important arenes are the polycyclic aromatic hydrocarbons (PAH); they are also called polynuclear aromatic hydrocarbons. They are composed of more than one aromatic ring. The simplest PAH is benzocyclobutene (C8H6).

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PAHs and the origins of life

Template:Main In January 2004 (at the 203rd Meeting of the American Astronomical Society), it was reported (as cited in Battersby, 2004) that a team led by A. Witt of the University of Toledo, Ohio studied ultraviolet light emitted by the Red Rectangle nebula and found the spectral signatures of anthracene and pyrene. (No other such complex molecules had ever before been found in space.) This discovery was considered confirmation of a hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's core to get caught in stellar winds, and radiate outward. As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms.

Witt and his team inferred (as cited in Battersby, 2004) that since they discovered PAHs—which may have been vital in the formation of early life on Earth—in a nebula, nebulae, by necessity, are where they originate.

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External links

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References

de:Aromaten et:Areenid es:Hidrocarburo aromático fr:Hydrocarbure aromatique ko:방향족 탄화수소 he:ארומטיות lv:Aromātiskie ogļūdeņraži nl:Aromatische verbinding ja:芳香族炭化水素 pl:Węglowodór aromatyczny ru:Арены sv:Aromatiska kolväten zh:芳烃

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