Dimerisation isobutene

Above we have mentioned several heterogeneous applications such as the OCT process and SHOP. Neohexene (3,3-dimethyl-1-butene), an important intermediate in the synthesis of fine chemicals, is produced from the dimer of isobutene, which consists of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl- 1-pentene. Cross-metathesis of the former with ethene yields the desired product. The catalyst is a mixture of W03/Si02 for metathesis and MgO for isomerisation at 370 °C and 30 bar. The isobutene is recycled to the isobutene dimerisation unit [48],... 

In cyclohexane the reaction was also first order in olefin and second order in acid, but the predominantly dimeric state of the acid in this solvent suggested that the one molecule of the aggregate was responsible for the attack on the olefin. Of course the transition state might again involve the concerted participation of two acid molecules arising from the opening of the dimer. In the second study, Latremouille and Eastham also obtained third order kinetics for the reaction of trifluoroacetic acid with isobutene in 1,2-dichloroethane. Given the low value of the dimerisation constant in this solvent it seems obvious that here too the Ad 3 mechanism should hold, a point which was in fact implicitly touched on by the authors in the discussion of their results. 

Recent work on the dimerisation of 1,1-diphenylethylene by aluminium chloride produced conclusive evidence that direct initiation does not lead to the total ctmsump-tion of the catalyst. This excellent piece of research diowed that about 2.5 aluminium atoms are needed to give rise to one carbenium ion. Similar indications were reported by Kennedy and Squires for the low temperature polymerisation of isobutene by aluminium chloride. They underlined the peculiar feature of limited yields obtained in flash polymerisations with small amounts of catalyst. The low conversions could be increased by further or continuous additions of the Lewis acid. Equal catalyst increments produced equal yield increments It was also shown that introductions of small amounts of moisture or hydrogen chloride in the quiescent system did not reactivate the polymerisation. This work was carried out in pentane and different purification procedures for this solvent resulted in the same proportionality between polymer yield and catalyst concentration. Experiments were also performed in which other monomers (styrene, a-methylstyrene, cyclopentadiene) were added to the quiescent isobutene mixture. The polymerisation of these olefins was initiated but limited yields were again obtained. Althou the full implications of these observations must await more precise data, we agree with the authors interpretation that allylic cations formed in the isobutene polymerisation, while incapable of activating that monomer, are initiators for the polymerisation of the more basic monomers added to the quiescent mixture. The low temperature polymerisation of isobutene by aluminium chloride was also studied... 

Myrcene is also accessible from isoprene, which derives from the steamcracker s C5-cracking fraction by dehydrogenation. In contrast, construction of its carhon skeleton, for example from acetylene and acetone or hy metathesis of isobutene and hut-2-ene, are declining in importance. [125] The sodium-catalysed dimerisation in the presence of diisopropylamine leads to myrcene. 

Fritsch, D., Randjelovic, 1., Keil, F. (2004). Application of a forced-flow catalytic membrane reactor for the dimerisation of isobutene. Catalysis Today, 98, 295—308. 

Ouni, T., Honkela, M., Kolah, A., Aittamaa, J. (2006). Isobutene dimerisation in a miniplant-scale reactor. Chemical Engineering Process Journal, 45, 329—339. 

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