Cycloalkanes torsional strain

Neighboring C—H bonds are eclipsed in any planar cycloalkane. Thus all planar conformations are destabilized by torsional strain. 

Section 3.13 Higher cycloalkanes have angles at carbon that are close to tetrahedral and are sufficiently flexible to adopt confonnations that reduce their torsional strain. They tend to be populated by several different confonna-tions of similar stability. 

In a similar way the potential constant method as described here allows the simultaneous vibrational analysis of systems which differ in other strain factors. Furthermore, conformations and enthalpies (and other properties see Section 6.5. for examples) may be calculated with the same force field. For instance, vibrational, conformational, and energetic properties of cyclopentane, cyclohexane and cyclodecane can be analysed simultaneously with a single common force field, despite the fact that these cycloalkanes involve different distributions of angle and torsional strain, and of nonbonded interactions 8, 17). This is not possible by means of conventional vibrational spectroscopic calculations. 

Ring strain the instability of cycloalkanes due to their cyclic structures. => angle strain and torsional strain. 

Torsional strain and van der Waals repulsions between hydrogen atoms across rings (transannular strain) cause the small instabilities of these higher cycloalkanes. 

Angle and torsional strain are major components of the total ring strain in fully reduced cyclic compounds. For cycloalkanes (see Table 1.2), the smaller the ring, the larger the overall strain becomes. What may appear at first to be surprising is that medium-sized rings containing 8-11 atoms... 

From heat of combustion data, cyclopropane has 26.7 kcal/mol (111.6 kJ/mol) of strain energy. Most of this strain is due to angle strain, but the contribution due to torsional strain is also significant. As we will see later, this strain energy causes cyclopropane to be more reactive than a normal alkane or cycloalkane. However, even though cyclopropane rings are reactive, they are fairly common in organic chemistry. 

Compare the energies of cycloalkanes, and explain how their angle strain and torsional strain combine to give the total ring strain. Problems 3-43,44, and 45... 

Besides torsional strain and steric strain, the conformations of cycloalkanes are also affected by angle strain. 

In addition to angle strain and torsional strain, steric strain is yet a third factor that contributes to the overall strain energy of cycloalkanes. As in gauche butane (Section 4.3), two nonbonded atoms in a molecule repel each other if they approach too closely and attempt to occupy the same... 

Cyclopropane is the only cycloalkane in which aU the ring carbons lie in the same plane. In all other cycloalkanes, the ring is nonplanar. A planar cycloalkane is destabilized by torsional strain and, in most cases, angle strain. 

The shape of cycloalkanes is determined by torsional strain, steric strain and angle strain. 

Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes also exist in nonplanar conformations. The small instabilities of these higher cycloalkanes appear to be caused primarily by torsional strain and repulsive dispersion forces between hydrogen atoms across rings, called transannular strain. The nonplanar conformations of these rings, however, are essentially free of angle strain. 

Angle strain and torsional strain are components of the total energy of a cycloalkane, which can be assessed by measuring heats of combustion per CH2 group. 

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