CHAPTER 3
CONFORMATIONS OF ALKANES
AND CYCLOALKANES
SOLUTIONS TO TEXT PROBLEMS
3.1 (b) The sawhorse formula contains four carbon atoms in an unbranched chain. The compound is
butane, CH
3
CH
2
CH
2
CH
3
.
(c) Rewrite the structure to show its constitution. The compound is CH
3
CH
2
CH(CH
3
)
2
; it is
2-methylbutane.
(d) In this structure, we are sighting down the C-3GC-4 bond of a six-carbon chain. It is
CH
3
CH
2
CH
2
CHCH
2
CH
3
, or 3-methylhexane.
=
CH
3
CH
2
CH
3
H
HH
CH
3
CH
2
CH
3
H11013 C
H
H
H
H
3
C
CH
3
CH
3
C
H
H
H
CH
3
CH
3
CH
3
HH
HH
CH
3
CH
3
46
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
3.2 Red circles gauche: 60° and 300°. Red circles anti: 180°. Gauche and anti relationships occur only
in staggered conformations; therefore, ignore the eclipsed conformations (0°, 120°, 240°, 360°).
3.3 All the staggered conformations of propane are equivalent to one another, and all its eclipsed con-
formations are equivalent to one another. The energy diagram resembles that of ethane in that it is a
symmetrical one.
The activation energy for bond rotation in propane is expected to be somewhat higher than that
in ethane because of van der Waals strain between the methyl group and a hydrogen in the eclipsed
conformation. This strain is, however, less than the van der Waals strain between the methyl
groups of butane, which makes the activation energy for bond rotation less for propane than for
butane.
3.4 (b) To be gauche, substituents X and A must be related by a 60° torsion angle. If A is axial as
specified in the problem, X must therefore be equatorial.
(c) For substituent X at C-1 to be anti to C-3, it must be equatorial.
(d) When X is axial at C-1, it is gauche to C-3.
3.5 (b) According to the numbering scheme given in the problem, a methyl group is axial when it is
“up” at C-1 but is equatorial when it is up at C-4. Since substituents are more stable when they
3
X
A
3
X
A
X and A are gauche.
X
A
Potential ener
gy
0 60 120 180 240 300 360
Torsion angle (degrees)
H
3
C H
HH
H
H
H H
HH
H
H
3
C
H CH
3
HH
H
H
H
3
C H
HH
H
H
H
H H
H
H
H
3
C
HH
H
CH
3
H
H
H
H H
H
CH
3
H
CONFORMATIONS OF ALKANES AND CYCLOALKANES 47
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
48 CONFORMATIONS OF ALKANES AND CYCLOALKANES
occupy equatorial rather than axial sites, a methyl group that is up at C-1 is less stable than one
that is up at C-4.
(c) An alkyl substituent is more stable in the equatorial position. An equatorial substituent at C-3
is “down.”
3.6 A tert-butyl group is much larger than a methyl group and has a greater preference for the equato-
rial position. The most stable conformation of 1-tert-butyl-1-methylcyclohexane has an axial methyl
group and an equatorial tert-butyl group.
3.7 Ethylcyclopropane and methylcyclobutane are isomers (both are C
5
H
10
). The less stable isomer has
the higher heat of combustion. Ethylcyclopropane has more angle strain and is less stable (has
higher potential energy) than methylcyclobutane.
3.8 The four constitutional isomers of cis and trans-1,2-dimethylcyclopropane that do not contain
double bonds are
3.9 When comparing two stereoisomeric cyclohexane derivatives, the more stable stereoisomer is the
one with the greater number of its substituents in equatorial orientations. Rewrite the structures as
chair conformations to see which substituents are axial and which are equatorial.
cis-1,3,5-Trimethylcyclohexane
H
H
H
H
3
C CH
3
CH
3
CH
3
CH
3
H
H
3
C
H H
CH
3
CH
3
1,1-Dimethylcyclopropane
CH
3
Methylcyclobutane
Ethylcyclopropane
CH
2
CH
3
Cyclopentane
CH
2
CH
3
CH
3
More stable
3352 kJ/mol
(801.2 kcal/mol)
Less stable
3384 kJ/mol
(808.8 kcal/mol)
Heat of combustion:
1-tert-Butyl-1-methylcyclohexane
CH
3
C(CH
3
)
3
H
H
3
C
Up
Down
H
CH
3
Up
Down
H
H
3
C
Up
Down
4
5
6
3
2
1
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
All methyl groups are equatorial in cis-1,3,5-trimethylcyclohexane. It is more stable than trans-
1,3,5-trimethylcyclohexane (shown in the following), which has one axial methyl group in its most
stable conformation.
