Conformers of cyclohexanes

Figure 8-6. Comparison of the radial distribution function of the ctiair, boat, and twist conformations of cyclohexane (hydrogen atoms are not considered).

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

The various conformations of cyclohexane are m rapid equilibrium with one another but at any moment almost all of the molecules exist m the chair conformation Not more than one or two molecules per thousand are present m the skew boat confer matron Thus the discussion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation... 

Section 3 7 Three conformations of cyclohexane have approximately tetrahedral angles at carbon the chair the boat and the skew boat The chair is by far the most stable it is free of torsional strain but the boat and skew boat are not When a cyclohexane ring is present m a compound it almost always adopts a chair conformation... 

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... 

One property of NMR spectroscopy is that it is too slow a technique to see the mdi vidual conformations of cyclohexane What NMR sees is the average environment of the protons Because chair-chair mterconversion m cyclohexane converts each axial pro ton to an equatorial one and vice versa the average environments of all the protons are the same A single peak is observed that has a chemical shift midway between the true chemical shifts of the axial and the equatorial protons... 

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... 

Birch reduction (Section 11 11) Reduction of an aromatic nng to a 1 4 cyclohexadiene on treatment with a group I metal (Li Na K) and an alcohol in liquid ammonia Boat conformation (Section 3 7) An unstable conformation of cyclohexane depicted as... 

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

A modified boat conformation of cyclohexane, known as the twist boat (Fig. 1.8), or skew boat, has been suggested to minimize torsional and nonbounded interactions. This particular conformation is estimated to be about 1.5 kcal moE (6 kJ moE ) lower in energy than the boat form at room temperature. 

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. 

FIGURE 3.14 (a) A ball-and-spoke model and (h) a space-filling model of the boat conformation of cyclohexane. Torsional strain from eclipsed bonds and van der Waals strain involving the "flagpole" hydrogens (red) make the boat less stable than the chair. 

FIGURE 3.18 Energy diagram showing the interconversion of various conformations of cyclohexane. 

Boat conformation (Section 3.7) An unstable conformation of cyclohexane, depicted as... 

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