Chair conformations of cyclohexane

Staggered arrangement of bonds in chair conformation of cyclohexane... 

The C—H bonds in the chair conformation of cyclohexane are not all equivalent but are divided into two sets of six each called axial and equatorial... 

Axial bond (Section 3 8) A bond to a carbon in the chair conformation of cyclohexane oriented like the six up and down bonds in the following... 

FIGURE 1.6 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. 

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. 

Ring inversion (Section 3.9) Process by which a chair conformation of cyclohexane is converted to a mirror-image chair. All of the equatorial substituents become axial, and vice versa. Also called ring flipping, or chair-chair interconversion. 

Figure 4.7 The strain-free chair conformation of cyclohexane. All C-C-C bond angles are 111.5°, close to the ideal 109.5C tetrahedral angle, and all neighboring C-H bonds are staggered.

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. 

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. 

The chair conformation of cyclohexane has many consequences. We ll see in Section 1.1.9, for instance, that the chemical behavior of many substituted cyclohexanes is influenced by their conformation. In addition, we ll see in Section 2S.5 that simple carbohydrates such as glucose adopt a conformation based on the cyclohexane chair and that their chemistry is directly affected as a result. 

Ring-flip (Section 4.6) A molecular motion that converts one chair conformation of cyclohexane into another chair conformation. The effect of a ring-flip is to convert an axial substituent into an equatorial substituent. 

Figure 4.12 Representations of the chair conformation of cyclohexane (a) Carbon skeleton only (b) Carbon and hydrogen atoms (c) Line drawing (d) Space-filling model of cyclohexane. Notice that there are two types of hydrogen substituents—those that project obviously up or down (shown in red) and those that lie around the perimeter of the ring in more subtle up or down orientations (shown in black or gray). We shall discuss this further in Section 4.13.

The chair conformation of cyclohexane is the most stable one and that it is the predominant conformation of the molecules in a sample of cyclohexane. 

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. 

You can only appreciate stereochemical features if you can draw a representation that correctly pictures the molecule. One of the most challenging is the chair conformation of cyclohexane. Practice makes perfect so this is how it is done. 

When one looks at the hydrogens in the chair conformation of cyclohexane, one can see that they are of two types. Six of them are parallel to the central rotational axis of the molecule, so are termed axial. The other six are positioned around the outside of the molecule and are termed equatorial. One might imagine, therefore, that these two t)T>es of hydrogen would have some different characteristics, and be detectable by an appropriate spectral technique. Such a technique is NMR spectroscopy but, at room temperature, only one type of proton is detectable. At room temperature, all hydrogens of cyclohexane can be considered equivalent this is a consequence of conformational mobility, and the interconversion of two chair conformations. 

The chair conformation of cyclohexane is the most stable conformer. In the chair conformation, the C-C-C angles can reach the strain free tetrahedral value (109.5°), and all neighbouring C-H bonds are staggered. Therefore, this conformation does not have any angle strain or torsional strain. 

Chair conformation of cyclohexane Six axiai (a) and six equatoriai (e) hydrogen atoms... 

The extent of deformation from the chair conformation of cyclohexane has been investigated by the R- value method (67JA1836). The R- value is based on vicinal coupling constants and is defined by the expression ... 

SAMPLE SOLUTION (a) The most stable conformation is the one that has the larger substituent, the fe/t-butyl group, equatorial. Draw a chair conformation of cyclohexane, and place an equatorial tert-butyl group at one of its carbons. Add a methyl group at C-3 so that it is trans to the tert-butyl group. 

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