3.10 In each of these problems, a tert-butyl group is the larger substituent and will be equatorial in the
most stable conformation. Draw a chair conformation of cyclohexane, add an equatorial tert-butyl
group, and then add the remaining substituent so as to give the required cis or trans relationship to
the tert-butyl group.
(b) Begin by drawing a chair cyclohexane with an equatorial tert-butyl group. In cis-1-tert-butyl-
3-methylcyclohexane the C-3 methyl group is equatorial.
(c) In trans-1-tert-butyl-4-methylcyclohexane both the tert-butyl and the C-4 methyl group are
equatorial.
(d) Again the tert-butyl group is equatorial; however, in cis-1-tert-butyl-4-methylcyclohexane the
methyl group on C-4 is axial.
3.11 Isomers are different compounds that have the same molecular formula. Compare the molecular
formulas of the compounds given to the molecular formula of spiropentane.
Only the two compounds that have the molecular formula C
5
H
8
are isomers of spiropentane.
3.12 Two bond cleavages convert bicyclobutane to a noncyclic species; therefore, bicyclobutane is
bicyclic.
Spiropentane (C
5
H
8
)
CH CH
2
C
5
H
8
C
6
H
10
C
5
H
8
C
5
H
10
CH
2
CH
3
H
C(CH
3
)
3
H
H
3
C
H
C(CH
3
)
3
H
H
H
3
C
H
C(CH
3
)
3
H
H
H
H
3
C CH
3
H
3
C
trans-1,3,5-Trimethylcyclohexane
CH
3
CH
3
H
H
CH
3
H
CONFORMATIONS OF ALKANES AND CYCLOALKANES 49
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
The two bond cleavages shown convert camphene to a noncyclic species; therefore, camphene is
bicyclic. (Other pairs of bond cleavages are possible and lead to the same conclusion.)
3.13 (b) This bicyclic compound contains nine carbon atoms. The name tells us that there is a five-
carbon bridge and a two-carbon bridge. The 0 in the name bicyclo[5.2.0]nonane tells us that
the third bridge has no atoms in it—the carbons are common to both rings and are directly
attached to each other.
(c) The three bridges in bicyclo[3.1.1]heptane contain three carbons, one carbon, and one carbon.
The structure can be written in a form that shows the actual shape of the molecule or one that
simply emphasizes its constitution.
(d) Bicyclo[3.3.0]octane has two five-membered rings that share a common side.
3.14 Since the two conformations are of approximately equal stability when R H11005 H, it is reasonable to
expect that the most stable conformation when R H11005 CH
3
will have the CH
3
group equatorial.
3.15 (a) Recall that a neutral nitrogen atom has three covalent bonds and an unshared electron pair.
The three bonds are arranged in a trigonal pyramidal manner around each nitrogen in
hydrazine (H
2
NNH
2
).
H
N
N
H
HH
H
H
H
H
H
H
H
H
H11013H11013
N
N
HH
HH
R H11005 H: both conformations similar in energy
R H11005 CH
3
: most stable conformation has CH
3
equatorial
R
N
N
R
Three-carbon
bridge
Three-carbon
bridge
One-carbon
bridge
One-carbon
bridge
Three-carbon
bridge
Bicyclo[5.2.0]nonane
CH
2
CH
3
CH
3
CH
2
CH
3
CH
3
CH
2
CH
3
CH
3
50 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
(b) The OGH proton may be anti to one NGH proton and gauche to the other (left) or it may be
gauche to both (right).
