Benzene Is an Aromatic Compound

An aromatic compound has a molecular structure containing cyclic clouds of delocalized tt electrons above and below the plane of the molecule, and the TT clouds contain a total of 4n -f 2) tt electrons (where n is a whole number). This is known as Hiickel s rule (introduced first by Erich Hiickel in 1931). Eor example, benzene is an aromatic compound. 

Benzaldehyde, a close structural relative of benzene, is an aromatic compound responsible for the odor of cherries. 

Benzene is an aromatic compound because it is cyclic and planar, every carbon in the ring has a p orbital, and the tt cloud contains three pairs of tt electrons. 

Science usage an organic compound with increased chemical stability due to the delocahzation of electrons Benzene is an aromatic compound. 

We can understand this because benzene is an aromatic compound. It has a special stability due to its aromaticity. If we add Bt2 across benzene, aromaticity will be lost. And that is why the reaction does not take place—it would be going uphiH in energy. But is it possible to try to force the reaction to happen ... 

In Chapter 8, you learned that benzene is an aromatic compound that can be represented by two resonance contributors. You saw that to be aromatic, a compound must be cyclic and planar and have an unintermpted cloud of rr electrons (called a tt cloud) above and below the plane of the molecule, and the tt cloud must contain an odd number of pairs of TT electrons (Section 8.8). You also learned that aromatic compounds are unusually stable compounds (Section 8.7). 

If n = 1, we have 4 x 1+2 = 6, which means that any compound containing a total number of six tt electrons is an aromatic compound. In the above structure of benzene, there are three double bonds and six tt electrons, and it is a planar molecule. Thus, benzene follows Hiickel s rule, and is an aromatic compound. 

Thiophenol is an aromatic compound (Table 6). One hydrogen atom in its benzene core is substituted for a SH-group. The SH-group has a weak negative induction and a positive mesomeric effects. In addition, the unshared pair of -electrons of a sulfur atom contributes to the 7i-electron cloud of the benzene ring. 

Thiophene is an aromatic compound. Its structure can be assumed to be derived from benzene by replacement of two annular CH groups with sulfur. The sulfur atom in this five-membered ring acts as an electron-donating heteroatom by contributing two electrons to the aromatic sextet and thiophene is thus considered to be an electron-rich heterocycle. 

C. .. chemists usually draw its skeletal structure instead. The presence of a benzene ring indicates that it is an aromatic compound. 

The first fact indicates that benzene is an aromatic member of a series of compounds. Another example is the following ... 

R is an abbreviation for any alkyl or aryl group aryl is used for aromatic compounds in the same way that alkyl is used for aliphatic compounds (for example, methyl, ethyl, isopropyl). An aryl group is an aromatic compound with one hydrogen removed (for example, phenyl—the phenyl group is benzene with one hydrogen removed). 

Musk ambrette (4), a synthetic musk, essential in perfumes to enhance and retain the odour, is an aromatic compound with five substituents on the benzene ring. The nitro groups are by far the most electron-withdrawing so we can disconnect them first. 

The two chapters in Part Five deal with aromaticity and the reactions of aromatic compounds. Aromaticity was first introduced in Chapter 7, where you saw that benzene, a compound with an unusually large resonance energy, is an aromatic compound. We will now look at the criteria that a compound must fulfill in order to be classified as aromatic. Then we will examine the kinds of reactions that aromatic compounds undergo. In Chapter 21, we will return to aromatic compounds when we look at the reactions of aromatic compounds in which one of the ring atoms is an atom other than a carbon. 

Aromaticity is one of the most fascinating popular qualitative chemical concepts in chemistry Michael Faraday isolated benzene by distillation in 1825. He noticed that although benzene is an unsaturated compound with H C 1 1 it is much less reactive than the related unsaturated aliphatic compounds. Moreover, it undergoes substitution reactions rather than addition reactions exhibited by alkenes and alkynes. Eilhard Mitscherlich synthesized benzene by heating benzoic acid with lime. 

