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Chapter

Cover Chemistry3

Benzene and other aromatic compounds  

Electrophilic substitution reactions

This chapter considers benzene as one of the most fascinating organic molecules. Six carbon atoms in benzene are linked in a planar hexagon and, as each carbon atom is bonded to only one hydrogen atom, benzene is an unsaturated hydrocarbon. The chapter determines what aromatic, antiaromatic, and nonaromatic compounds are and give examples of each. It identifies reagents and reaction mechanisms to explain how benzene undergoes halogenation, nitration, sulfonation, Friedel–Crafts alkylation, and Friedel–Crafts acylation. It also talks about how the electronic and steric effects of substituents on benzene rings influence the rates and regioselectivities of electrophilic substitution reactions and how substituents on benzene rings can be converted into other substituents by redox reactions or by forming diazonium ions.

Chapter

Cover Organic Chemistry

Aromatic heterocycles 1: reactions  

This chapter investigates the reactions of aromatic heterocycles. Benzene is aromatic because it has six electrons in a cyclic conjugated system. Most organic compounds belong to the class of aromatic heterocycles, and they number among them some of the most significant compounds for human beings. The chapter begins by considering how aromaticity survives when parts of benzene’s ring are replaced by nitrogen atoms. How can we insert a heteroatom into the ring and retain aromaticity? What kind of atom is needed? If we want to replace one of the carbon atoms of benzene with a heteroatom, we need an atom that can be trigonal to keep the flat hexagonal ring, and that has a p orbital to keep the six delocalized electrons. Nitrogen fits all of these requirements. This is what happens if we replace a CH group in benzene with a nitrogen atom. The chapter then looks at reactions of five-membered heterocycles, as well as benzo-fused heterocycles.

Chapter

Cover Organic Chemistry

Electrophilic aromatic substitution  

This chapter assesses electrophilic aromatic substitution. Formation of the enol tautomer is catalysed by acid or by base, and because the ketone and enol are in equilibrium, enolization in the presence of D2O can lead to replacement of the protons in the α positions of ketones by deuterium atoms. Because the enolization and deuteration process can be repeated, eventually all of the α-protons are replaced by deuterium. The way this ketone is deuterated provides evidence that its enol form exists, even though the keto/enol equilibrium greatly favours the ketone form at equilibrium. The chapter discusses similar reactions of a compound that exists entirely in its enol form. That very stable enol is phenol and its stability is a consequence of the aromaticity of its benzene ring. The chapter then looks at alkyl benzenes, halogens, and the Friedel–Crafts chemistry.

Chapter

Cover Applied Organometallic Chemistry and Catalysis

Nylon intermediates: buta-1,3- diene hydrocyanation  

This chapter examines benzene, which has been used as the primary hydrocarbon source for the manufacture of Nylon via the intermediates adipic acid and hexamethylenediamine. It reviews the condensation polymerization of comonomers giving Nylon 6,6', which is the most common form of Nylon. It also outlines the multi-step sequence of reactions required for the manufacture of Nylon from benzene, which has provided a commercial incentive to introduce step changes by the development of simpler routes based on alternative feedstocks. The chapter recounts recent developments at BASF, the largest chemical producer in the world, which have demonstrated a viable alternative production route to adipic acid via the selective dicarbonylation of buta 1,3 diene. It mentions propylene that is converted into acrylonitrile by ammoxidation using well-known mixed oxide.

Chapter

Cover Organic Chemistry

Conjugation, π-Electron Delocalization, and Aromaticity  

This chapter begins with a discussion on extended π bonds and the concept of conjugation. It stresses that the C=C double bond is made up of a σ bond and a π bond, and the characteristic chemical properties of ethene come mainly from the π bond. The chapter also highlights that two C=C double bonds separated by a single σ bond are also able to interact by sideways overlap of their π molecular orbitals or MOs (called a conjugative interaction) in a conjugated system. The chapter then introduces stabilization or delocalization energy. It investigates how constituent π and p orbitals of appropriate energy and symmetry within a molecule (or ion) interact to give the new MOs of a conjugated system, and how the conjugation can be described by resonance. Next, the chapter considers the electronic structure of benzene and the nature of aromaticity.

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.

