Substitution reactions of aromatic compounds

AROMATIC SUBSTITUTION The most common reaction for aromatic compounds is substitution. This process is much like a soccer substitution one group (in this case, a person) replaces another. 

5 Carbon-Carbon Bond Formation Friedel-Crafts Alkylation 

9 Disubstituted Benzenes Ortho, Meta, and Para Substitution 

15 Special Topic Biological Synthesis of Aromatic Rings Phenylalanine 

Even electrons, supposedly the paragons of unpredictability, are tame and obsequious little creatures that rush around at the speed of light, going precisely where they are supposed to go. They make faint whistling sounds that when apprehended in varying combinations are as pleasant as the wind flying through a forest, and they do exactly as they are told. Of this, one can be certain. 

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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. 

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... 

Basically, three experimental problems are involved in the substitution reactions of aromatic compounds (1) proof of structure of the isomers that are formed (2) determination of the percentage of each isomer formed, if the product is a mixture and (3) measurement of the reactivity of the compound being substituted relative to some standard substance, usually benzene. 

Tab. 5.1 Substitution Reactions of Aromatic Compounds Mechanistic Alternatives ...

Practical use is often made of the nucleophilic behaviour of jff-diketone-derived monomacrocyclic complexes. Sueh compounds undergo many reactions reminiscent of the electrophilic substitution reactions of aromatic compounds. For example, by interaction of [Ni(L1790)]I with isophthaloyl dichloride, the binuclear complex [Ni2(L1791)]l2 H20 was obtained [134], whose structure has been confirmed by X-ray diffraction (Eq. 8.56) [135],... 

In Chapters 12 and 13, we examined the structural consequences of conjugation, the overlap of 2p orhitals. This story culminates in the enormously stable aromatic molecules, with the parent compound, benzene, being the prototype. In this chapter, we win focus mostly on substitution reactions of aromatic compounds, an idea introduced in Section 13.11a (p. 606).The chemistry of aromatic compounds is dominated by a tendency to retain the stable aromatic sextet of electrons. It takes a great deal of energy to destroy this arrangement, and it is very easily regained. 

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