Return to Contents Chapter 3: Conformations of Alkanes and Cycloalkanes  Ch 3 contents


The most stable conformations of the first three cycloalkanes (the smallest three) are shown below where they can be contrasted with the planar conformation. In order to be able to compare the strain in each member of the cycloalkane series, the heat of combustion per methylene (i.e. -CH2-) is also given (see note below the table). The smaller this number is the less ring strain there is.
In each case, you should manipulate the 3D-JSMOL images to visualise the deviation from planarity and the effect this has on the eclipsing interactions of adjacent C-H bonds and C-C bonds (hint turn the model to "see" the Newman projections).

most stable structure  
ΔHc / CH2 = -697 kJ/mol
(-166.6 kcal/mol)
see left

ΔHc / CH2 = -681 kJ/mol
162.7 kcal/mol)
ΔHc / CH2 = -658 kJ/mol
157.3 kcal/mol)
Note: since the structures are not isomeric, we can't just compare the heats of combustion. However, simple cycloalkanes have have the generic molecular formula (CH2)n, where n = the number of C atoms in the ring. Therefore dividing the heat of combustion for each cycloalkane by n means we are able to make a reasonable comparison.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary