Larger cycloalkanes

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. 

The isomerization of open-chain alkanes with more than six carbon atoms gives isobutane as the main product, together with disproportionated materials, even though the reaction proceeds by the monomolecular pathway [144]. On the other hand, for cyclic alkanes the monomolecular process with preservation of the cyclic structure seems to be the most probable, judging from the results for cyclohexane. The absence of isobutane in the products indicates that the reaction path does not involve open-chain intermediate species. Therefore, it is of interest to try cycloalkanes larger than cyclohexane for clarification of the reaction mechanism along with the catalytic action of S04/Zr02. 

In all cycloalkanes larger than cyclopropane, nonplanar conformations are favored. 

There are no metisured enthalpies of formation for cycloalkanes larger than cycloheptadecane. Cycloalkanes between Ce and C15 exhibit some degree of angle and/or transannular strain. 

Bond-angle, eclipsing, and other strain in the cycloalkanes larger than cyclopropane (which is by necessity flat) can be accommodated by deviations from planarity. 

Larger cycloalkanes have increasing rotational freedom, and the very large rings (C25 and up) are so floppy that they are nearly indistinguishable from open-chain alkanes. The common ring sizes (C3-C7L however, are severely restricted in their molecular motions. 

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

Thiourea clathrates may also be applied for geochemical separations. They are similar to the molecnlar complexes of nrea bnt owing to the larger dimensions of the thiourea crystal channels, only branched alkanes, cycloalkanes, and their derivatives participate in clathrate formation. In contrast, the unbranched molecules are too small and hence cannot be held rigidly [77]. 

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... 

Angle and torsional strain are major components of the total ring strain in fully reduced cyclic compounds. For cycloalkanes (see Table 1.2), the smaller the ring, the larger the overall strain becomes. What may appear at first to be surprising is that medium-sized rings containing 8-11 atoms... 

The spectrum of cyclopropane is rather similar in that it has only Rydberg transitions. The spectrum of cyclobutane is also dominated by Rydberg transitions, and its spectrum resembles that of the larger cycloalkanes, and is quite different to that for cyclopropane.104 Cyclopropene and cyclobutene have also been examined, and here a n -> n transition is seen along with the Rydberg transitions.101... 

To examine the possibilities for knotted isomers in larger rings we note that two 5-fold knots (Figure 2) require 83 and 92 methylenes the three 6-fold -100, -102, and -105. Thus a Cioo cycloalkane might exist as one of -5 isomeric knots. 

This sparse set of choices becomes richer with larger rings. With more crossings, n, die number of possible knots increases exponentially [21]. There are 21 for n = 8, 165 for n= 10, and 2176 for n=12 [22]. Models indicate that many 12-fold knots should be stable in a cycloalkane of C20o- The rapid increase in the number of possible knots between Ci0o ( 5) and C2oo (-3000) results from the dependence of allowed values of n on ring size. 

In Table 27, CH dissociation enthalpies and differences of DH or DE values are compared89, l65,76a. They clearly confirm that CH dissociation for 1 requires a significantly larger energy (up to 11 kcalmol"1) than for other cycloalkanes or propane. It isjust4kcalmor smaller than the CH dissociation enthalpy for ethene or benzene, as can be seen from the... 

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