Cycloalkanes strain

Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free.

Fig. 25 Idealised EM-profiles calculated extrathermodynamically from (67) for reactions series whose transition states or products have a constant fraction a of the cycloalkane strain energies (see also Fig. 26)...

Fig. 28 Nucleophilic ring opening of cyclic dimethylammonium ions [38], Free energies of activation relative to ring-size 6 against cycloalkane strain-energies. Energy units kcal mol"1. (Reproduced with permission from Di Vona et al., 1985)...

The simplest way to determine cycloalkane strain energies is to measure their heats of combustion, the amount of heat released when a compound burns completely with oxygen. The more energy (strain) a compound contains, the more energy (heat) is released on combustion. 

Although it s sometimes possible to find relative stabilities of alkene isomers by establishing a cis-trans equilibrium through treatment with strong acid, there are easier ways to gain the same information. One way is simply to measure the heats of combustion for the two isomers, as we did in determining cycloalkane strain energies (Section 4.5). cis-2-Butene is found to be more strained than [Pg.202]

FIGURE 4.3 Cycloalkane strain energies, calculated from thermodynamic heats of formation. Small and medium rings are strained, but cyclohexane rings are strain-free. 

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