Angle strain, conformational

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

A second but much less stable nonplanar conformation called the boat is shown in Eigure 3 14 Like the chair the boat conformation has bond angles that are approximately tetrahedral and is relatively free of angle strain It is however destabi hzed by the torsional strain associated with eclipsed bonds on four of its carbons The... 

Cyclobutane has less angle strain than cyclopropane (only 19.5°). It is also believed to have some bent-bond character associated with the carbon-carbon bonds. The molecule exists in a nonplanar conformation in order to minimize hydrogen-hydrogen eclipsing strain. 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planai geometry by adopting the nonplanai puckered conformation shown in Figure 3.11. 

Cyclohexane adopts a strain-free, three-dimensional shape, called a chair conformation because of its similarity to a lounge chair, with a back, a seat, and a footrest (Figure 4.7). Chair cyclohexane has neither angle strain nor torsional strain—all C—C-C bond angles are near 109°, and all neighboring C-H bonds are staggered. 

In addition to the chair conformation of cyclohexane, a second arrangement called the twist-boat conformation is also nearly free of angle strain. It does, however, have both sleric strain and torsional strain and is about 23 kj/mol (5.5 kcal/mol) higher in energy than the chair conformation. As a result, molecules adopt the twisl-boat geometry only under special circumstances. 

Boat cyclohexane (Section 4.5) A conformation of cyclohexane that bears a slight resemblance to a boat. Boat cyclohexane has no angle strain but has a large number of... 

Cyclobutanc, angle strain in. 115-116 conformation of. 115-116 molecular model of, 116 photochemical synthesis of, 1190 strain energy of, 114 torsional strain in, 115-116 Cyclodecane, strain energy of. 114 Cyclodecapentaene, molecular model of, 525, 540... 

Boat conformation of cyclohexane is free of angle strain. 

In Table 7 the six-membered monocyclic dienes are represented by the conjugated 1,3-cyclohexadiene and its isomer 1,4-cyclohexadiene. 1,3-Cyclohexadiene has a nonplanar equilibrium conformation that is primarily influenced by three factors -electron interaction (optimal for a planar conformation) angle strain and torsion strain (both optimal for a planar conformation). The reduced overlap between the two --orbital systems is, for the observed C=C—C=C angle of 18°, estimated at ca 10% and should therefore not influence the conjugation stabilization drastically, compared to a conformation with coplanar C=C bonds. 

The reason for the minimum energy conformer of 6 cannot be as simple as that proposed for 5, as the former is far more puckered than what is necessary for minimizing the H- - -H nonbonded repulsions. Valence angle strain is another factor that might be important in this case. The similarity to the chair conformer of cyclohexane is striking, although the calculated [6]radialene conformation is less puckered. 

When we study the case of cyclodecane, we find that x-ray crystallographic studies reveal that the most stable conformation has C—C bond angles of 117° which indicates some angle strain from the normal 109°28. So the wide bond angles allow the molecule to expand and minimise unfavourable repulsions between hydrogens across the ring. 

Generally six-membered rings are more stable than five-membered ones and on the basis of the tetrahedral theory Sachse in 1890 pointed out that cyclohexane could exist in two forms known as boat and chair conformations and neither of which is planar. They are actually puckered i.e., non-planar and are free of angle strain and so are called strainless rings. 

Such high EM s (> 1010 M) are scarcely to be expected for the formation of three-, or especially four-membered rings, because of the angle strain associated with the small ring of the product, and they are not found in any system where the open-chain form has significant conformational flexibility. An effective molarity of 1010 M or more may therefore be taken as prima facie evidence for strain in the ground state which is relieved in the cyclic product. 

Is there any ground-state destabilization of the substrate by bond-angle strain or binding in a high-energy conformation ... 

The strain in ring structures is of two types—angle strain and conformational strain. Ring structures of less than five atoms are highly strained due to the high degree of angle strain,... 

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