Cyclohexane axial hydrogens

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

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

Figure 4.8 Axial (red) and equatorial (blue) positions in chair cyclohexane. The six axial hydrogens are parallel to the ring axis, and the six equatorial hydrogens are in a band around the ring equator.

E2 Elimination Where There Are Two Axial Cyclohexane P-Hydrogens... 

E2 Elimination Where The Only Eligible Axial Cyclohexane P-Hydrogen is From a Less Stable Conformer... 

The case becomes interesting when one studies mono substituted cyclohexanes. For example, methyl cyclohexane exists in two forms, one in which the methyl group is axial and the other in which it is equatorial. The latter is more stable than the former by 1.6 K cals/mole and in an equilibrium mixture, it is present to the extent of 98%. This is because that when the CH3 group is axial, it is so close to the two axial hydrogens on the same side of the molecule, that the van der Waals, forces between them are repulsive ... 

Valuable information on conformational equilibrium can be obtained particularly by N.M.R. technique. When a molecule can exist in several conformations which rapidly interchange, then any proton which assumes all possible positions in a very short time, the n.m.r. spectrum would show only one peak. This happens in most open chain compounds and even in cyclohexanes where the interconversion is very rapid. But if the interconversion is slowed or prevented, either by cooling or due to the inherent structure in the molecule, the hydrogens of each conformer appear separately and so more than one peak would appear. For example by cooling cyclohexane to -110°C, two peaks appear, one due to equatorial and the other to the axial hydrogens. 

Figure 12-5 Chair form of cyclohexane showing equatorial and axial hydrogens. Top, space-filling model center, ball-and-stick model bottom, sawhorse representation. Notice that all the axial positions are equivalent and all the equatorial positions are equivalent. By this we mean that a substituent on any one of the six axial positions gives the same axial conformation, whereas a substituent on any one of the six equatorial positions gives the same equatorial conformation.

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). 

In the space provided on the Report Sheet (2e), draw the structure of cyclohexane in the chair conformation with all 12 hydrogens attached. Label all the axial hydrogens, Ha, and all the equatorial hydrogens, He. How many hydrogens are labeled Ha (2f) How many hydrogens are labeled He (2g) ... 

In the chair conformation cyclohexane has two different types of hydrogens. The bonds to one type are parallel to the axis of the ring. These are called axial hydrogens. The axial bonds alternate up and down around the ring. 

What happens when there is a substituent on the cyclohexane ring Let s consider methylcyclohexane as a simple example. As before, there are two chair conformations, which interconvert by the ring-flipping process. In this case, however, the two conformations are not identical. As shown in Figure 6.16, the methyl group is equatorial in one conformation and axial in the other. The conformation with the axial methyl is less stable than the conformation with the equatorial methyl by 1.7 kcal/mol (7.1 kJ/rnol) because of steric interactions between the methyl and the axial hydrogens on C-3 and C-5. (These are often called 1,3-diaxiaI interactions.)... 

---