Enthalpy of cycloalkanes

Calculations of Conformations, Vibrational Spectra, and Enthalpies of Cycloalkane and H-AIkane Molecules. 

S. Lifson and A. Warshel, A Consistent Force Field for Calculation on Conformations, Vibrational Spectra and Enthalpies of Cycloalkanes and n-Alkane Molecules. J, Chem. Phys., 49 (1968) 5116. 

Figure 1.3. (a) Reactivity profile for lactone formation, (b) Enthalpy of cycloalkanes [CnH2n]. [Reprinted with permission from Illuminati, G. Mandolini, L. Acc. Chem. Res., 1981, 14, 95. Copyright 1981 American Chemical Society.]... 

Landis C R, T Cleveland and T K Firman 1995. Making Sense of the Shapes of Simple Metal Hydrides Journal of the American Chemical Society 117T859-1860 Landis C R, T K Firman, D M Root and T Cleveland 1998. A Valence Bond Perspective on the Molecular Shapes of Simple Metal Alkyls and Hydrides Journal of the American Chemical Society 120T842-1854 Lifson S and A Warshel 1968. Consistent Force Field for Calculations of Conformations, Vibrational Spectra and Enthalpies of Cycloalkane and n-Alkane Molecules Journal of Chemical Physics 49 5116-5129. 

Lifson S, Warshel A (1968) Consistent force field fpr calculations of conformations vibrational spectra and enthalpies of cycloalkane and n-alkane molecules. J Chem Phys 49 5116... 

Lifson S, Warshel A (1968) Consistent force field for calculations of conformations, vibrational Spectra and enthalpies of cycloalkane and n-alkane molectrles. J Chem Phys 49 5116-5129 Lubin MI, Bylaska EJ, Weare JH (2000) Ab initio molecttlar dynamics simulations of aluminum ion solvation in water clusters. Chem Phys Lett 322 447-453 Matsui M (1988) Molecular dynamics study of MgSiOs perovskite. Phys Chem Miner 16 234-238 Matsui M, Busing WR (1984) Computational modehng of the structrrre and elastic constants of the olivine and spinel forms of Mg2Si04. Phys Chem Miner 11 55-59 Matsrri M, Materrmoto T (1982) An interatomic potential-function model for Mg, Ca and CaMg olivines. 

Lifson, S., Warshel, A. (1968). Consistent force field for calculations of conformations, vibrational spectra, and enthalpies of cycloalkane and -alkane molecules. Journal of Chemical Physics, 49, 5116. 

RoCek et al. have also measured rate coefficients for a series of cyclo-alkanes, (CH2) (/i = 4 to 14), and find the analogue of kcHj in equation (25) to fluctuate with ring size in a manner corresponding exactly to the enthalpy of combustion of the cycloalkane concerned per methylene group, provided n is greater than five, i.e. there exists a direct correlation between reactivity and thermochemical strain. 

Standard enthalpies of formation and strain-energies of cycloalkanes 1... 

The enthalpy of fomation of two such species has been measured, namely the cyclopropane and cycloheptane derivatives. The difference between the values for these two species, both as solids, is 238.1 kJmol . Is this difference plausible Consider the difference between the enthalpies of formation of the parent cycloalkanes as solids, 194 kJ mol . The ca 44 kJ mol discrepancy between these two differences seems rather large. However, there are idiosyncracies associated with the enthalpies of formation of compounds with three-membered rings and almost nothing is known at all about the thermochemistry of compounds with seven-membered rings. Rather, we merely note that a seemingly well-defined synthesis of cycloheptyl methyl ketone was shown later to result in a mixture of methyl methylcyclohexyl ketones, and superelectrophilic carbonylation of cycloheptane resulted in the same products as methylcyclohexane, namely esters of 1-methylcyclohexanecarboxylic acid. The difference between the enthalpies of formation of the unsubstituted alicyclic hydrocarbons cycloheptane and methylcyclohexane as solids is 33 kJmol . This alternative structural assignment hereby corrects for most of the above 44 kJ mol discrepancy in the enthalpies of formation of the two oximes. More thermochemical measurements are needed, of oximes and cycloheptanes alike. 

The single most important factor that determines whether a cyclic monomer can be converted to linear polymer is the thermodynamic factor, that is, the relative stabilities of the cyclic monomer and linear polymer structure [Allcock, 1970 Sawada, 1976]. Table 7-1 shows the semiempirical enthalpy, entropy, and free-energy changes for the conversion of cycloalkanes to the corresponding linear polymer (polymethylene in all cases) [Dainton and Ivin, 1958 Finke et al. 1956]. The Ic (denoting liquid-crystalline) subscripts of AH, AS, and AG indicate that the values are those for the polymerization of liquid monomer to crystalline polymer. 

