Cycloalkanes cyclohexane

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

FIGURE 11. Infra-red low-resolution textbook spectra of (a) a typical normal alkane (n-hexane) and (b) a cycloalkane (cyclohexane)... 

CoHMA Four-coordinated framework Co + on A1 sites UV-vis Presence of Br0nsted and Lewis sites no leaching of Co with calcination Oxidation of cycloalkanes (cyclohexane, cyclooctane, cyclododecane), mild conditions (296- 298)... 

Cyclopentane and cyclohexane are present m petroleum but as a rule unsubsti tuted cycloalkanes are rarely found m natural sources Compounds that contain rings of various types however are quite abundant... 

The name naphthenic acid is derived from the early discovery of monobasic carboxyUc acids in petroleum, with these acids being based on a saturated single-ring stmcture. The low molecular weight naphthenic acids contain alkylated cyclopentane carboxyUc acids, with smaller amounts of cyclohexane derivatives occurring. The carboxyl group is usually attached to a side chain rather than direcdy attached to the cycloalkane. The simplest naphthenic acid is cyclopentane acetic acid [1123-00-8] (1, n = 1). 

In addition to the ahphatic (chain) molecules, the saturates contain cycloalkanes, called naphthenes, having mainly five or six carbons in the ring (Fig. 5). Methyl derivatives of cyclopentane and cyclohexane ate commonly found in greater quantity than the parent unsubstituted stmctures and can be present at levels above 2% (2). Fused-ting dicycloalkanes such as decahydronaphthalenes (decalins) and hexahydroindans are also common, but nonfused bicyhc naphthenes, eg, cyclohexyl cyclohexane, are not. 

In this chapter we ll examine the confonnations of various alkanes and cycloalkanes, focusing most of our attention on three of them ethane, butane, and cyclohexane. A detailed study of even these three will take us a long way towar d understanding the main ideas of conformational analysis. 

Physical properties of cycloalkanes [49, p. 284 50, p. 31] show reasonably gradual changes, but unlike most homologous series, different members exhibit different degrees of chemical reactivity. For example, cyclohexane is the least reactive member in this family, whereas both cyclopropane and cyclobutane are more reactive than cyclopentane. Thus, hydrocarbons containing cyclopentane and cyclohexane rings are quite abundant in nature. 

We JI look in the next chapter at cycloalkanes—saturated cyclic hydrocarbons— and we ll see that the molecules generally adopt puckered, nonplanar conformations. Cyclohexane, for instance, has a puckered shape like a lounge chair rather than a flat shape. Why ... 

Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free.

The data in Figure 4.3 show that Baeyer s theory is only partially correct. Cyclopropane and cyclobutane are indeed strained, just as predicted, but cyclopentane is more strained than predicted, and cyclohexane is strain-free. Cycloalkanes of intermediate size have only modest strain, and rings of 14 carbons or more are strain-free. Why is Baeyer s theory wrong ... 

Substituted cyclohexanes are the most common cycloalkanes and occur widely in nature. A large number of compounds, including steroids and many pharmaceutical agents, have cyclohexane rings. The flavoring agent menthol, for instance, has three substituents on a six-membered ring. 

Petroleum contains hydrocarbons other than the open-chain alkanes considered to this point. These include cycloalkanes in which 3 to 30 CH2 groups are bonded into closed rings. The structures of the two most common hydrocarbons of this type are shown in Figure 22.5 (p. 585). Cyclopentane and cyclohexane, where the bond angles are close to the ideal tetrahedral angle of 109.5°, are stable liquids with boiling points of 49°C and 81°C, respectively. 

Cycloalkanes. Cyclopentane and cyclohexane, the two most common cycloalkanes. 

To name an alkane in which the carbon atoms form a single chain, we combine a prefix denoting the number of carbon atoms with the suffix -ane (Table 18.1). For example, CH,—CH, (more simply, CH,CH,) is ethane and CH,—CH2—CH, (that is, CH,CH2CH,) is propane. Cyclopropane, C,H6 (15), and cyclohexane, C6H12 (16), are cycloalkanes, alkanes that contain rings of carbon atoms. 

Curie temperature, 696 cyanide ion, 615 cycles, 4 cycloalkane, 731 cyclohexane, 731 cyclone, 144 cyclopropane, 542, 731 cysteine, 774 cytosine, 438, 777... 

The amount of strain in cycloalkanes is shown in Table 4.6, which lists heats of combustion per CH2 group. As can be seen, cycloalkanes larger than 13 membered are as strain-free as cyclohexane. 

It is possible to perform the conversion CH2 C=0 on an alkane, with no functional groups at all, though the most success has been achieved with substrates in which all CH2 groups are equivalent, such as unsubstituted cycloalkanes. One method uses H2O2 and bis(picolinato)iron(II). With this method, cyclohexane was converted with 72% efficiency to give 95% cyelohexanone and 5% cyclohexanol. ... 

O-M-O bond angles required. One can relate bite sizes (distances between the donor atoms) of strain-free chelate rings of different sizes to the dimensions of the chair form of the cyclohexane ring (39), which has the minimum steric strain possible for a cycloalkane. The bite size of strain-free four- and six-membered chelate rings both correspond to... 

A conformational effect was detected for the H-transfer reactions from cycloalkanes to a series of attacking radicals. The data of Table 6 show that cyclopentane is generally a better H-donor than cyclohexane. The rate ratio is generally largest for the least reactive radicals because the change in hybridization at transition state... 

In the linear versus cyclic case, n-hexane oxidizes 18.9 times as fast as cyclohexane (see Fig. 6-6) however, under slightly different conditions (same temperature and pressure, acetone solvent) and a slightly different preparation of TS-1, n-hexane oxidizes only 4.8 times as fast as cyclohexane.45 These differences in TOFs between the linear and cyclic isomers are also attributed to the size restrictions of the zeolite. When the channel diameter is increased, as in the Ti-(1 catalyst (-6.5 A), larger cycloalkanes, such as cyclododecane, can be oxidized.45... 

In a similar way the potential constant method as described here allows the simultaneous vibrational analysis of systems which differ in other strain factors. Furthermore, conformations and enthalpies (and other properties see Section 6.5. for examples) may be calculated with the same force field. For instance, vibrational, conformational, and energetic properties of cyclopentane, cyclohexane and cyclodecane can be analysed simultaneously with a single common force field, despite the fact that these cycloalkanes involve different distributions of angle and torsional strain, and of nonbonded interactions 8, 17). This is not possible by means of conventional vibrational spectroscopic calculations. 

---