Aromatic Compounds, definition

The above definition implies that the reactivity of an aromatic compound depends upon the reaction which is used to measure it, for the rate of reaction of an aromatic compound relative to that for benzene varies from reaction to reaction (table 7.1). However, whilst a compoimd s reactivity can be given no unique value, different substitution reactions do generally set aromatic compoimds in the same sequence of relative reactivities. 

Another characteristic of aromatic compounds is a relatively large HOMO-LUMO gap, which can be expressed in terms of hardness, t] (see p. 21 for the definition of hardness) ... 

A brief overview of the historical developments that led to the dichotomy between aliphatic and aromatic compounds delineates that the former are characterized by additivity for heats of formation and practically all other properties, whereas the situation for aromatic systems is less straightforward. Definitions, criteria, and properties of aromatic systems will be discussed. Notwithstanding controversies over aro-... 

Based on the above information, the CAMD problem definition is revised as follows - The solvent can be acyclic hydrocarbons and ketones (aromatic compounds, chlorides, dioxanes are not considered for EH S concerns). The normal boiling point should be higher than that of chloroform (334 K), the molecular weight could be between 70-120, the solvent must not form azeotrope with either acetone or chloroform, and, must be totally miscible with the binary mixture of acetone and chloroform. 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

The conclusion appears to be that furan, if regarded as an aromatic compound, probably has less than about 20% of the aromaticity of benzene, and perhaps has none. Only a definition of aromaticity that identifies it with NMR shielding by ring currents, and little else, clearly suggests an aromaticity of 60% or more. 

Wolfsberg-Helmholtz, 43 zero-overlap, 38, 40 Aromaticity, 150 Aromatic compounds, 150-160 Arsenium ions, 119 Asymmetric synthesis definition, 9 Avoided crossing, 206... 

D is correct. Anthracene is a larger version of benzene, a prototvpic aromatic compound. It satisfies Huckel s rule, which states that if a compound has planar, monocyclic rings with 4n + 2 n electrons (n being any integer, including zero), it is by definition an aromatic compound. Benzene houses six x electrons, a pair for each double bond (while anthracene has 14 n electrons). 

Aromatic compounds, sometimes called arenes, have a functional group called a benzene ring. Actually, this is too narrow a definition, because there are non-ben-zenoid aromatic compounds. However, this definition is a good starting point. Aromatic compounds are very common in nature because aromatic compounds are especially stable. Aromatic rings are frequently a structural element in medications. Benzene is the archetype aromatic compound and has the molecular formula C H. The carbon atoms form a six-membered ring, and the entire molecule is flat. Models of benzene are shown in Figure 11.15. 

Substrates containing an electron-rich double bond, such as enol ethers and enol acetates, are easily oxidized by means of PET to electron-deficient aromatic compounds, such as dicyanoanthracene (DCA) or dicyanonaphthalene (DCN), which act as photosensitizers. Cyclization reactions of the initially formed silyloxy radical cation in cyclic silyl enol ethers tethered to an olefinic or an electron-rich aromatic ring, can produce bicyclic and tricyclic ketones with definite stereochemistry (Scheme 9.14) [20, 21]. 

A complete definition of aromatic compounds must wait until Chapter 16. For the present we will define them as benzene and its substituted derivatives. They are also called arenes. These aromatic compounds have a six-membered ring with three conjugated doable bonds. It is this cycle of conjugated double bonds that makes arenes special. Examples include the following ... 

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