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Benzene: Introduction to Aromatic Compounds

Reactions of Dienes

The chemical behavior of acyclic and cyclic dienes is not significantly different. 1,3-Pentadiene and 1,3-cyclohexatriene, for instance, are marked by similar reactions. The differences in their chemical and physical properties are rather small. They can easily be hydrogenated and they are typical starting products of Diels-Alder reactions. Furthermore, they participate in a number of addition reactions.

Movie: Electrophilic addition of bromine to cyclohexene

Fig.1
Reactions of acyclic dienes.
Fig.2
Reactions of cyclic dienes.

Physical and chemical properties change considerably if the cyclic oligoene is aromatic, such as is the case with benzene. According to "triene" nomenclature, benzene would be classified as "1,3,5-cyclohexatriene". However, in order to prevent any misunderstandings, this name is not used, as the chemical and physical properties of benzene are completely different from that of oligoenes. Bromine and hydrogen, for instance, can be added to benzene only very slowly even if catalysts are applied.

Movie: Catalytic bromination of benzene

Fig.3
Reactions of benzene.

However, cyclic oligoenes do not universally have benzene-like chemical and physical properties. The cyclic oligoenes 1,3,5-cycloheptatriene and 1,3,5,7-cyclooctatetraene, for example, are non-aromatic compounds whose reactions, such as additions and hydrogenations, are typical for oligoenes. Cyclobutadiene, the archetype of cyclic oligoenes, is antiaromatic and does not react like benzene, as well. Cyclobutadiene is very instable and can only be isolated at extremely low temperatures (< 20 K).

Tab.1
Cyclic, non-aromatic and antiaromatic polyenes
Cyclobutadiene1,3,5-Cycloheptatriene1,3,5,7-Cyclooctatetraene
Fig.4

Cyclobutadiene is an antiaromatic compound.

Fig.5

1,3,5-Cycloheptatriene is a non-aromatic compound.

Fig.6

1,3,5,7-Cyclooctatetraene is a non-aromatic compound.

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