Arenes and aromaticity

In this chapter and the next we extend our coverage of conju-5 

I gated systems to include arenes. Arenes are hydrocarbons based on the benzene ring as a structural unit. Benzene, toluene, and naphthalene, for example, are arenes. 

Let s begin by tracing the history of benzene, its origin, and its sttucture. Many of the terms we use, including aromaticity itself, are of historical origin. We ll begin with the discovery of benzene. 

In 1825, Michael Faraday isolated a new hydrocarbon from illuminating gas, which he called bicarburet of hydrogen. Nine years later Eilhardt Mitscherlich of the University of Berlin prepared the same substance by heating benzoic acid with lime and found it to be a hydrocarbon having the empirical formula C, H . 

Faraday is better known in chemistry for his laws of electrolysis and in physics for proposing the relationship between electric and magnetic fields and for demonstrating the principle of electromagnetic induction. 

Eventually, because of its relationship to benzoic acid, this hydrocarbon came to be named benzin, then later benzene, the name by which it is known today. 

Benzoic acid had been known for several hundred years by the time of Mitscher-lich s experiment. Many trees exnde resinous materials called balsams when cuts are made in their bark. Some of these balsams are very fragrant, which once made them highly prized articles of commerce, especially when the trees that produced them could be found only in exotic, faraway lands. Gum benzoin is a balsam obtained from a tree that grows in Java and Snmatra. Benzoin is a word derived from the French equivalent, benjoin, which in tnm comes from the Arabic luban jawi, meaning incense from Java. Benzoic acid is itself odorless but can easily be isolated from gum benzoin. 

Componnds related to benzene were obtained from similar plant extracts. For example, a pleasant-smelling resin known as tolu balsam was obtained from the South American tolu tree. In the 1840s it was discovered that distillation of tolu balsam gave a methyl derivative of benzene, which, not surprisingly, came to be named toluene. 

1 Toluene is C6H5CH3 it has a methyl group attached to a benzene ring. 

Benzoic acid has a —C02H substituent on the benzene ring. 

Kekule forms of benzoic acid Robinson symbol 

The value given in the text for the resonance energy of benzene (152 kJ/mol) is six times larger than this. 1,3,5-Cycloheptatriene is not aromatic. 

3 (b) The parent compound is styrene, C6H5CH=CH2. The desired compound has a chlorine in the 

4 The most stable resonance form is the one that has the greatest number of rings that correspond to Kekuld formulations of benzene. For chrysene, electrons are moved in pairs from the structure given to generate a more stable one  

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Carey Organic Cheimstry 11 Arenes and Aromaticity Text Fifth Edition... 

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