Ring strain cycloalkanes, Table

Table 4.6 Heats of Combustion and ring Strain of Cycloalkanes...

In marked contrast to the n-alkanes, the cycloalkanes exhibit thermodynamic properties where such regularities are no longer present. Heats of formation (AH ) for a substantial number of cycloalkanes are available from heats of combustion. With the exception of cyclohexane, AH°f is always more positive than the quantity — 4.926n. The difference between the two quantities leads to a quantitative assessment of the important notion of ring strain. The AH -values and strain energy data listed in Table 1 were taken from Skinner and Pilcher (1963). Other references give different but usually comparable... 

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 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... 

Comparison of the heats of combustion of cycloalkanes (Table 9.1) shows that cyclopropane, cyclobutane, and cyclononane yield more energy per methylene group than the other cycloalkanes. This can be attributed to strain resulting from bond-angle distortion (Baeyer strain), eclipsed conformations (Pitzer strain), and trans-annular, repulsive van der Waals interactions. Common (five- and six-membered) rings and large (more than twelve-membered) rings have little or no strain. This... 

A screening of the vibrational spectra of 1,3,2-diazaboracycloalkanes has indicated a relationship between the ring size and the frequency of a ring pulsation mode 28>. It was noted that the frequency position of the ring pulsation appears to reflect the ring size (Table 4) but does not give an indication about ring strain. This observation correlates well with the situation noted for cycloalkanes. 

In Table 2, the ring strains, bond lengths, and bond angles for some 3-and 4-membered heterocydics are compared with these of corresponding cycloalkanes [63]. 

In Table 1 the ring strains of some heterocyd arrf their cycloalkane analogs are listed. The values of the rir strain were calculated as the difference between the calculated and measured enthalpies of formation of the correspondir cyclic compounds. 

Thus, as shown in Table 2.4 for unsubstituted cycloalkanes, ring strain is high for small rings (n = 3, 4), approaches zero at n = 6, increases again to a shallow maximum and decreases to a small value for large rings. The data for unsubstituted cycloalkanes are compared with recently determined values of AHp by Yamashita 35) for... 

Like protons, transition-metal ions are strongly acidic and they can, in principle, add to both the C—H and C—C bonds of alkanes. As already noted in the section on proton affinities (Table 1) strained cycloalkanes are intrinsically more basic than open-chain alkanes, and the reaction of cyclopropane with Pt((II) to form a platinacyclo-butane (equation 14) was the first reaction of a formally saturated hydrocarbon with a transition-metal ion . The driving force in this reaction is relief of the strain associated with the small ring. The resulting metallacyclobutane is essentially free of ring strain. Many low-valent transition-metal complexes have been found to react with cycloalkanes. Metal ions convet the strained hydrocarbons quadricyclane , cubane , bicyclo-[2.1.0]pentanes , bicyclo[3.1.0]hexanes , bicyclo[4.1.0]heptanes and bicyclo-butanes into less strained isomers (usually cyclohexanes). 

Enthalpy of polymerization is related mainly (but not exclusively) to the ring strain. As the bond lengths and the bond angles for C-O and C-C bonds are similar, the ring strains of cycloalkanes provide a good approximation for ring strains of cyclic ethers. Ring strains of cycloalkanes are listed in Table 1. 

TABLE 26.5 Heats of Combustion and Ring Strain in Some Cycloalkanes (All Values in kJ mol... 

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