Strain per methylene

The molar heat of combustion of cyclohexane is nearly twice that of cyclopropane, simply because cyclohexane contains twice as many methylene (CH2) groups per mole. To compare the relative stabilities of cycloalkanes, we divide the heat of combustion by the number of methylene (CH2) groups. The result is the energy per CH2 group. These normalized energies allow us to compare the relative amounts of ring strain (per methylene group) in the cycloalkanes. 

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. 

TABLE 2-7 Heats of Combustion and Strains of Cycloalkanes per Methylene Group"... 

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... 

Since straining the ring involves an increase in the heat content, the heat of combustion per methylene group should increase as the straiu... 

PROBLEM 5.5 How would one get the value of for a CH2 group in an infinite chain, (CHj) Use the following data to estimate the value of A/ of a strain-free methylene group in kilocalories per mole hexane = —39.9,... 

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