Organic Chemistry

98 CHAPTER 2 An Introduction to Organic Compounds

CH 3 ring flip

CH 3

the methyl group is in

an equatorial position

the methyl group is

in an axial position

less stable

chair conformer

more stable

chair conformer

Figure 2.11N

A substituent is in an equatorial

position in one chair conformer and

in an axial position in the other. The

conformer with the substituent in

the equatorial position is more

stable.

CH 2

CH 2

H

H

H

H

3

1

H

CH 3

H

H

2

methyl is anti to C-3

CH 2

CH 2

H

H

CH 3

H

5

2

1

H

H

H

H

6

5

4

6

4

3

equatorial

substituent

methyl is anti to C-5

Figure 2.12

An equatorial substituent on the C-1 carbon is anti to the

C-3 and C-5 carbons.

CH 2

CH 2

H

H

H

H

3

1

H

axial substituent

H

CH 3 CH 3

H

CH 2

CH 2

H

H

H

5

1

H

H

H

H

2 6

methyl is gauche to C-3 methyl is gauche to C-5

62

Figure 2.13

An axial substituent on the C-1 carbon is gauche to

the C-3 and C-5 carbons.

a chair conformation, no more than two molecules are in the next most stable

conformation—the twist-boat.

2.13 Conformations of Monosubstituted

Cyclohexanes

Unlike cyclohexane, which has two equivalent chair conformers, the two chair con-

formers of a monosubstituted cyclohexane such as methylcyclohexane are not equiva-

lent. The methyl substituent is in an equatorial position in one conformer and in an

axial position in the other (Figure 2.11), because substituents that are equatorial in one

chair conformer are axial in the other (Figure 2.8).

Build a model of methylcyclohexane,

and convert it from one chair conformer

to the other.

The chair conformer with the methyl substituent in an equatorial position is the

more stable conformer because a substituent has more room and, therefore, fewer

steric interactions when it is in an equatorial position. This can be best understood by

examining Figure 2.12, which shows that when the methyl group is in an equatorial

position, it is anti to the C-3 and C-5 carbons. Therefore, the substituent extends into

space, away from the rest of the molecule.

In contrast, when the methyl group is in an axial position, it is gauche to the C-3

and C-5 carbons (Figure 2.13). As a result, there are unfavorable steric interactions be-

tween the axial methyl group and both the axial substituent on C-3 and the axial sub-

stituent on C-5 (in this case, hydrogens). In other words, the three axial bonds on the

same side of the ring are parallel to each other, so any axial substituent will be rela-

tively close to the axial substituents on the other two carbons. Because the interacting

substituents are on 1,3-positions relative to each other, these unfavorable steric inter-

actions are called 1,3-diaxial interactions. If you take a few minutes to build models,

you will see that a substituent has more room if it is in an equatorial position than if it

is in an axial position.

Go to the Web site for three-

dimensional representations of the

conformers of cyclohexane.

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