Strain in cyclohexane

The experimentally measured H-C-H bond angles of cyclohexane are tetrahedral, or about 109.5°. There is virtually no Baeyer strain in the chair conformation of cyclohexane (48D). There is very little torsion strain in cyclohexane. The lack of torsion strain and Baeyer strain makes chair conformation (48B) the lowest energy conformation of cyclohexane. 

Studies of cyclodecane derivatives by X-ray crystallographic methods have demonstrated that the boat-chair-boat conformation is adopted in the solid state. (Notice that boat is used here in a different sense than for cyclohexane.) As was indicated in Table 3.7 (p. 146), cyclodecane is significantly more strained than cyclohexane. Examination of the boat-chair-boat conformation reveals that the source of most of this strain is the close van der Waals contacts between two sets of three hydrogens on either side of the molecule. 

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.

Boat conformation (cyclohexane), steric strain in, 118 Boc (fcJrf-butox carbonyl amide). 

The amount of strain in cycloalkanes is shown in Table 4.6, which lists heats of combustion per CH2 group. As can be seen, cycloalkanes larger than 13 membered are as strain-free as cyclohexane. 

Cyclopentane has a smaller amount of strain than does cyclohexane when forced to have all its carbons in a plane (flat). Cyclopentane is nearly flat since the bond angles in a regular pentagon (108°) are near the tetrahedral angle (109.5°). The bond angles in a flat cyclohexane are 120°, which are not near the 109.5° bond angle. To avoid this strain, the cyclohexane ring buckles and becomes nonplanar. 

We know that all cycloalkanes do not have the same relative stability. Cyclohexane is most stable while cyclopropane and cyclobutane are much less stable, because they have a ring strain in their molecules. 

As an illustration, the ring strain in cyclopropane, cyclobutane and cyclohexane and cyclodecane has been calculated ... 

The study of S -1 reaction in cyclohexane becomes difficult because of the ease with which the elimination reactions occur at the same time. However it has been shown that SN reaction will be sterically accelerated for an axial substituent, because the formation of carbocation will relieve the steric strain due to 1, 3 interactions which is absent with equatorial substituent. 

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