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Chapter 3 145
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47.
Two 1,3-diaxial (gauche) interactions cause the chair conformer of fluorocyclohexane to be 0.25 kcal/mol
less stable when the fluoro substituent is in the axial position than when it is in the equatorial position.
The gauche conformer of 1-fluoropropane has one gauche interaction (see Figure 3.15 on page 128 of the
text). Therefore, the gauche conformer is
1
0.25
>
2
2
=
0.13 kcal
>
mol
less stable than the anti conformer
that has no gauche interactions.
48.
If both substituents point downward or both point upward, it is a cis isomer.
If one substituent points upward and the other downward, it is a trans isomer.
a.
cis
b.
cis
c.
cis
d.
trans
e.
trans
f.
trans
49.
Both
trans
-1,4-dimethylcyclohexane and
cis
-1-
tert
-butyl-3-methylcyclohexane have a conformer with two
substituents in the equatorial position and a conformer with two substituents in the axial position.
cis
-1-
tert
-Butyl-3-methylcyclohexane will have a higher percentage of the diequatorial-substituted
conformer because the bulky
tert
-butyl substituent will have a greater preference for the equatorial position
than will a less bulky methyl substituent, since the larger substituent will have greater destabilizing
1,3-diaxial interactions when it is in an axial position.
50.
a.
CH
2
CH
3
CH
3
b.
CH
2
CH
3
CH
3
c.
trans
-1-Ethyl-2-methylcyclohexane is more stable because both substituents can be in equatorial
positions.
51.
a.
one equatorial and one axial in each
b.
both equatorial in one and both axial in the other
c.
both equatorial in one and both axial in the other
d.
one equatorial and one axial in each
e.
one equatorial and one axial in each
f.
both equatorial in one and both axial
in the other
52.
Solved in the text.
53.
a.
H
H
H
H
H
H
H
H
H
H
H
H
CH
3
CH
3
CH
3
CH
3
b.
There will be equal amounts of the two conformers at equilibrium because they have the same
stability—each one has one methyl group on an equatorial bond and one methyl group on an axial bond.
54.
a.
One chair conformer of
trans
-1,4-dimethylcyclohexane has both substituents in equatorial positions,
so it does not have any 1,3-diaxial interactions. The other chair conformer has both substituents in
axial positions. When a substituent is in an axial position, it experiences two 1,3-diaxial interactions,
so this chair conformer has a total of four 1,3-diaxial interactions.