Energy, cyclohexane conformations

Figure 3 Shapes and relative energies of cyclohexane conformations...

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Quantitatively, the anomeric effect is defined as the difference in conformational free energy for the process shown in Scheme 28 and the corresponding process in cyclohexane. Conformational energies for a range of substituents are available (82JA3635). 

Draw the conformational isomers of cis-1,2-dimethylcyclohexane and cis-3,4-dimetliylcyclohexanone. While the cyclohexane conformers are of equal energy, the cyclohexanone conformers are not. Indicate which con-former is favored and explain why. 

Disubstituted Cyclohexanes Conformational Energies of Substitutents Conformation and Chemical Reactivity Six Membered Heterocyclic Rings Cyclooctane and Cyclononane Cyclodecane... 

The boat conformation of cyclohexane (18) can be constructed from a molecular model of the chair form by holding the right-hand three carbons C(2), C(3) and C(4) of 15, clamped from the top with the hand and moving the left-hand three carbons upward. A Newman projection of the boat form looking along the C(l)-C(2) bond, and shown in 19, is reminiscent of the highest energy cis conformation of butane. 

The various conformations of cyclohexane are in rapid eqnilibrinm with one another, but at any moment almost all of the molecnles exist in the chair conformation. Not more than one or two molecnles per thonsand are present in the higher energy skew boat and boat conformations. Thns, the discnssion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation. 

Surprisingly, the lowest energy boat conformation is more stable than the lowest energy half-chair by more than 4 kcal/mol. This inversion of conformational preference is a direct consequence of the sp hybridization of the cyclohexane carbons where they join the polycyclic ring system, hold the prow and stem of the boat in close proximity, and raise the energy of the half-chair conformation. ... 

The lower-energy, chair conformation of cyclohexane has a sixfold axis of alternating S3mimetry, which divides the carbon-hydro-gen bonds into two types, shown in Mg.3. Six bonds are parallel to the axis the axial bonds, and six extend radially outward at angles of 109 5° to the axis the so-called equatorial bonds. This fact, that the carbon-hydrogen bonds divide into two types is important in considering energy differences between substituted cyclohexanes, referred to later. 

The experimental data in Table 3.1 show that the calculated values of total angle strain are approximately correct only for cyclopropane, cyclobutane, and cyclopentane. Cyclohexane is definitely not the strained compound Baeyer s theory predicts, and the larger ring compounds are also not very strained. Any chemist today can explain the discrepancy between these calculated and experimental values of strain energy cyclohexane is not planar. In either the chair or boat conformations (Figure 3.12), all bond angles can be approximately 109.5°. In the chair conformation of cyclohexane, all bonds are staggered, and there are no apparent van der Waals repulsions in the molecule. ... 

FIGURE 4.16 The relative energies of the various conformations of cyclohexane. The positions of maximum energy are conformations called half-chair conformations, in which the carbon atoms of one end of the ring have become coplanar. 

Figure 23 Structure of the model compound tranS l,2-bis-(N-metliylureido)-cyclohexane (a), and energy minimized conformation of trans-l,2-bis-(N-methylureido)-cyclohexane with a antiparallel orientation of the urea groups (b). The shaded areas in (b) are the contour levels for a interaction energy of-80 kJ/mol with a second molecule of l,2-bis ( N-methylureido)-cyclohexane.

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