Cyclohexane equatorial hydrogen atoms

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

FIGURE 1.7 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. The middle and bottom panels show methylcyclohexane in the chair form with the methyl group equatorial (middle) and axial (bottom). 

Axial and equatorial hydrogen atoms in chair cyclohexane. The six axial hydrogens (red) are parallel to the ring axis, and the six equatorial hydrogens (blue) are in a band arourtd the ring equator. 

FIGURE 4.17 The chair conformation of cyclohexane. Axial hydrogen atoms are shown in red, equatorial hydrogens are shown in black. 

Figure 8.8 Axial and equatorial hydrogen atoms in chair cyclohexane.

In the figure, two of the H atoms are shown in red to emphasize that when the cyclohexane ring converts from one chair conformation to another, the equatorial hydrogen atoms are converted into axial hydrogen atoms and vice versa. The interconversion of the two chair forms proceeds through other conformations, including the boat form. 

Cyclodecane molecule is a typical representative of the medium ring. While in cyclohexane all carbon atoms are equivalent, in cyclodecane we have three types of carbon atoms and hydrogen atoms occupy six different positions unlike cyclohexane where we have only two positions, axial and equatorial. 

The stereochemistry of enolisation has been mostly examined in cyclic systems where the relative positions of the enolisable hydrogen atoms are fixed. Over the last decade, these studies have benefited from important improvements in the experimental methods, namely mass spectrometry and nmr spectroscopy. Of great interest is the comparison of the relative mobilities of diastereoisomeric axial and equatorial protons from ketones in the cyclohexane series. Indeed, since axial a(C—H) bonds of rigid cyclohexanones are closer than equatorial a(C—H) bonds to the desirable conformation in which the breaking C—H bond is perpendicular to the direction of the C=0 bond, it can be expected that the axial a(C—H) bond-breaking is easier than that of the equatorial one. 

Each carbon atom in cyclohexane is bonded to two hydrogen atoms, one directed upward and one downward. As the carbon atoms are numbered in Figure 3-22, Cl has an axial bond upward and an equatorial bond downward. C2 has an equatorial bond upward and an axial bond downward. The pattern alternates. The odd-numbered carbon atoms have axial bonds up and equatorial bonds down, like Cl. The even-numbered carbons have equatorial bonds up and axial bonds down, like C2. This pattern of alternating axial and equatorial bonds is helpful for predicting the conformations of substituted cyclohexanes, as we see in Sections 3-13 and 3-14. 

Figure 15. Representation of the interaction of cyclohexane (chair form) with Ru(0001) as suggested by Madey and Yates (31) in their classic study of hydrocarbon chemisorption on this basal plane of ruthenium. Three Ru—H—C three-center bonds are formed with three of the axial hydrogen atoms on one side of the chair form of cyclohexane. Weaker three-center Ru—H—C bonds may also be extant with equatorial C—H hydrogen atoms.

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