Torsional strain cyclobutane

Cyclopropane is planar and destabilized by angle strain and torsional strain Cyclobutane is nonplanar and less strained than cyclopropane... 

Cyclobutane has less angle strain than cyclopropane. However, it has more torsional strain, because there are four sets of eclipsing H s rather than just three. To alleviate some of this additional torsional strain, cyclobutane can adopt a sfightly puckered conformation without gaining too much angle strain ... 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planar geometry by adopting the nonplanar puckered confer matron shown m Figure 3 11... 

Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. As a result, the total strain for the two compounds is nearly the same—110 kj/mol (26.4 kcal/mol) for cyclobutane versus 115 kj/mol (27.5 kcal/mol) for cyclopropane. Experiments show that cvclobutane is not quite flat but is slightly bent so that one carbon atom lies about 25° above the plane of the other three (Figure 4.5). The effect of... 

Cyclopropane (115 kj/mol strain) and cyclobutane (110.4 kj/mol strain) have both angle strain and torsional strain. Cyclopentane is free of angle strain but has a substantial torsional strain due to its large number of eclipsing interactions. Both cyclobutane and cyclopentane pucker slightly away from planarity to relieve torsional strain. 

The Origin of Ring Strain in Cyclopropane and Cyclobutane Angle Strain and torsional Strain... 

The cyclobutane ring is not plannar but is slightly folded => considerably larger torsional strain can be relieved by sacrificing a little bit of angle strain. 

Cyclobutane, in fact, is not a planar molecule. To reduce torsional strain, this compound attains the above nonplanar folded conformation. Hydrogen atoms are not eclipsed in this conformation and torsional strain is much less than in the planar structure. However, in this form angles are less than 90°, which means a slight increase in angle strain. 

Evidence of maoy kinds strongly indicates that cyclobutane is not planar, but rapidly changes between equivalent, slightly folded conformations (Fig. 9.7). Here, too, torsional strain is partially relieved at the cost of a little angle strain. 

Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. As a result, the total... 

Spectroscopic measurements indicate that cyclobutane is slightly bent (puckered), so that one carbon atom lies about 25° above the plane of the other three. This slightly increases the angle strain (-88° internal bond angle) but decreases the torsional strain (< 8 kcal/mol) until a minimum energy balance between the two opposing effects is reached. Total strain is -26 kcal/mol. 

Cyclopentane is appreciably less strained than cyclobutane and cyclopropane, and the strain energy relative to cyclohexane is ca. 6.45 kJ mol-1 per CH2 group. In order to lessen the torsion strain that would occur in a planar conformation, in which every C-H bond is involved in two eclipsing interactions, cyclopentane adopts a puckered conformation (see Dunitz, Further Reading). This has four carbons approximately planar, with the fifth carbon bent out of this plane in such a way that the molecule resembles a small near-square envelope 9. A Newman projection of 9 is shown in 10. 

Cyclopropane has to be planar and therefore has very strained bond angles of 60° and a great deal of torsional energy. However, cyclobutane and cyclopentane can adopt non-planar (puckered) shapes which decrease the torsional strain by staggering the C-H bonds. However, this is at the expense of angle strain, and the butterfly and open-envelope shapes represent the best compromise between the two opposing effects. 

---