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The chemical behavior of acyclic and cyclic dienes does not differ significantly. Thus, 1,3-pentadiene and 1,3-cyclohexatriene are similar in their reaction behavior. The differences in the physical and chemical properties are small: Both groups of substances are easy to hydrogenate, a large number of addition reactions are known, and Diels-Alder reactions are also used.
This situation changes markedly when the cyclic compound - as with benzene - is "conjugated through". The logical name 1,3,5-cyclohexatriene suggests the chemical behavior of a triene, but benzene just does not behave like a triene. Benzene only adds bromine or hydrogen very slowly, even in the presence of catalysts. Diels-Alder reactions are not possible with benzene.
However, not all cyclic polyenes behave like benzene. The cyclic polyenes 1,3,5-cycloheptatriene and 1,3,5,7-cyclooctatetraene are, for example, substances that behave like normal polyenes. They undergo addition reactions and are easy to hydrogenate. The archetype of cyclic polyenes, cyclobutadiene, is very unstable and can only be isolated at very low temperatures. Cyclobutene also behaves like a polyene
- Tab. 1
- Cyclic, non-aromatic polyenes