Substituted benzenes aromatic substitution reactions

It is a typically aromatic compound and gives addition and substitution reactions more readily than benzene. Can be reduced to a series of compounds containing 2-10 additional hydrogen atoms (e.g. tetralin, decalin), which are liquids of value as solvents. Exhaustive chlorination gives rise to wax-like compounds. It gives rise to two series of monosubstitution products depending upon... 

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

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. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

REPRESENTATIVE ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS OF BENZENE... 

Representative Electrophilic Aromatic Substitution Reactions of Benzene... 

With this as background let us now examine each of the electrophilic aromatic substitution reactions presented m Table 12 1 m more detail especially with respect to the electrophile that attacks benzene... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Section 12 17 Polycyclic aromatic hydrocarbons undergo the same kind of electrophilic aromatic substitution reactions as benzene... 

The aromatic ring has high electron density. As a result of this electron density, toluene behaves as a base, not only in aromatic ring substitution reactions but also in the formation of charge-transfer (tt) complexes and in the formation of complexes with super acids. In this regard, toluene is intermediate in reactivity between benzene and the xylenes, as illustrated in Table 2. 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Unsaturated hydrocarbons undergo a variety of reactions. Experimentally, alkenes and alkynes undergo addition reactions, whereas aromatic molecules, such as benzene, undergo substitution reactions instead. Why ... 

Aromatic substitution reactions occur by addition of an electrophile such as Br+ to the aromatic ring to yield an allylic carbocation intermediate, followed by loss of H+. Show the structure of the intermediate formed by reaction of benzene with Br+. 

Among the most useful electrophilic aromatic substitution reactions In the laboratory is alkylation—the introduction of an alkyl group onto the benzene ring. Called the Friedel-Crafts reaction after its discoverers, the reaction is carried out... 

Another drawback to the use of amino-substituted benzenes in electrophilic aromatic substitution reactions is that Friedel-Crafts reactions are not successful (Section 16.3). The amino group forms an acid-base complex with the AICI3 catalyst, which prevents further reaction from occurring. Both drawbacks can be overcome, however, b3 carrying out electrophilic aromatic substitution reactions on the corresponding amide rather than on the free amine. 

Unlike benzene, pyridine undergoes electrophilic aromatic substitution reactions with great difficulty. Halogenation can be carried out under drastic conditions, but nitration occurs in very low yield, and Friedel-Crafts reactions are not successful. Reactions usually give the 3-substituted product. 

Bifunctional catalysis has also been observed by Pietra and Vitali35 for a more typical nucleophilic aromatic substitution reaction, that of 2,4-dinitrofluorobenzene and piperidine in benzene. For this reaction triethylamine does not have an... 

The chemical reactions of benzene and all aromatic compounds, with few exceptions, are unlike those of unsaturated aliphatic compounds (olefins) that is, addition reactions do not occur. Instead, the hydrogens on the ring are replaced by other atoms or groups of atoms. The aromatic ring remains unchanged by these substitution reactions. All six of the hydrogens in benzene can be replaced by other atoms. 

For many years phenol was made on a large industrial scale from the substitution reaction of benzene sulfonic acid with sodium hydroxide. This produced sodium sulfite as a by-product. Production and disposal of this material, contaminated with aromatic compounds, on a large scale contributed to the poor economics of the process, which has now been replaced by the much more atom economic cumene route (see Chapter 2, Schemes 2.2 and 2.3). 

If we are correct in our assumption that the electrophilic substitution of aromatic species involves such a complexes as intermediates—and it has proved possible actually to isolate them in the course of some such substitutions (p. 136)—then what we commonly refer to as aromatic substitution really involves initial addition followed by subsequent elimination. How this basic theory is borne out in the common electrophilic substitution reactions of benzene will now be considered. 

Naphthalene intermediates [61] are always built up by substitution reactions starting from the cheap and plentiful hydrocarbon using, in the main, only seven basic reactions. Most of these reactions are generally familiar from benzene chemistry but with some modification, since naphthalene has two different possible positions of substitution. These positions are often designated a and [3, the four a-positions being ortho and the four P-positions meta to the nearest carbon atom of the central bond. A further modifying influence is the lower level of aromaticity of naphthalene compared with benzene, leading to increased reactivity. 

The reactions between 2,4-dinitrohalogenobenzenes and X-substituted anilines in benzene produce the usual diphenylamines 109 by nucleophilic aromatic substitution reaction 28. The inspection of reaction mixtures by UV/VIS spectroscopy at zero reaction... 

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