Organic Chemistry

Section 2.12 Conformations of Cyclohexane 97

H H

H H H H

H H

H

H

H

H

CH 2

CH 2

HH

H

H H

H

HH

flagpole hydrogens

ball-and-stick model of the boat

conformer of cyclohexane

Newman projection of

the boat conformer

boat conformer of

cyclohexane

Figure 2.9

The boat conformer of cyclohexane, a Newman projection of the boat conformer, and a

ball-and-stick model showing that some of the bonds are eclipsed.

5.3 kcal/m

22 kJ/m

6.8 kcal/m

28 kJ/m

12.1 kcal/m

50.6 kJ/m

half-chair

energy

chair chair

twist-

boat

twist-

boat

boat

half-chair

>Figure 2.10

The conformers of cyclohexane—

and their relative energies—as one

chair conformer interconverts to

the other chair conformer.

Cyclohexane can also exist in a boat conformation, shown in Figure 2.9. Like the

chair conformer, the boat conformer is free of angle strain. However, the boat con-

former is not as stable as the chair conformer because some of the bonds in the boat

conformer are eclipsed, giving it torsional strain. The boat conformer is further desta-

bilized by the close proximity of the flagpole hydrogens(the hydrogens at the “bow”

and “stern”of the boat), which causes steric strain.

The conformations that cyclohexane can assume when interconverting from one

chair conformer to the other are shown in Figure 2.10. To convert from the boat con-

former to one of the chair conformers, one of the topmost carbons of the boat

conformer must be pulled down so that it becomes the bottommost carbon. When

the carbon is pulled down just a little, the twist-boat(or skew-boat) conformeris

obtained. The twist-boat conformer is more stable than the boat conformer because

there is less eclipsing and, consequently, less torsional strain and the flagpole hydro-

gens have moved away from each other, thus relieving some of the steric strain.

When the carbon is pulled down to the point where it is in the same plane as the

sides of the boat, the very unstable half-chair conformeris obtained. Pulling the

carbon down farther produces the chair conformer.The graph in Figure 2.10 shows

the energy of a cyclohexane molecule as it interconverts from one chair conformer

to the other; the energy barrier for interconversion is 12.1 kcal mol (50.6 kJ mol).

From this value, it can be calculated that cyclohexane undergoes ring flips

per second at room temperature. In other words, the two chair conformers are in

rapid equilibrium.

Because the chair conformers are the most stable of the conformers, at any instant

more molecules of cyclohexane are in chair conformations than in any other confor-

mation. It has been calculated that, for every thousand molecules of cyclohexane in

105

> >

3-D Molecule:

Boat cyclohexane

Build a model of cyclohexane, and con-

vert it from one chair conformer to the

other. To do this, pull the topmost car-

bon down and push the bottommost

carbon up.

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