Cyclohexane shifting equilibrium

Itoh, N., Y. Shindo, K. Haraya and T. Hakuta. 1988. A membrane reactor using microporous glass for shifting equilibrium of cyclohexane dehydrogenation. J. Chem. Eng. Japan 21(4) 399-404. 

One of the earliest applications of membrane to shift equilibrium was developed by Wood(5) (1960). He showed that by imposing a nonequilibrium condition on a hydrogen-porous palladium silver alloy membrane, an otherwise stable cyclohexane vapor is rapidly dehydrogenated to cyclohexene. 

Fig. 35. The reaction paths for pure benzene and pure cyclohexane initial compositions for different hydrogen partial pressures. Experimental points obtained with pure benzene initial composition are shown. The shifting equilibrium points are shown by O. The experimental points for the different pressures are indicated as follows — A 10 atm 0 20 atm S 30 atm A 40 atm ID 50 atm.

TABLE 2. IR carbonyl wavenumber shifts" and equilibrium constants for the reactions of sec-BuIi with substituted phenyl sec-butyl ketones in cyclohexane at 25°C ... 

Booth and Little40 observed a chemical shift difference (Aae) of about 0.4 ppm for the C-2 and C-6 methylene protons in 4-methylpiperidine, which was assumed to exist as an equilibrium containing 95% of the C-methyl equatorial conformation. N-Methylation of 4-methylpiperidine caused an upfield shift of 0.22 ppm for the C-2 equatorial proton and of 0.68 ppm for the C-2 axial proton. Comparison of these values with those observed for the proton adjacent to methyl groups in cyclohexane led these authors to the... 

For the ABBB monomer, two diastereoisomeric chiral capsules were possible, with a diastereoisomeric ratio that varies from 6 1 in cyclohexane-di2 to 1 1 in chloroform-d, with a strong solvent effect the latter was also the guest and therefore influenced the stability of the capsules both from the inside and outside of the capsule. The ABAB monomer gave rise to only one chiral racemic capsule, while for AABB two regioisomeric structures were possible, with a relative amount from 1 1 to 2 1 increasing the steric difference between A and B from n-hexyl to adamantyl residues. Moreover, the equilibrium between the regioisomeric forms could be shifted entirely on one side if two adjacent A and two B groups are covalently connected. 

The benzene yields given by the data of Figures 4 and 5, 87% at 204°C and 88% at 227°C, may be compared with computed equilibrium yields of 13% and 19%, based on inlet conditions. This clearly shows the advantage of the continuous annular chromatographic reactor over, say, a tubular reactor. The comparison is not entirely straightforward, because dilution of the cyclohexane by He carrier as it disperses circumferentially shifts the equilibrium toward products this would have to be taken into account in any quantitative comparison. The data show only partial separation of benzene and cyclohexane. This partial separation must result in partial suppression of the back reaction, and must also contribute to the observed yield enhancement (in addition to the dilution effect). ... 

Time-dependent effects The NMR signals are sometimes influenced by time-dependent phenomena such as conformational or prototropic changes, which take place at a rate faster than the line width and comparable to (or faster than) the inverse of the differences between the frequencies of the transitions of the different sites. This means that kinetic phenomena may be studied by the NMR technique, especially if the temperature of the sample can be adapted. As an example we mention the two chair forms of cyclohexane, which are energetically stable there is a fast inversion from the one into the other via the boat form. At room temperature the PNMR spectrum exhibits one sharp peak, corresponding to the mean of the two chemical shifts. At -120 °C the dynamic equilibrium is frozen and the spectrum exhibits two sharp peaks, whereas at -60 °C the inversion is slow and the spectrum exhibits one broad peak. 

Nonbonded complexes. The equilibrium constants, enthalpies, and entropies for the weak complexation of pyrazine with dichloromethane, chloroform, or carbon tetrachloride have been determined from changes in the n — 77 absorptions of solutions at various concentrations (in cyclohexane) and temperatures 568 similar data for pyrazine-aromatic hydrocarbon complexes were obtained from variations in the H NMR chemical shift values.1037 The spectral effects of complexation with borane have been studied in the pyrazine diborane adduct and its methyl derivatives.254... 

Open-chain y -keto carboxylic esters with two mesityl substituents, such as methyl 3-hydroxy-2,3-dimesityl-2-propenoate, exist in solution only as the (Z)-isomer (4b) and the (ii)-isomer (4c) no keto form (4a) has been observed [223]. The (Z)-form predominates in nonpolar solvents such as cyclohexane (90 cmol/mol) and benzene (87 cmol/mol). Increasing solvent polarity shifts this Z)( E) equilibrium in favour of the more polar (7i)-isomer, up to 76 cmol/mol ( )-form in ethanol [223], The introduction of mesityl substituents stabiUzes enols of simple monocarbonyl compounds such as 2,2-dimesitylethenol, Mes2C=CH—OH [42, 224]. 

IR and UV/Vis [65a], mass spectrometric [65b], photoelectron [65c], microwave [65d], as well as low-temperature matrix-isolation IR spectroscopic measurements [65e] reveal that 2- and 4-hydroxypyridine (as well as 2- and 4-mercaptopyridine [65f]) exist in the gas phase and in inert matrices (N2, Ar) under equilibrium conditions mainly in the lactim (hydroxy or mercapto) form, in contrast to the situation in solution. While in nonpolar solvents such as cyclohexane and chloroform both tautomers exist in comparable amounts, the tautomeric equilibrium is shifted entirely in favour of the lactam (0x0 or thioxo) form in polar solvents such as water, as well as in the crystalUne state [66, 67, 141-145, 251-255], Supercritical-fluid 1,1-difluoroethane can be used to adjust the tautomeric constant Ki = [(llb)]l[(lla)] iso thermally over a continuum from gas-phase values to those measured in polar solvents, simply by increasing the pressure [254]. The gas-phase and solution equilibrium constants of 2- and 4-hydroxypyridine are given in Table 4-4. 

The CD spectrum of the thiolactam (R)-5-methylpyrrolidine-2-thione reveals a pronounced solvent dependence [490], Its long-wavelength n k thiocarbonyl CD band at Imax = 326 nm (in cyclohexane) undergoes a large solvent- and concentration-dependent blue shift of Av = +4880 cm (A1 = —54 nm) on going from cyclohexane to water. This corresponds to the blue shifts usually observed for n n bands in UV absorption spectra (see Section 6.2.3) and is best explained in terms of a monomer dimer equilibrium of the thiolactam involving the formation of intermolecular solute/ solute hydrogen bonds in nonpolar solvents. In HBA solvents e.g. DMSO) and HBD solvents [e.g. EtOH), this equilibrium is disturbed by competitive solute/solvent H-... 

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equilibrium Ethane to ethylene, propane to propylene, water-gas shift reaction CO + Fl20 CO2 + IT2, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

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