3.16 Conformation (a) is the most stable; all its bonds are staggered. Conformation (c) is the least stable;
all its bonds are eclipsed.
3.17 (a) First write out the structural formula of 2,2-dimethylbutane in order to identify the substituent
groups attached to C-2 and C-3. As shown at left, C-2 bears three methyl groups, and C-3
bears two hydrogens and a methyl group. The most stable conformation is the staggered one
shown at right. All other staggered conformations are equivalent to this one.
(b) The constitution of 2-methylbutane and its two most stable conformations are shown.
Both conformations are staggered. In one (left), the methyl group at C-3 is gauche to both of
the C-2 methyls. In the other (right), the methyl group at C-3 is gauche to one of the C-2
methyls and anti to the other.
(c) The hydrogens at C-2 and C-3 may be gauche to one another (left), or they may be anti (right).
3.18 The 2-methylbutane conformation with one gauche CH
3
. . .
CH
3
and one anti CH
3
. . .
CH
3
rela-
tionship is more stable than the one with two gauche CH
3
. . .
CH
3
relationships. The more stable
conformation has less van der Waals strain.
CH
3
H
3
C
CH
3
HH
H
CH
3
CH
3
H
3
C
H
HH
More stable Less stable
CH
3
CH
3
H
3
C
CH
3
H
H
CH
3
H
CH
3
CH
3
H
3
C
H
CCH
3
C
CH
3
CH
3
CH
3
H
H
Sight along this bond.
CH
3
CH
3
H
H
3
C
HH
CH
3
H
CH
3
H
3
C
HH
CCH
3
H
3
H
CH
3
H
H
Sight along this bond.
CH
3
CH
3
CH
3
H
3
C
HH
CCH
3
H
3
CH
3
CH
3
H
H
Sight along this bond.
N
H
O
H
H
N
HH
H
O
CONFORMATIONS OF ALKANES AND CYCLOALKANES 51
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
3.19 All the staggered conformations about the C-2GC-3 bond of 2,2-dimethylpropane are equivalent to
one another and of equal energy; they represent potential energy minima. All the eclipsed confor-
mations are equivalent and represent potential energy maxima.
The shape of the potential energy profile for internal rotation in 2,2-dimethylpropane more
closely resembles that of ethane than that of butane.
3.20 The potential energy diagram of 2-methylbutane more closely resembles that of butane than that of
propane in that the three staggered forms are not all of the same energy. Similarly, not all of the
eclipsed forms are of equal energy.
Potential ener
gy
0 60 120 180 240 300 360
Torsion angle (degrees)
H
3
CCH
3
HH
H
CH
3
H
3
C H
HH
H
3
C
CH
3
HCH
3
HH
H
3
C
CH
3
CH
3
H H
HH
3
C
CH
3
CH
3
HH
CH
3
H
3
C
H
CH
3
H H
CH
3
H
CH
3
Potential ener
gy
0 60 120 180 240 300 360
Torsion angle (degrees)
H H
H
3
C
CH
3
H
CH
3
H H
H
3
C CH
3
CH
3
H
H H
H
3
C CH
3
CH
3
H
H H
CH
3
CH
3
H
3
C
H
CH
3
H
3
C CH
3
HH
H
CH
3
H
3
C CH
3
HH
H
CH
3
H
3
C CH
3
HH
H
52 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
3.21 Van der Waals strain between the tert-butyl groups in 2,2,4,4-tetramethylpentane causes the
C-2GC-3GC-4 angle to open to 125–128°.
3.22 The structure shown in the text is not the most stable conformation, because the bonds of the methyl
group are eclipsed with those of the ring carbon to which it is attached. The most stable conforma-
tion has the bonds of the methyl group and its attached carbon in a staggered relationship.
3.23 Structure A has the hydrogens of its methyl group eclipsed with the ring bonds and is less stable
than B. The methyl group in structure B has its bonds and those of its attached ring carbon in a stag-
gered relationship.
Furthermore, two of the hydrogens of the methyl group of A are uncomfortably close to two axial
hydrogens of the ring.
