Strain in Cycloalkane Rings

Many of the properties of cyclopropane and its derivatives are similar to the properties of alkenes. In 1890, the famous German organic chemist, A. Baeyer, suggested that cyclopropane and cyclobutane derivatives are different from 

Cycloalkane. (CH2) n Angle strain at each CH2, dega Heat of combustion,b A/-/0, kcal mole-1 Heat of combustion per CH2, AH°/n, kcal Total strain,0 kcal mole-1 

the result, now known as the Sachse-Mohr theory, was complete confirmation of the idea of nonplanar large rings. 

Because cyclopentane and cyclobutane (Sections 12-3E and 12-3F) also have nonplanar carbon rings, it is clear that the Baeyer postulate of planar rings is not correct. Nonetheless, the idea of angle strain in small rings is important. There is much evidence to show that such strain produces thermodynamic instability and usually, but not always, enhanced chemical reactivity. 

We expect that the total strain in cycloalkanes of the type (CH2) should decrease rapidly in the order = 2 ra = 3 = 4. However, the data of Table 12-3 show that the order actually is 3 = 4 2. This difference in order often is disguised by dividing the heats of combustion by the numbers of CH2 groups and showing that the heats of combustion per CH2 are at least in the order expected from bond-angle strain. This stratagem does not really solve the problem. 

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