Cyclohexenes isomerisation

Two structure insensitive reactions have been selected cyclohexene hydrogenation [6] oil surface Pt sites upon silica and but-l-ene isomerisation on acidic sites in bentonite. Both reactions were studied in differential reactors The former was investigated at 273-313K and lOlkPa Samples (5-lGmg) of catalyst were flushed with N2, pre-reduced in H at 423K for lh, flushed with N2 and then the reactant stream (lOlkPa total pressure cyclohexene N2 H2 = 1,7 89.5 10 1 200cm3.min 1 total flow rale) was introduced and analysed... 

Methanol reacts with limonene over acidic catalysts in a batch reactor to 1-methyl-4-[alpha-methoxy-isopropyl]-l -cyclohexene (alpha-terpinyl methyl ether) as the main reaction product (see Eq. 15.3.3 R- = CH3-). Besides the desired methoxylation, isomerisation reactions leading to terpinolene and traces of alpha-and gamma-terpinene can be observed. Furthermore, the addition of methanol to the methylterpinylether leads to the undesired cis- or trans-1,8-dimethoxy-p-menthane. The amount of unidentified products does not exceed 1%. At high temperatures and long reaction times the reverse reaction of the alpha-terpinyl methyl ether and the other addition products to limonene and its isomers can be observed. The reaction scheme of the alkoxylation oflimonene is illustrated in Equation 15.3.5. 

It is interesting to note that the destruction of the structure of beta zeolite by treatment with strong acids or high temperature leads to a complete deactivation of the catalyst for limonene alkoxylation. By using a higher reaction temperature only isomerisation and polymerisation products have been obtained. 1-methyl-4-[alpha-methoxy-isopropyl]-1 -cyclohexene or other addition products cannot be found. 

Following the generation of initial reactive surface species from hydrocarbon feedstocks the product distribution is largely governed by the balance of moncroolecular processes such as isomerisation and Beta scission and bimolecular processes (for example oligomerisation and bimolecular hydrogen transfer). A measure of the relative contribution of bimolecular versus monomolecular carbenium-ion like processes can be provided from the distribution of products from cyclohexene conversion... 

A clear example of the repulsive forces that exist between alkyl substituents in adsorbed cycloalkenes is provided by the observation " that 1,2-dimethylcyclopentene gives much more of the Z-product than does the corresponding cyclohexene the lack of flexibility in the C5 ring means that adjacent Z-substituents experience a strong repulsion, while in the more flexible Ce ring this is lessened. This effect, felt in the cycloalkyl intermediates, is sufficient to persuade the C5 reactant to give the stabler isomer, perhaps via isomerisation to the 2,3-isomer. 

Kowalak et al (42) used Pd (SALEM) complexes encapsulated in zeolites X and Y as room temperature hydrogenation catalysts. Residual protons, present in such catalysts and formed during the in situ syntesis (reaction VIII) cause always secondary isomerisation (42). Hex-1-ene and cyclohexene are hydrogenated on Pd(SALEN)X and Y zeolite. In a competitive hydrogenation experiment, however, the hydrogenation rate of cyclohexene is completely suppressed by the presence of hex-1-ene, which is partly hydrogenated, partly isomerized to t-hex-2-ene (42). This selective size-exclusion, apparently is not just the mere result of the presence of bulky SALEM complexes, as it is observed as well for the zeolite loaded with Pd (42). Once more, one deals with a zeolite effect which is not understood. 

Another use for polyphosphoric acid is for dehydration of cyclohexanol to cyclohexene (12.263), or for isomerisation of straight-chain to branched-chain hydrocarbons (12.264). 

Evers, J.T.M. and Mackor, A., Photocatalysis III. Photochemical isomerization of cyclohexenes and cycloheptene in the presence of copper(I) trifluoromethanesidfonate. Identification and acid-catalyzed isomerisation of the products, Tetrahedron Lett., 2321-2324,1978. 

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