3.24 Conformation B is more stable than A. The methyl groups are rather close together in A, resulting in
van der Waals strain between them. In B, the methyl groups are farther apart.
3.25 (a) By rewriting the structures in a form that shows the order of their atomic connections, it is
apparent that the two structures are constitutional isomers.
CH
3
CH
3
H
3
C
H
HH
is equivalent to CH
3
CCH
3
CH
3
CH
3
(2,2-Dimethylpropane)
CH
3
CH
3
H
3
C H
HH
is equivalent to CH
3
CH
2
CHCH
3
CH
3
(2-Methylbutane)
HH
CH
3
CH
3
A
Van der Waals strain between cis methyl groups.
HH
H
3
C CH
3
B
Methyl groups remain cis, but are far apart.
A (less stable) B (more stable)
H
H
H
H
H
H
H
H
H
H
Bonds of methyl group eclipsed
with those of attached carbon
Bonds of methyl group staggered
with those of attached carbon
H
H
H
H
H
H
H
H
is equivalent to
CH
2
CCH
3
C CH
3
CH
3
CH
3
CH
3
CH
3
This angle is
enlarged.
CONFORMATIONS OF ALKANES AND CYCLOALKANES 53
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
(b) Both models represent alkanes of molecular formula C
6
H
14
. In each one the carbon chain is
unbranched. The two models are different conformations of the same compound,
CH
3
CH
2
CH
2
CH
2
CH
2
CH
3
(hexane).
(c) The two compounds have the same constitution; both are (CH
3
)
2
CHCH(CH
3
)
2
. The Newman
projections represent different staggered conformations of the same molecule: in one the
hydrogens are anti to each other, whereas in the other they are gauche.
(d) The compounds differ in the order in which the atoms are connected. They are constitutional
isomers. Although the compounds have different stereochemistry (one is cis, the other trans),
they are not stereoisomers. Stereoisomers must have the same constitution.
(e) Both structures are cis-1-ethyl-4-methylcyclohexane (the methyl and ethyl groups are both
“up”). In the structure on the left, the methyl is axial and the ethyl equatorial. The orientations
are opposite to these in the structure on the right. The two structures are ring-flipped forms of
each other—different conformations of the same compound.
( f ) The methyl and the ethyl groups are cis in the first structure but trans in the second. The two
compounds are stereoisomers; they have the same constitution but differ in the arrangement
of their atoms in space.
Do not be deceived because the six-membered rings look like ring-flipped forms. Remember,
chair–chair interconversion converts all the equatorial bonds to axial and vice versa. Here the
ethyl group is equatorial in both structures.
(g) The two structures have the same constitution but differ in the arrangement of their atoms in
space; they are stereoisomers. They are not different conformations of the same compound,
because they are not related by rotation about CGC bonds. In the first structure as shown
here the methyl group is trans to the darkened bonds, whereas in the second it is cis to these
bonds.
Methyl is trans to
these bonds.
CH
3
H
Methyl is cis to
these bonds.
H
CH
3
cis-1-Ethyl-4-methylcyclohexane
(both alkyl groups are up)
trans-1-Ethyl-4-methylcyclohexane
(ethyl group is down; methyl group is up)
CH
3
CH
3
CH
2
CH
3
CH
3
CH
2
CH
3
CH
3
CH
3
CH
3
cis-1,2-Dimethylcyclopentane trans-1,3-Dimethylcyclopentane
and
are different conformations of
2,3-dimethylbutane
Hydrogens at C-2 and
C-3 are gauche.
Hydrogens at C-2
and C-3 are anti.
CH
3
CH
3
H
H
H
3
C
H
3
C
CH
3
CH
3
H
HH
3
C
H
3
C
54 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
3.26 (a) Three isomers of C
5
H
8
contain two rings and have no alkyl substituents:
(b) Five isomers of C
6
H
10
contain two rings and have no alkyl substituents:
3.27 (a) The heat of combustion is highest for the hydrocarbon with the greatest number of carbons.
Thus, cyclopropane, even though it is more strained than cyclobutane or cyclopentane, has the
lowest heat of combustion.