Chapter

Cover Making the Transition to University Chemistry

Bonding and Molecular Shape  

This chapter discusses bonding and molecular shape of electrons. A covalent bond occurs when atoms share a pair of similar electrons. Modern theories on covalent bonding are dominated by molecular orbital theory. Polar covalent bonds occur when different atoms share the electron pair unequally due to electronegativity. With benzene being the most familiar molecule of the bonding, delocalization happens when more than two atoms are involved in the bonding. The valence-shell electron-pair repulsion (VSEPR) theory can help us to visualize the shapes of simple molecules. Additionally, ionic bonding occurs when an atom transfers an electron to another atom and the ions formed a crystal lattice electrostatically.

Chapter

Cover Organic Chemistry

Electrophilic Aromatic Substitution  

This chapter focuses on aromaticity - the parent member of a large class of so-called aromatic compounds. It shows that benzene is a conjugated unsaturated compound which has special stability attributed to aromaticity. However, an organic compound does not have to be either aromatic, or not, as a whole: molecules of many compounds, including natural products and biologically important compounds, contain aromatic and non-aromatic residues linked together. The chapter then displays the distinguishing feature of an aromatic compound: its cyclic system of delocalized ? electrons. It looks at electrophilic aromatic substitution and the main reactions of benzene: halogenation, nitration, sulfonation, and Friedel-Crafts alkylation and acylation. Next, the chapter analyzes the regioselectivity of electrophilic substitution directed by substituents and the reactivity of phenol and aniline. It also looks at the preparation of substituted benzenes.

Chapter

Cover Reactive Intermediates

Arynes  

This chapter describes arynes as neutral intermediates derived from aromatic rings by removing two substituents and leaving behind two electrons to be distributed between two orbitals. It highlights C6H4. This is based on benzene as the most common aryne, although aromatic and heteroaromatic systems can give similar species. The chapter also mentions that the need to postulate C6H4 started in the 1870s, which led to the formation of biphenyl in certain reactions. It was not until the 1940s that advanced clear and convincing arguments for the intermediacy of arynes were developed. The chapter analyzes ortho-Benzyne, which is usually represented as a singlet molecule with a carbon-carbon triple bond. It looks at theoretical calculations that support the view that the symmetrical triple bond structure is lowest in energy and suggests that the structure is distorted from benzene.

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 Foundations of Organic Chemistry

Reactions with electrophiles  

This chapter assesses organic reactions with electrophiles. Electrophiles have centres of low electron density, which will accept an electron pair to make a covalent bond. There are three types: positively charged cations, neutral molecules, and radicals. We could also include as electrophiles all the organic molecules which react with the nucleophilic reagents in the previous chapter, e.g. the polarized haloalkanes and carbonyl compounds. The chapter then looks at the addition of hydrogen halides to alkenes; the reactions of alkenes with sulfuric acid; the addition of halogens to alkenes; the cationic polymerization of alkenes; and the oxidation of alkenes. It also considers benzene and related compounds, studying the electrophilic substitution of benzene; the oxidation of benzene and related compounds; and the comparison between benzene and alkenes.

Chapter

Cover Chemical Bonding

Hybrid orbital description of bonding  

This chapter assesses the hybrid orbital description of bonding. The hybrid orbital view of bonding is a localized view that involves the construction of two-centre, two-electron bonds. Hybrid orbitals for an atom are linear combinations of its atomic orbitals combined in appropriate proportions so that they point towards the attached atoms or groups for that atom. An appropriate hybridization scheme for any particular atom within a molecule is linked with its geometry. The linear geometry is associated with sp hybridization, the trigonal geometry is associated with sp2 hybridization, and the tetrahedral geometry is associated with sp3 hybridization. The chapter then considers benzene, which is one of the most important molecules in organic chemistry.

Chapter

Cover Chemical Structure and Reactivity

Organic chemistry 3: reactions of π systems  

This chapter evaluates reactions in which C=C double bonds are formed and the reactions which such molecules undergo, including elimination reactions. Given that C=C double bonds are electron rich, their most characteristic reactions are with electrophiles. The chapter looks at typical examples of these reactions and the factors that control which carbon is attacked in an unsymmetrical double bond. It then turns to the enols and enolates, which are formed from aldehydes and ketones under acidic and basic conditions. The chapter also considers aromatic systems, exemplified by benzene. The special stability of the π system in a benzene ring means that the molecule tends to react in such a way that the aromatic ring is preserved. As a result, the reactions of benzene are quite different to those of simple π systems, being mainly electrophilic substitution rather than addition.

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.