Cycloalkane methylene groups

Bischler-Napieralski ring closure with cycloalkane methylene groups... 

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... 

Table9.1. Heats of combustion (AHC, kcal mol-1) in the gas phase of n-membered cycloalkanes per methylene group, and enthalpies (AH, kcal mol-1) and entropies (AScy0, cal mol- K- ) forthe cyclization H(CH2) H —> cyclo-(CH2) + H2[42, 43],...

Know the meaning of saturated hydrocarbon, alkane, cycloalkane, homologous series, methylene group. 

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. 

Examination of the data for a great many compounds has shown that the heat of combustion of an aliphatic hydrocarbon agrees rather closely with that calculated by assuming a certain characteristic contribution from each structural unit. For open-chain alkanes each methylene group, —CH2—, contributes very close to 157.4 kcal/mole to the heat of combustion. Table 9.2 lists the heats of combustion that have been measured for some of the cycloalkanes. 

The principal mass spectral fragmentation path for the homologous tricyclic derivatives 454 n = 0-2) involves cleavage of the C—C bond between positions a and b (see structure), with subsequent loss of the methylene groups that formed the original cycloalkane ring. ... 

Between, 2850 and 3000 cm one finds the C-H stretching of alkanes. There must be methylene groups (-CH2-) present because the band for their characteristic C-H bending is found at 1400 to 1470 cm . When there are no alkyl substituents on the ring, the characteristic bending frequencies of methyl groups at 1380 cm are absent. Such is the case here. If there are no alkyl substituents on the cycloalkane CeHi2/ the only possible structure is... 

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