|Chapter 3: Conformations of Alkanes and Cycloalkanes|
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
ΔHc / CH2 = -681 kJ/mol
ΔHc / CH2 = -658 kJ/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.|
|© Dr. Ian Hunt, Department of Chemistry|