BENZENE AND ITS COMPOUNDS

TABLE OF CONTENTS

Title Page    –         –         –         –         –         –         –         –         –         i

Certification –         –         –         –         –         –         –         –         –         ii

Dedication   –         –         –         –         –         –         –         –         –         iii

Acknowledgments –         –         –         –         –         –         –         –         iv

Table of Contents  –         –         –         –         –         –         –         –         v

CHAPTER ONE

1.0     INTRODUCTION           –         –         –         –         –         –         1

1.2     General Properties of Aromatic Hydrocarbons        –         –         2

CHAPTER TWO

2.1     Aromatic Reactions         –         –         –         –         –         –         4

2.2     Aromatic Substitution      –         –         –         –         –         –         4

2.3     Coupling Reactions          –         –         –         –         –         –         5

2.4     Hydrogenation       –         –         –         –         –         –         –         5

2.5     Cycloadditions       –         –         –         –         –         –         –         6

2.6     Dearomatization     –         –         –         –         –         –         –         7

2.7     The Structure of Benzene –         –         –         –         –         –         7

2.8     Nomenclature of Benzene Derivatives –         –         –         –         8

2.9     Monosubstituted Benzenes        –         –         –         –         –         8

2.10   Disubstituted Benzenes    –         –         –         –         –         –         9

2.11   Polysubstituted Benzenes –         –         –         –         –         –         10

CHAPTER THREE

3.0     Interesting Aromatic Compounds        –         –         –         –         12

3.1     The Criteria for Aromaticity-Huckel’s Rule –         –         –         15

3.2     Examples of Aromatic Compounds     –         –         –         –         18

3.2.1  Aromatic Compounds with Single Ring        –         –         –         18

3.2.2  Aromatic Compounds With More Than One Ring   –         –         20

3.2.3  Aromatic Heterocycles    –         –         –         –         –         –         21

3.2.4  Pyridine       –         –         –         –         –         –         –         –         21

3.2.5  Pyrole          –         –         –         –         –         –         –         –         22

3.2.6  Histamine    –         –         –         –         –         –         –         –         24

3.2.7  Charged Aromatic Compounds –         –         –         –         –         24

3.2.8  Cyclopentadienyl Anion  –         –         –         –         –         –         24

3.3     Basis of Huckel’s Rule    –         –         –         –         –         –         25

3.3.1  Bonding and Antibonding Orbitals      –         –         –         –         26

CHAPTER FOUR

SUMMARY AND CONCLUSION

4.1     Summary     –         –         –         –         –         –         –         –         28

4.2     Conclusion  –         –         –         –         –         –         –         –         28

REFERENCES

 

CHAPTER ONE

1.0     INTRODUCTION        

Aromatic compounds, also known as “mono-and polycyclic aromatic hydrocarbons” (Jensen, 2009) are organic compounds containing one or more aromatic rings. The parent member is benzene. Heteroarenes are closely related, since at least one carbon atom of CH group is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.

benzene, C₆ H₆, is the least complex aromatic hydrocarbon, and it was the first one named as such. The nature of its bonding was first recognized by August kekule in the 9^th century. Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one to each of the two neighboring carbons. This leaves one electrons to share with one of the two neighboring carbon atoms, thus creating a double bond with one carbon and leaving a single bond with the other, which is why some representations of the benzene molecule portray it as a hexagon with alternating single and double bonds.

Other depictions of the structure portray the hexagon with a circle inside it, to indicate that the six electrons are floating around in delocalized molecular orbitals the size of the ring itself. This represents the equivalent nature of the six carbon-carbon bonds all of bond order (Lafrane et al., 2006). The equivalency is explained by resonance forms. The electrons are visualized as floating above and below the ring, with the electromagnetic fields the generate acting to keep the ring flat.

1.2     General Properties of aromatic hydrocarbons

1. They display aromaticity
2. The carbon-hydrogen ratio is high
3. They burn with a strong sooty yellow flame because of the high carbon-hydrogen ratio
4. They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions.

The circle symbol for aromaticity was introduced by Sir Robert Robinson and his student James Armit in 1925 (Jensen, 2009) and popularized starting in 1959 by the Morrison and Boyd textbook or organic chemistry. The proper use of the symbol is debated some publications use it to any cyclic π systems that obey tiṻcket’s rule. Jensen (Webb, 1995) argues that, in line with Robinson’s original proposal, the use of the circle symbol should be limited to monocyclic 6π-electron systems. In this way the circle symbol for a six-center six electron bond can be compared to the y symbol for a three-center two-electron bond.

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