Hydrogenation of cyclohexane

When one looks at the hydrogens in the chair conformation of cyclohexane, one can see that they are of two types. Six of them are parallel to the central rotational axis of the molecule, so are termed axial. The other six are positioned around the outside of the molecule and are termed equatorial. One might imagine, therefore, that these two t)T>es of hydrogen would have some different characteristics, and be detectable by an appropriate spectral technique. Such a technique is NMR spectroscopy but, at room temperature, only one type of proton is detectable. At room temperature, all hydrogens of cyclohexane can be considered equivalent this is a consequence of conformational mobility, and the interconversion of two chair conformations. 

One way to investigate the stability of benzene is to compcire the cimount of heat produced by the reactions of benzene to similar compounds that are not aromatic. For example, a simple comparison of the heat of hydrogenation for a series of related compounds allows us to see the difference. Figure 6-6 shows the hydrogenation of cyclohexane, 1,3-cyclohexadiene, and benzene, which make a suitable set because all three yield cyclohexane. 

Besser et al. [53] have studied the hydrogenation of cyclohexane over a platinum catalyst. They used microstructured chips with channel widths of 100 pm and 5 pm and showed that the conversion in the smaller channels is larger, as expected, due to a larger surface-volume ratio. The conversion data were consistent with data from macro-scale reactors. 

Figure 3.52 Conversion and selectivity data for the hydrogenation of cyclohexane on a platinum catalyst in micro structured channels with widths of 100 and 5 pm [133]...

Hydrogenation of cyclohexane in an aqueous suspension of 30-fun palladium black particles 107... 

Figure 6-6 The hydrogenation of cyclohexane, 1,3-cyclo-hexadiene, and benzene.

In this case the activity decreases if benzene is hydrogenated at 229°, up to 23% cyclohexane is formed, and no cyclohexene is found. This probably occurs because the latter quickly hydrogenates to cyclohexane, which follows from the special experiments on hydrogenation of cyclohexane. 

E. Segal, R.J. Madon, and M. Boudart. Catalytic Hydrogenation of Cyclohexane. I. Vapor-Phase Reaction on Supported Platinum. J. Catal. 52 45 (1978). 

In the second hydrogenation step during the asymmetric hydrogenation of cyclohexane-1,2-dione over Pt-alumina-Cnd an enantiomeric excess of over 80% of (li ,2/ )-tra 5-cyclohexane-l,2-diol was obtained due to kinetic resolution (Sonderegger et al. ). 

Sonderegger, 0., Buergi, T., Baiker, A. (2003) Asymmetric hydrogenation of cyclohexane-1,2-dione over cinchonidine-modified platinum, J. Catal. 215, 116-121. 

Hydrogenation of cyclohexane Rha(PPh3>3 H-AfAlAl -t-ATBlBlb Osborn et al. (1966)... 

Hydrogenation of cyclohexane In an aqueous suspension of 30 pm palladium black particles... 

A very important application of zeolites includes in the hydration/dehy-dration reactions. A very interesting example is the Asahi process for the hydration of cyclohexene to cyclohexanol over a high silica (Si/Al>20), H-ZSM-5 type catalyst [44]. The process has been operated successfully on a 60,000 tons per year since 1990. However, this process has a problem of catalyst deactivation. The hydration of cyclohexanene is a key step in an alternative route to cyclohexanone (and phenol) from benzene (Figure 11.11). The conventional route involves hydrogenation of cyclohexane followed by auto-oxidation to a mixture of cyclohexanol and cyclohexanone and subsequent dehydrogenation of the former. A disadvantage of this process is only 75-80% selectivity at very low conversions (ca. 5%), thus one has to the recycle of enormous quantities of cyclohexane. 

Subsequendy, Khan and Rao (1990) studied the photocatalytic transfer hydrogenation of cyclohexane, in the absence of hydrogen gas flow, using water as the hydrogen source and ethanol as the electron donor. The authors reported that a very low yield of cyclohexane (7—8%) was obtained in aqueous ethanol solutions (80 20 ethanol-water) with K[Ru(H-EDTA)Cl] 2H2O and Pt/CdS/Ru02 as the semiconductor. 

Earlier, cationic Ir catalysts suffered from the hydrogenolysis of the phosphine-carbon bond occurring at reaction temperatures above 130 C [108,109]. In contrast, the iridium bis(phosphine) PCP-pincer complexes showed higher thermal stabilities [110, 111]. For example, iridium ( PCP)Ir(H2), Ir-2(H2), quantitatively catalyzed the transfer hydrogenation of cyclohexane into cyclohexene in the presence of tbe at 200 °C. Moreover, Ir-pincer complex Ir-2(H2) displayed TONs up to 1000 at 200 °C. 

FIGURE 15.59 At low temperature, ring flipping is slowed and the axial and equatorial hydrogens of cyclohexane can be resolved in the NMR spectrum. As the temperature is raised, the — 89 °C spectrum approaches the 25 °C spectrum. 

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