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In organic chemistry, the term aromaticity is formally used to describe an unusually stable nature of some flat rings of atoms. These structures contain a number of double bonds that interact with each other according to certain rules. As a result of their being so stable, such rings tend to form easily, and once formed, tend to be difficult to break in chemical reactions. Since one of the most commonly encountered aromatic system of compounds in organic chemistry is based on derivatives of the prototypical aromatic compound benzene (common in petroleum), the word “aromatic” is occasionally used to refer informally to benzene derivatives, and this is how 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. 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 do have odors (unlike pure saturated hydrocarbons). Today, there is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds, 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 we recognize today are similar to unsaturated petroleum hydrocarbons like benzene. In terms of the electronic nature of the molecule, aromaticity describes the way a conjugated ring of unsaturated bonds, lone pairs of electrons, or empty molecular orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. Aromaticity can be considered a manifestation of cyclic delocalization and of resonance. This is usually considered to be because electrons are free to cycle around circular arrangements of atoms that are alternately single- and double-bonded to one another. These bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring identical to every other. 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 section 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. ==Theory== As is standard for resonance diagrams, the use of a double-headed arrow indicates that two structures are not distinct entities but merely hypothetical possibilities. Neither is an accurate representation of the ''actual'' compound, which is best represented by a hybrid (average) of these structures, as can be seen at right. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon-carbon bonds have the same length, intermediate between that of a single and that of a double bond. A better representation is that of the circular π-bond (Armstrong's ''inner cycle''), in which the electron density is evenly distributed through a π-bond above and below the ring. This model more correctly represents the location of electron density within the aromatic ring. The single bonds are formed with electrons in line between the carbon nuclei— these are called σ-bonds. Double bonds consist of a σ-bond and a π-bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. The following diagram shows the positions of these p-orbitals: Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalized. This means that, instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital is considered to have ''π symmetry''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Aromaticity」の詳細全文を読む スポンサード リンク
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