Cyclopropane Baeyer strain energies

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... 

The experimental data in Table 3.1 show that the calculated values of total angle strain are approximately correct only for cyclopropane, cyclobutane, and cyclopentane. Cyclohexane is definitely not the strained compound Baeyer s theory predicts, and the larger ring compounds are also not very strained. Any chemist today can explain the discrepancy between these calculated and experimental values of strain energy cyclohexane is not planar. In either the chair or boat conformations (Figure 3.12), all bond angles can be approximately 109.5°. In the chair conformation of cyclohexane, all bonds are staggered, and there are no apparent van der Waals repulsions in the molecule. ... 

Experimental data on strain energy in cycloalkanes show that Baeyer s theory is only partially correct (Figure 4.3). Cyclopropane and cyclobutane are indeed strained, just as predicted, but cyclopentane is more strained than predicted and cyclohexane is strain-free. Cycloalkanes of intermediate size have only modest strain, and rings of more than 14 carbons are strain-free. Why is Baeyer s theory wrong ... 

Perhaps it is appropriate that this review be written some 100 years after the report of the first syntheses of cyclopropane derivatives by von Baeyer and Perkin and the formulation of the theory of ring strain by von Baeyer. The chemistry of small ring compounds has risen to prominence in the last 30 years. The popularity especially of the three-membered rings as intermediates in synthetic transformations has been due primarily to their latent energy content and to the almost endless number of chemical transformations in which these compounds and their derivatives can participate. New applications and novel permutations of the basic systems continue to appear at a fast pace. The fascinating chemistry... 

---