Electra
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Does anyone understand the mechanism of catalytic dehydrocyclisation of parafins?
Cannot find too many specific details on the catalytic reforming of various parrafins such as Hexane and Heptane to form Benzene and Toluene,
respectively. One site I saw discussing this transformation said it would skip the explanation since it is so widely known.
This process is supposed to occur at high temperatures over a hydrogen adsorbing catalyst such as platinum or palladium. Hydrogen is produced in the
process.
Is this sort of catalytic reforming simply that at high temperatures the catalyst abstracts hydrogen from the parrafin, forcing ring formation, until
it is in the most dehydrogenized state possible? Is it that simple? How is the hydrogen to be vented out with the formed hydrocarbon evaporating? The
hydrogen can't remain in the pressure/heated vessel for it could rehydrogenate it, sending toluene back to methylcyclohexane, for instance? Platinum
is known for hydrogenating aromatic rings, even turning benzaldehyde and benzoic acid into methylcyclohexane.
One question I have is why is the benzene ring most likely to form instead of a 5-carbon ring?
[Edited on 13-2-2014 by Electra]
[Edited on 13-2-2014 by ScienceSquirrel]
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ScienceSquirrel
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A six membered all carbon ring is preferred on steric and thermodynamic grounds over a five membered ring if you have a six membered or longer chain.
The mechanism is almost certainly radical and the final product is the most thermodynamically stable.
A related reaction is the formation of thiophene and hydrogen sulphide from butane and sulphur. Here hydrogen abstraction and sulphurisation yields a
five membered aromatic ring.
Look also at the preparation of adamantane where successive rearrangements give you a very stable molecule from a less stable molecule in 60% yield.
http://en.wikipedia.org/wiki/Adamantane#History_and_synthesi...
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Nicodem
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Thread Moved
13-2-2014 at 06:50 |
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