Reactions of Aromatic Compounds Electrophilic Substitution

The most common reaction of aromatic compounds is electrophilic aromatic substitution, a process in which an electrophile (E+) reacts with an aromatic ring and substitutes for one of the hydrogens  

The reaction is characteristic of all aromatic rings, not just benzene and substituted benzenes. In fact, the ability of a compound to undergo electrophilic substitution is a good test of aromaticity. 

Before seeing how electrophilic aromatic substitutions occur, let s briefly recall what we said in Chapter 6 about electropbilic alkene additions. Wben a reagent such as HCl adds to an alkene, the electrophilic hydrogen approaches the V electrons of the double bond and forms a bond to one carbon, leaving a positive charge at the other carbon. This carbocation intermediate then reacts with the nucleophilic Cl ion to yield the addition product. 

An electrophilic aromatic substitution reaction begins in a similar way, but there are a number of differences. One difference is that aromatic rings are less reactive toward electrophiles than alkenes are. For example, Br2 in CH2CI2 solution reacts instantly with most alkenes but does not react with benzene at room temperature. For bromination of benzene to take place, a catalyst such as FeBrs is needed. The catalyst makes the Br2 molecule more electrophilic by polarizing it to give an FeBr4 Br species that reacts as if it were Br . The 

Although more stable than a typical alkyl carbocation because of resonance, the intermediate in electrophilic aromatic substitution is nevertheless much less stable than the starting benzene ring itself, with its 150 kj/mol of aromatic stability. Thus, the reaction of an electrophile with a benzene ring is endergonic, has a substantial activation energy, and is rather slow. 

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As discussed in section IV, fullerenes are deprived of the archetypical reactions of aromatic compounds, electrophilic or nucleophilic substitution, due to the lack of substituents. Rather, the reactivity of Ceo and Cjo has been compared to that of electron-deficient olefins (see refs 4—11 and 20—26 for reviews on... 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

The most common reaction of aromatic compounds is electrophilic aromatic substitution. That is, an electrophile reacts with an aromatic ring and substitutes for one of the hydrogens. The reaction is characteristic of all aromatic rings, not just benzene and substituted benzenes. In fact, the ability of a compound to undergo electrophilic substitution is a good test of aromaticity- . 

Volume 8 Volume 9 Volume 10 Volume 12 Volume 13 Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)... 

In Volume 13 reactions of aromatic compounds, excluding homolytic processes due to attack of atoms and radicals (treated in a later volume), are covered. The first chapter on electrophilic substitution (nitration, sulphonation, halogenation, hydrogen exchange, etc.) constitutes the bulk of the text, and in the other two chapters nucleophilic substitution and rearrangement reactions are considered. 

In Part 11 we concentrate on aromatic systems, starting with the basics of structure and properties of benzene and then moving on to related ciromatic compounds. We even throw in a section of spectroscopy of aromatic compounds. Chapters 7 and 8 finish up this pcirt by going into detail about substitution reactions of aromatic compounds. You find out all you ever wanted to know (and maybe more) about electrophilic and nucleophilic substitutions, along with a little about elimination reactions. 

One major reaction of aromatic compounds is electrophilic substitution and it was therefore natural to submit the present systems to electrophiles. Thus l,2-dihydro-2-methylbenz[e][l,2]azaborine (113 R = Me) is brominated and chlorinated in its 3-position to give compounds (186), of which (186 X = C1) was prepared authentically for identification, starting from w-chloro-2-aminostyrene. The main reaction was, however, deboronation to w-halo-2-aminostyrenes. 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

The most common reaction of aromatic compounds is electrophilic aromatic substitution. Many different substituents can be introduced. .. Starting from only a few simple materials, we can prepare many thousands of substituted aromatic compounds. J. McMurry, Organic Chemistry, 5 th Ed., Brooks/Cole, Pacific Grove, CA, 2000, p. 592. 

Aromatic compounds undergo many reactions, but relatively few reactions that affect the bonds to the aromatic ring itself. Most of these reactions are unique to aromatic compounds. A large part of this chapter is devoted to electrophilic aromatic substitution, the most important mechanism involved in the reactions of aromatic compounds. Many reactions of benzene and its derivatives are explained by minor variations of electrophilic aromatic substitution. We will study several of these reactions and then consider how substituents on the ring influence its reactivity toward electrophilic aromatic substitution and the regiochemistry seen in the products. We will also study other reactions of aromatic compounds, including nucleophilic aromatic substitution, addition reactions, reactions of side chains, and special reactions of phenols. 

Naphthalene is classified as aromatic because its properties resemble those of benzene (see Sec. lO.lO). Its molecular formula, C,oHs, might lead one to expect a high degree of unsaturation yet naphthalene is resistant (although less so than benzene) to the addition reactions characteristic of unsaturated compounds. Instead, the typical reactions of naphthalene are electrophilic substitution reactions, in which hydrogen is displaced as hydrogen ion and the naphthalene ring system is preserved. Like benzene, naphthalene is unusually stable its heat of combustion is 61 kcal lower than that calculated on the assumption that it is aliphatic (see Problem 10.2, p. 323). 

In aromatic electrophilic substitution, " the initial interaction between an electrophile and the aromatic n system is a multicenter interaction (of n-complex nature). The lack of substrate selectivity observed in some reactions of aromatic compounds with strong electrophiles (e.g., N()2 ) indicates that the initial multicenter complex is a separate well-defined intermediate,- " " Its nature was much discussed. Schofield et al. suggested it to be a solvent cage, whereas Perrin preferred a radical ion pair. There was general agreement of an initial intermediate involving the aromatic as an entity. The subsequent step affords a trivalent benzenium ion intermediate or a complex (Scheme 6.42). 

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