Aromaticity | Wikipedia audio article
Channel: wikipedia tts
Date: 2019-10-04

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Aromaticity


00:02:56 1 Theory
00:05:05 2 History
00:05:14 2.1 The term "aromatic"
00:06:32 2.2 The structure of the benzene ring
00:09:27 3 Characteristics of aromatic (aryl) compounds
00:13:43 4 Importance of aromatic compounds
00:14:59 5 Types of aromatic compounds
00:15:17 5.1 Neutral homocyclics
00:15:42 5.2 Heterocyclics
00:17:30 5.3 Fused aromatics and polycyclics
00:18:21 5.4 Substituted aromatics
00:18:45 5.5 Aromatic ions
00:19:59 5.6 Atypical aromatic compounds
00:21:49 6 Other symmetries
00:23:34 7 See also



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SUMMARY
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In organic chemistry, aromaticity is a property of cyclic (ring-shaped), planar (flat) structures with a ring of resonance bonds that gives increased stability compared to other geometric or connective arrangements with the same set of atoms. Aromatic molecules are very stable, and do not break apart easily to react with other substances. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have special stability (low reactivity).
Since the most common aromatic compounds are derivatives of benzene (an aromatic hydrocarbon common in petroleum and its distillates), the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined. Nevertheless, many non-benzene aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the double-ringed bases in RNA and DNA. An aromatic functional group or other substituent is called an aryl group.
The earliest use of the term aromatic was in an article by August Wilhelm Hofmann in 1855.  Hofmann used the term for a class  of benzene compounds, many of which have odors (aromas), unlike pure saturated hydrocarbons. Aromaticity as a chemical property bears no general relationship with the olfactory properties of such compounds (how they smell), although in 1855, before the structure of benzene or organic compounds was understood, chemists like Hofmann were beginning to understand that odiferous molecules from plants, such as terpenes, had chemical properties that we recognize today are similar to unsaturated petroleum hydrocarbons like benzene.
In terms of the electronic nature of the molecule, aromaticity describes a conjugated system often made of alternating single and double bonds in a ring. This configuration allows for the electrons in the molecule's pi system to be delocalized around the ring, increasing the molecule's stability. The molecule cannot be represented by one structure, but rather a resonance hybrid of different structures, such as with the two resonance structures of benzene. These molecules cannot be found in either one of these representations, with the longer single bonds in one location and the shorter double bond in another (see Theory below). Rather, the molecule exhibits bond lengths in between those of single and double bonds. This commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by August Kekulé (see History below). The model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to produce six one-and-a-half bonds.  Benzene is a more stable molecule than would be expected without accounting for charge delocalization.