TABLE 13.2. Standard Enthalpy of Formation of Cycloalkanes (kcal/mol)... 

The enthalpies of formation of the cycloalkylmagnesium bromides that have been determined by reaction calorimetry are listed in Table 3. As with other functionalized cycloalkanes and the cycloalkanes themselves, there is no regularity to these values with respect to carbon number as there are for their acyclic analogs because of the influence of ring strain on the enthalpies. Unfortunately, there are no enthalpies of formation for the bromocycloalkanes with which to compare these values there are, however, enthalpies of formation for liquid phase cycloalkanes. Figure 3 is a plot of the enthalpies of formation for the cycloalkyl-MgBr vs. those for cycloalkyl-H. There is a linear relationship with... 

Indeed, the enthalpies of formation of the cycloalkylmagnesium bromides were calculated from the enthalpies of formation of the cycloalkanes themselves by way of the protonation reaction (equation 15). 

FIGURE 3. Enthalpies of formation of cycloalkylmagnesium bromides vs. those of cycloalkanes (kJ moU )... 

Not surprisingly, the enthalpy of reaction for cyclopropyhnagnesium bromide, —282.8 kJmol , is somewhat of an outlier, given the numerous anomalies associated with this small ring . For example, cyclopropane is the most olefinic and most acidic of the cycloalkanes—which correctly suggests that cyclopropyl forms the most polar C—Mg bond and, accordingly, is the thermodynamically most stable cycloalkylmagnesium species. 

In this section we consider the energetics of molecules with the generic formula cis- or (Z)-cyclo-[(CH2) -2 (CH=CH)] (1). A natural comparison of the enthalpies of formation of these olefins is with their saturated analogs, the cycloalkanes, with the generic formula, cyclo-[(CH2) -2 (CH2CH2)] [i.e. (CFL), 2], Using equation 13, let us now define the difference quantity < 13 (n) as the difference of the gas-phase enthalpies of formation of the cycloalkene of interest and the related cycloalkane ... 

We start with the unsubstituted cycloalkanones (38), compounds of the generic formula cyclo-[(CH2 ) -1 CO]. The gas-phase enthalpies of formation of all of these species from n = 3 to 12, 15 and 17 are available from experiment. An obvious comparison is with the corresponding n-membered ring cycloalkanes, cyclo-[(CH2)n]. Consider equation 38 in which the difference quantity 83% n) is defined as the difference of the gas-phase enthalpies of formation of the ketone of interest and the related cycloalkane. 

What can be said about the thermochemistry of methylated cyclic ketones in any phase other than liquid In that we would have discussed gas-phase species if the data were available, what we are really asking is what can be said about the species of interest as solids , Let us generalize this to see what can be said about the thermochemistry of cyclic ketones as solids. In that the cyclic ketones in the previous section were all formally quite strain-free derivatives of cyclopentanone or cyclohexanone, let us now consider only such species and their cycloalkane analogs. There are no enthalpy-of-fusion data available for cyclopentanone. For cyclohexanone, the temperature-uncorrected fusion enthalpy is about 1.3 kJmol-1—we arbitrarily ignore in this discussion enthalpies of any crystal - different crystal phase enthalpy. The same treatment for cyclohexane discloses the fusion enthalpy of 2.7 kJmol-1. Accordingly, the earlier reported value of 638(6) = 104.4 for the gas phase is sequentially modified to ca 115 kJ mol-1 for the liquid and to 113 kJmol-1 for the solid. For 2-norbomanone, adamantanone and diamantanone (39a, 42 and 43), the gas-phase differences of 115.7 6.8, 96.0 5.1 and 90.6 3.2 become for the solids 124.4 6.8, 116.9 4.5 and 97.8 3.1 kJmol-1. Very roughly, 639( ) is some 10 kJmol-1 more positive for solids than for gases and 639(8) is about 113 kJmol-1 for all of the solids discussed here. 

This statement is not quite correct—in Reference 14, perfluorocyclohexane was used as a reference species for the thermochemical understanding of perfluorocyclopropane and perfluorocyclobutane. Enigmatically there are no enthalpy-of-formation data on perfluorinated (or even partially fluorinated) derivatives of any other cycloalkane. 

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