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Cycloalkanes: Structural Variety

Cycloalkanes: Ring Strain

An inspection of the enthalpy of combustion of cycloalkanes sometimes shows a clear difference between calculated and observed values. This difference, observed particularly in smaller cycloalkanes, points to ring strain. In the homologous sequence of alkanes, the enthalpy of combustion increases steadily by an average of -659 kJmol-1 per additional CH2 group. Similarly, the enthalpy of combustion of cycloalkanes should exhibit the same values. However, a closer inspection of the observed data show that these are actually higher .

Tab.1
Ring strain
Ring size (Cn) ΔH° (combustion) calculated [ kJmol-1 ] ΔH° (combustion) observed [ kJmol-1 ]Total ring strain [ kJmol-1 ]Strain per CH2 group [ kJmol-1 ]
3-1977.1 -2092.7 115.6 38.5
4-2636.1 -2746.1 110.0 27.5
5-3295.2 -3322.4 27.2 5.4
6-3954.0 -3954.4 0.4 ~0
7-4613.2 -4640.0 26.8 3.8
8-5272.3 -5314.1 41.8 5.2
9-5931.3 -5985.3 54.0 6.1
10-6590.3 -6640.6 50.3 5.0
11-7249.4 -7295.4 46.0 4.2
12-7908.4 -7918.4 10.0 0.8
14-9226.5 -9226.5 0.0 0.0

For example, the difference for cyclobutane is 110.0 kJmol-1 which is caused by the existing ring strain of the molecule. Nearly strain-free are cyclohexane and larger rings with at least 13 carbon atoms. Based on these observations, cycloalkanes can be roughly divided into four groups:

  • Small rings (cycloproprane, cyclobutane)
  • Normal rings (cyclopentane, cyclohexane, cycloheptane)
  • Medium rings (cyclooctane to cyclodecane)
  • Large rings (cyclotridecane or larger)

Compared to cyclohexane, medium-sized rings show a higher ring strain. This is due to conformations which exhibit increased hydrogen-hydrogen repulsion.

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