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Aromatisation of Six-Membered Rings

Aromatization of Six-Membered Rings: Catalytic Dehydrogenation

In the laboratory, benzene derivatives are prepared by dehydrogenation of either cyclohexane, cyclohexene or cyclohexadiene and their respective derivatives. The reactions are carried out at temperatures of approximately 300 °C with platinum or palladium catalysts that are either finely powdered or deposited on activated charcoal. The reaction is driven towards dehydrogenation by removal of hydrogen from the equilibrium. The reactions can also be carried out under milder conditions if a hydrogen acceptor, such as maleic acid, is present to remove hydrogen as it is formed. Likewise, higher temperatures shift the equilibrium towards dehydrogenation because a larger number of molecules causes an increase in entropy.

Presumably, the mechanism of catalytic dehydrogenation is a reversal of double bond hydrogenation.

Driven by the formation of an aromatic system, the "last" double bond in a six-membered ring is formed under milder conditions than the first two.

Fig.1
Formation of the aromatic system

The same principle applies to the preparation of condensed aromatic compounds, such as naphthalene.

Fig.2
Synthesis of naphthalene
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