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Chapter

Cover Aromatic Chemistry

Polycyclic arenes  

This chapter discusses polycyclic arenes. The fusion of benzene rings leads to an array of polycyclic arenes, of which naphthalene, anthracene, and phenanthrene are the best known. Other compounds in the series include chrysene, pyrene, tetracene, and coronene. Many polycyclic aromatic hydrocarbons are found in coal tar, and are formed when plant materials are partially combusted. Some are procarcinogens, that are oxidized within the body to the true cancer-promoting compounds. Very large polycyclic systems are known, and graphite consists of planes derived from an infinite linear fusion of benzene nuclei. Naphthalene is much more easily hydrogenated than benzene, but the reduction only proceeds as far as 1,2,3,4-tetrahydronaphthalene (tetralin). Meanwhile, anthracene can be synthesized in several ways, two of which parallel the constructions of naphthalenes.

Chapter

Cover Organometallics 2

Arene complexes  

This chapter assesses arene complexes. Following the recognition in the early 1950s of the sandwich bonding principle for metallocenes, the existence of related sandwich complexes of neutral arenes such as benzene was demonstrated through the synthesis of bis-(benzene)chromium. The complex is isoelectronic to ferrocene and follows the 18-electron rule. Bonding in bis-(arene)metal complexes resembles that in metallocenes, and the MO diagram is qualitatively very similar. Since the aromatic ligands in bis-(arene) complexes are not negatively charged, there is no electrostatic contribution to bonding, and bis-(arene) complexes are in general less stable than metallocenes and more easily oxidized. The chapter then considers arene half-sandwich complexes; seven and eight membered ring ligands; heteroarene complexes; and multidecker complexes.

Chapter

Cover Making the Transition to University Chemistry

Hydrocarbons: Arenes  

This chapter discusses arenes, a type of hydrocarbon. Benzene is known to be the archetypal arene as it features the original Kekulé structure with alternating double and single bonds. The electrophilic substitution reactions of benzene go in line with the high electron density above and below the benzene ring. Nitration is a particularly vital reaction undergone by benzene. This involves a nitrating mixture of concentrated nitric acid and sulfuric acid. Additionally, the electrophilic substitution of Friedel–Crafts acylation involves reagents of acyl chloride and aluminium chloride , the latter which acts as a Lewis acid. On the other hand, the electrophilic substitution of halogenation pertains to how benzene needs a catalyst for halogenation.

Book

Cover Aromatic Chemistry

Malcolm Sainsbury

Aromatic Chemistry presents all the basic principles of this important topic in an account which takes as its examples many compounds of industrial and biological significance. Consideration is given to the structure, reactions, and properties of benzene and classes of aromatic compounds derived from it, and topics such as thermodynamic versus kinetic control and pericyclic reactions are introduced. The text also covers polycyclic arenes and the small and large ring systems which are embraced by the wider definition of aromaticity.

Chapter

Cover Aromatic Chemistry

Aromatic character and the structure of benzene  

This chapter discusses the aromatic character and the structure of benzene. Benzene C6H6 is a component of coal tar distillate and an aerial pollutant found in the exhaust gases of automobiles. It is the parent of a group of compounds known as arene, which exhibit different reactions to those shown by other unsaturated hydrocarbons, i.e. alkenes. Arenes are also described as aromatic molecules. The term aromatic is historical and derives from the fact that many naturally occurring fragrant compounds were observed to contain a benzene unit. Nowadays aromaticity has a wider scientific meaning and refers to planar cyclically conjugated structures having (4n+2) π electrons (where n is 0, 1, 2 etc.), Hückel's rule. The chapter discusses resonance and the π electron system of benzene, the representation of benzene, the importance of Hückel's rule, and nomenclature and numbering of benzenes.

Chapter

Cover The f Elements

Organometallics  

This chapter focuses on organometallics. Important advances in understanding the behaviour of the f elements have arisen from the study of their organometallic complexes. The chapter describes the methods that are used to synthesize these compounds, the structures that they adopt, and the physical and chemical properties that they display. The sensitivity of most f element organometallics to oxygen and water is a property which they share with the early d block compounds, but this problem is exacerbated by the large atomic radii of the lanthanides and actinides. It is with the advent of modern techniques for the rigorous exclusion of air and the development of sterically demanding ligands that the field has expanded to the point where, like the d block organometallics, applications in catalysis and other areas are beginning to emerge. The chapter then looks at alkyls; cyclopentadienyls; carbonyls and related complexes; arene complexes; and cyclooctatetraene complexes.

Chapter

Cover Aromatic Chemistry

Reactions of arenes  

This chapter examines the reactions of arenes. If benzene is reacted with a reagent R, it is reasonable to conclude that should R be positively charged (an electrophile), the process will be easier than when R is negative (a nucleophile). For these reasons, electrophilic substitution is commonplace, whereas nucleophilic substitution only occurs under special circumstances. The energy profile of an electrophilic substitution reaction with benzene as the substrate can be represented diagrammatically. In order that the reaction should progress to the final product, another transition state is traversed in which the sigma bond to the proton of the tetrahedral carbon atom weakens and eventually breaks. There is much evidence for the formation of sigma complexes. The chapter then looks at π complexes.