Molecular modeling cyclohexane chair

Figure 4-5 Conversion of the (A) hypothetical planar cyclohexane into the (B) chair conformation, showing bond lengths and angles (C) molecular model. The chair conformation is strain free.

Make molecular models of the two chair conformations of cis 1 tert butyl 4 phenyl cyclohexane What is the strain energy calculated for each conformation by molecular mechanics Which has a greater preference for the equatorial onentation phenyl or tert butyD... 

The most stable conformation of cyclohexane is the chair. Electron diffraction studies in the gas phase reveal a slight flattening of the chair compared with the geometry obtained when tetrahedral molecular models are used. The torsion angles are 55.9°, compared with 60° for the ideal chair conformation, and the axial C—H bonds are not perfectly parallel but are oriented outward by about 7°. The length of the C—C bonds is 1.528 A, the length of the C—H bonds is 1.119 A, and the C—C—C angles are 111.05°. ... 

Make a molecular model of the chair conformation of cyclohexane, and turn it so that you can look down one of the C—C bonds. 

The easiest way to visualize chair cyclohexane is to build a molecular model. (In fact do it now.) Two-dimensional drawings like that in Figure 4.7 are useful, but there s no substitute for holding, twisting, and turning a three-dimensional model in your own hands. The chair conformation of cyclohexane can be drawn in three steps. 

The same distortion, however, is not required for chelation of transA, -cyclohexanediamine (chxn) molecular models show that the two NH2 groups are now ideally situated for bidentate formation to a metal ion (Fig. 8). 7rart -l,2-cyclohexanediamine differs from the ethylenediamine system in that, when it is coordinated, only one chair conformation of the cyclohexane ring is possible. If the other conformer is assumed, the NH2... 

Cyclohexane achieves tetrahedral bond angles and staggered conformations by assuming a puckered conformation. The most stable conformation is the chair conformation shown in Figure 3-19. Build a molecular model of cyclohexane, and compare its shape... 

It is easy to see from inspection of molecular models that two distinct conformations of cyclohexane can be formed when the tetrahedral angle is maintained at each carbon atom. These are called the boat and chair conformations because of their resemblance to these objects (Fig. 7.7). The chair conformation has four carbon atoms in a plane with one above and one below that plane, located on opposite sides of the molecule. The boat conformation also has four carbon atoms in a plane, but both of the remaining atoms are located above this plane. Both conformations exist and appear to interconvert rapidly at room temperature through a sequence of rotations about single bonds (see Fig. 7.2). The chair conformation is significantly more stable than the boat, because the hydrogen atoms can become... 

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. 

Secondly, make a mode of the hindered alkene 43 this shows that it is sterically impossible for bromine to bond with the alkene carbons other than by a top or bottom approach. To construct a molecular model of 43, make two models of bicyclo[3.3.1]nonane, with both cyclohexane rings in a chair conformation. Identify carbons C(3) and C(7) in each of these structures then take a molecule of ethene and use C(l) of ethene to bond to C(3) and C(7) of one bicyclic structure, and C(2) of the alkene to bond to C(3) and C(7) of the other structure. The result is the hindered alkene 43, Examination of this model, together with an understanding of the orientation of the double bond, indicates that steric factors permit isolation of the bromonium ion. 

The cyclohexane ring has a unique property that is familiar to everyone who has handled molecular models. There is one form, the chair form (point group >3 ), that appears to be rigid, and there is a family of flexible forms that includes the... 

In 1890 Ulrich Sachse had pointed out that the ring strains referred to by Baeyer in his strain theory would be eliminated altogether if six-membered (and larger) rings adopted a puckered arrangement. Molecular models showed that two arrangements (usually termed boat and chair) were possible for the cyclohexane molecule (Figure 10.17). 

---