Aromatic compounds other than benzene

Benzene is not the only compound that exhibits aromatic stabilization. A compound will be aromatic if it satisfies the following two criteria. 

The compound must contain a ring comprised of continuously overlappingp orbitals. 

The number of it electrons in the ring must be a Hiickel number. 

Compounds that fail the first criterion are called nonaromatic. Below are three examples, each of which fails the first criterion for a different reason. 

The first compound (1,3,5-hexatriene) is not aromatic, because it does not possess a ring. The second compound is not aromatic because the ring is not a continuous system ofp orbitals (there is an intervening r/ -hybridized carbon atom). The third compound is not planar, and the p orbitals do not effectively overlap. 

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Some authors have used aromatic compounds other than benzene, and the behavior is generally similar to that described in this section. 

The apparent polarity of the dinonylphthalate column, expressed by a Al of 84, is a measure of retardation of aromatic and olefinic substances. Since the chromatographer is interested in the selectivity of a column for a variety of functional groups, it is important to classify each of the stationary phases by their ability to retard compounds other than benzene. This has been done by Rohrschneider (22) and further developed by McReynolds (23). The system is discussed in detail with many examples by Supina (6). The Rohrschneider/McReynolds (R/M) system involves the measurement of the retention indices for several compounds on a given column to determine the degree to which each is retarded. In each case, the retention indices are compared to... 

Cyclic compounds other than benzene can, however, possess aromatic or benzenoid properties. This arises when they are planar and possess some double bonds which enable their formulae to be expressed in alternative Kekule-type structures. In such compounds an overlap of p orbitals occurs between adjacent atoms, and this allows the n electrons to become delocalised and form a continuous ring of electron density (Figure 6.21). An aromatic compound of this kind will sustain a magnetically induced ring current, and the bond lengths are all equivalent, lying between single and double bond values. Benzene with 6 n electrons was the first of these aromatic compounds to be encountered and seriously studied by chemists. 

Some papers report on the influence of aromatic compounds on the polymerization of vinyl compounds other than vinyl acetate. Mayo et al.48 found that bromo-benzene acts as a chain transfer agent in the polymerization of styrene, although no fragments of bromobenzene are incorporated into the polymer. They concluded that a complex is formed between the solvent molecule and either the propagating poly-styryl radical or hydrogen atom derived from the latter. 

The hexane spectrum (Figure 14.15) shows that the C-H stretch at 2850-2960 cm-i and the signals at 1350-1470 cm-i correlate with the absorptions in Table 14.3. Benzene derivatives and other aromatic compormds usually show absorption for the C-H units at 3000-3100 cm" but also at 675-870 cm i (in the fingerprint region). Note the subtle shift of the C-H absorption to lower energy for the aromatic compounds. Other compounds that have a C-H absorption are those for alkenes and alkynes. The CsC-H absorption is at lower energy than the C=C-H absorption, which is lower in energy than the C-C-H absorption for alkanes. 

In Section 21.1, benzene was identified as an aromatic compound with the resonance-stabilized delocalization shown in 1. Benzene is also identified as an aromatic compound because it meets certain unique criteria. There are six 7i-electrons confined to a ring, and every carbon atom in that ring is sp hybridized with a p-orbital attached. Further, the p-orbitals are contiguous and continuous. In other words, every carbon in the ring has a p-orbital and there are no intervening sp atoms. In principle, if a molecule other than benzene meets the criteria, it is aromatic. There are several such molecules, both neutral molecules and charged intermediates. 

Fullerenes are aromatic structures and dissolve readily in the archetypal aromatic compound, i.e., benzene and in other aromatic solvents. They oxidize slowly in a mixture of concentrated sulfuric and nitric acids at temperatures above 50°C. In pure oxygen, Cqo begins to sublime at350°C and ignites at 365°C in air, it oxidizes rapidly to CO and CO2 and is more reactive than carbon black or any other form of graphite. ... 

The Hiickel approximation is especially useful in understanding the chemical stability of benzene, and by extension other aromatic compounds. Recall that benzene (Figure 15.21) is more stable than expected for a cyclohexatriene, and its chemistry is representative of an entire class of aromatic hydrocarbons as opposed to the nonaromatic aliphatic hydrocarbons. The Hiickel approximation provides some clues for benzene s distinctions. 

Other atoms or groups may be substituted for H atoms on the benzene molecule, and to name these compounds, we use a numbering system for the C atoms in the ring. If the name of an aromatic compound is based on a common name other than benzene (such as toluene), the characteristic substituent group (for example. 

Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het erocyclic aromatic compounds Some representative heterocyclic aromatic compounds are pyridine pyrrole furan and thiophene The structures and the lUPAC numbering system used m naming their derivatives are shown In their stability and chemical behav lor all these compounds resemble benzene more than they resemble alkenes... 

Toluene (methylbenzene) is similar to benzene as a mononuclear aromatic, but it is more active due to presence of tbe electron-donating metbyl group. However, toluene is much less useful than benzene because it produces more polysubstituted products. Most of tbe toluene extracted for cbemical use is converted to benzene via dealkylation or disproportionation. Tbe rest is used to produce a limited number of petro-cbemicals. Tbe main reactions related to tbe cbemical use of toluene (other than conversion to benzene) are the oxidation of the methyl substituent and the hydrogenation of the phenyl group. Electrophilic substitution is limited to the nitration of toluene for producing mono-nitrotoluene and dinitrotoluenes. These compounds are important synthetic intermediates. 

Aromatic hydrocarbons substituted by alkyl groups other than methyl are notorious for their tendency to disproportionate in Friedel-Crafts reactions. This tendency has previously limited the application of the isomerization of para- or ortho-) m ky -benzenes to the corresponding meta compounds. At the lower temperature of the present modification, disproportionation can be minimized. 

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