Benzene is a simple hydrocarbon with a unique structure of carbon-carbon bonds of equal length and strength. Overlap and delocalization of the n-electrons in benzene, coupled with its cyclic nature and continuous array of sp atoms, leads to special stability called aromaticity, and benzene is an aromatic molecule. A molecule is aromatic if it is cyclic, has a continuous and contiguous array of sp hybridized atoms, and has 4n + 2 n-electrons (the Hiickel rule). Monocyclic aromatic compounds are called annulenes. Both anions and cations derived from cyclic hydrocarbons can be aromatic if they fit the usual criteria 1, 56, 57,104,105,106,107,108,109. 

Ferrocene is an aromatic compound similar to benzene. As the aromatic rings of ferrocene are coordinated by iron, ferrocene shows a higher basicity than that of benzene. Then an electrophilic reaction is liable to proceed. For example, Friedel-Crafts acylation, Mannich reaction (aminomethylation) and lithiation easily proceeds as shown in eqs. (15.12)-(15.14) [26]. 

Capsaicin is responsible for the characteristic spicy flavor of jalapeho and habanero peppers. Although it first produces a burning sensation on contact with the mouth or skin, repeated application desensitizes the area to pain. This property has made it the active ingredient in several topical creams for the treatment of chronic pain. Capsaicin has also been used as an animal deterrent in pepper sprays, and as an additive to make birdseed squirrel-proof. Capsaicin is an aromatic compound because it contains a benzene ring. In this chapter, we learn about the characteristics of aromatic compounds like capsaicin. 

As we have seen, pyridine is an aromatic compound with a nitrogen in place of one of the carbons in a benzene ring (Section 8.10). 

The answer is 1 [II Alb, C 2], Benzene is an aromatic hydrocarbon. Aromatic hydrocarbons are metabolized by hydroxylation and glucuronidation, which facilitates excretion. Because cats lack glucuronide conjugating ability, the aromatic compounds would nx>st likely be more toxic to cats than to other domestic animals at comparable body weight dosages. 

The deductions above indicate that this is an aromatic compound, which is consistent with a DBE of 4. Monosubstitution is confirmed by the IR spectrum and by both NMR spectra. This means that the two chlorines caimot be substituted on the ring. Since the carbon external to the ring has only a single hydrogen, both the chlorines must also be attached to it. The compound is therefore dichloromethyl benzene, CI2CH-C4H5 (benzylidene chloride). 

The second and the most important approach is calculation of aromatic stabilization using a range of computational procedures. Hiickel delocalization energy of a series of aromatic compounds is first reported by Roberts [72]. In Hiickel s approach, the n energy of aromatic compound is compared with the n energy of the same number of double bonds in its isolated form, if we take benzene as an aromatic compound. 

Although Dopamine is an aromatic compound, its conversion to norepinephrine does not involve the hydroxylation of the benzene ring but rather the hydroxylation of the side chain. It would appear, therefore, to be a matter of choice as to whether this reaction should be included in a discussion of aromatic hydroxylations. It was decided to include it because the reaction shows some basic similarities to the aromatic hydroxylation just considered, and also, because it does differ in some important details. A comparison of the two reactions may serve to emphasize some features which could be basic to all aerobic hydroxylation reactions. 

When an aromatic compound having an aliphatic side chain is subjected to oxidation, fission of the side chain occurs between the first and second carbon atoms from the benzene ring, the first carbon atom thus becoming part of a carboxyl ( -COOH) group. For example ... 

The behaviour of benzene is the datum from which any discussion of aromatic compoimds must start the reactivity of an aromatic compound is its rate of reaction relative to that of benzene when both are taking part in reactions occurring under the same conditions and proceeding by the same mechanism. 

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. 

Styrene undergoes many reactions of an unsaturated compound, such as addition, and of an aromatic compound, such as substitution (2,8). It reacts with various oxidising agents to form styrene oxide, ben2aldehyde, benzoic acid, and other oxygenated compounds. It reacts with benzene on an acidic catalyst to form diphenylethane. Further dehydrogenation of styrene to phenylacetylene is unfavorable even at the high temperature of 600°C, but a concentration of about 50 ppm of phenylacetylene is usually seen in the commercial styrene product. 

One approach by Monsanto (described in the basic patent BP 1037111) is to prepare a modified polypenylene by reacting an aromatic sulphonyl halide such as benzene-1,3-disulphonyl dichloride with an aromatic compound having replaceable nuclear hydrogen (e.g. bisphenoxybenzene, sexiphenyl and diphenyl ether). This was discussed in Chapter 21. 

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