Cyclohexane Newman projection

Structural Considerations.—The crystal structure of the asymmetrically substituted cis, trans-2,5-di-t-butylcyclohexyl toluene-p-sulphonate (l)has been determined. The mean torsion angle of the cyclohexane ring in (1) is 55.1°, slightly less than the value of 55.9° for cyclohexane. Newman projections along the C-2—C-7 and C-5—C-11 bonds... 

For example, chemists are required to interpret diagrams of a molecule of cyclohexane, or of substituted cyclohexanes, when it is represented as a flat-looking hexagonal projection, or as a side-on view to show the chair form, or as a Newman projection (Fig. 1.8). 

Figure 4.13 (a) A Newman projection of the chair conformation of cyclohexane. 

TABLE 9. Conformational structures (a) and Newman projection diagrams (b) of (IS,25)- and (IR,2ft)-cyclohexane diol. Bis-p-dimethylaminobenzoate derivatives (c), bisignate CD Cotton effect data and torsion angles (d)14... 

The two main conformational shapes for cyclohexane are called the chair and the boat (Fig.M). The chair form is more stable than the boat form since the latter has eclipsed C-C and C-H bonds. This can be observed in the Newman projections (Fig.N) that have been drawn in such a way that we are looking along two bonds at the same time-bonds 2-3 and 6-5. In the chair conformation, there are no eclipsed C-C bonds. But, in the boat conformation, bond 1-2 is eclipse with bond 3-4, and bond 1-6 is eclipsed with bond 5-4. This indicates that the boat conformation is less stable than the chair conformation and the majority of... 

Fig.N. Newman projections of the chair and boat conformations of cyclohexane.

Table XI. Conformational structures (a and b) and Newman projection diagrams (c and d) of (15,25) and (lR,2R)-cyclohexane diol (a) and their bis-p-dimethylaminobenzoate derivatives (b). CD data for the bisignate Cotton effects of the latter are shown below.

Viewed from the side, the chair conformation of cyclohexane appears to have one methylene group puckered upward and another puckered downward. Viewed from the Newman projection, the chair has no eclipsing of the carbon-carbon bonds. The bond angles are 109.5°. 

Name an Alkane Using the lUPAC System 122 Name a Cycloalkane Using the lUPAC System 126 Draw a Newman Projection 132 Draw the Chair Form of Cyclohexane 140 Draw the Two Conformations for a Substituted Cyclohexane 143 Draw Two Conformations for a Disubstimted Cyclohexane 146 Stereochemistry... 

Carbons 2, 3, 5, and 6 in boat cyclohexane lie in a plane, with carbons 1 and 4 above the plane. The inside hydrogen atoms on carbons 1 and 4 approach each other closely enough to produce considerable steric strain, and the four eclipsed pairs of hydrogens on carbons 2, 3, 5, and 6 produce torsional strain. The Newman projection in Figure 4.23, obtained by sighting along the C2-C3 and C5-C6 bonds, shows this eclipsing clearly. 

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. 

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. 

The chair conformer of cyclohexane, a Newman projection of the chair conformer, and a ball-and-stick model showing that all the bonds are staggered. 

Newman projections of mono-substituted cyclohexane conformations. 

Aspects of cyclohexane conformational analysis. A. Interconverting chair forms of cyclohexane, with axial and equatorial locations labeled. In the left structure letters x are axial while letters y are equatorial. Note that the chair flip moves axial substituents to the equatorial position and vice versa. B. Newman projections down the C1-C2/ C5-C4 bonds of methylcyclohexane. In the axial form, there is a gauche butane interaction between the methyl and C3. C. Views of cyclohexane, equatorial methylcyclohexane, and axial methylcyclohexane. Note that chair cyclohexane is a relatively disk-shaped molecule, and an equatorial methyl does little to disrupt this shape. In contrast, an axial substituent puts a "kink" into the structure. Also evident is the steric interaction between one methyl hydrogen and the two axial hydrogens on C3 and C5. 

A double Newman projection of the chair form of cyclohexane note the perfect staggering of the C—H bonds... 

WORKED PROBLEM 5.8 Draw a Newman projection looking down the side H2C—CH2 bond of the full-boat cyclohexane. 

Figure 2.48. Newman projection of Cg on Ci in a cyclohexane ring showing axial-axial (aa), axial-equatorial (ae), and equatorial-equatorial (ee) interactions.

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