A comparison of heats of combustion can only be used to assess relative stability when the
compounds are isomers.
(b) All these compounds have the molecular formula C
7
H
14
. They are isomers, and so the one
with the most strain will have the highest heat of combustion.
H
3
CCH
3
CH
3
H
3
C
1,1,2,2-Tetramethylcyclopropane
(high in angle strain; bonds are
eclipsed; van der Waals strain
between cis methyl groups)
Heat of combustion
4635 kJ/mol
(1107.9 kcal/mol)
H
3
CCH
3
HH
cis-1,2-Dimethylcyclopentane
(low angle strain; some torsional
strain; van der Waals strain
between cis methyl groups)
Heat of combustion
4590 kJ/mol
(1097.1 kcal/mol)
Methylcyclohexane
(minimal angle, torsional, and
van der Waals strain)
Heat of combustion
4565 kJ/mol
(1091.1 kcal/mol)
CH
3
Cyclopentane
Heat of combustion 3291 kJ/mol
(786.6 kcal/mol)
Cyclobutane
Heat of combustion 2721 kJ/mol
(650.3 kcal/mol)
Cyclopropane
Heat of combustion 2091 kJ/mol
(499.8 kcal/mol)
Spirohexane Bicyclo[2.2.0]hexane
Bicyclo[2.1.1]hexane Cyclopropylcyclopropane
Bicyclo[3.1.0]hexane
Spiropentane Bicyclo[2.1.0]pentane Bicyclo[1.1.1]pentane
CONFORMATIONS OF ALKANES AND CYCLOALKANES 55
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
(c) These hydrocarbons all have different molecular formulas. Their heats of combustion de-
crease with decreasing number of carbons, and comparisons of relative stability cannot be
made.
(d) Bicyclo[3.3.0]octane and bicyclo[5.1.0]octane are isomers, and their heats of combustion
can be compared on the basis of their relative stabilities. The three-membered ring in bicy-
clo[5.1.0]octane imparts a significant amount of angle strain to this isomer, making it less
stable than bicyclo[3.3.0]octane. The third hydrocarbon, bicyclo[4.3.0]nonane, has a greater
number of carbons than either of the others and has the largest heat of combustion.
3.28 (a) The structural formula of 2,2,5,5-tetramethylhexane is (CH
3
)
3
CCH
2
CH
2
C(CH
3
)
3
. The sub-
stituents at C-3 are two hydrogens and a tert-butyl group. The substituents at C-4 are the same
as those at C-3. The most stable conformation has the large tert-butyl groups anti to each
other.
(b) The zigzag conformation of 2,2,5,5-tetramethylhexane is an alternative way of expressing
the same conformation implied in the Newman projection of part (a). It is more complete,
Anti conformation of
2,2,5,5-tetramethylhexane
C(CH
3
)
3
C(CH
3
)
3
HH
HH
H
H
H
H
H
H
Bicyclo[4.3.0]nonane
(C
9
H
16
)
Bicyclo[5.1.0]octane
(C
8
H
14
)
Bicyclo[3.3.0]octane
(C
8
H
14
)
Heat of combustion 5652 kJ/mol
(1350.9 kcal/mol)
Heat of combustion 5089 kJ/mol
(1216.3 kcal/mol)
Heat of combustion 5016 kJ/mol
(1198.9 kcal/mol)
Cyclopropylcyclopropane
(C
6
H
10
)
Heat of combustion
3886 kJ/mol
(928.8 kcal/mol)
Spiropentane
(C
5
H
8
)
Heat of combustion
3296 kJ/mol
(787.8 kcal/mol)
Bicyclo[1.1.0]butane
(C
4
H
6
)
Heat of combustion
2648 kJ/mol
(633.0 kcal/mol)
56 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
however, in that it also shows the spatial arrangement of the substituents attached to the main
chain.
(c) An isopropyl group is bulkier than a methyl group, and will have a greater preference for an
equatorial orientation in the most stable conformation of cis-1-isopropyl-3-methylcyclohexane.
Draw a chair conformation of cyclohexane, and place an isopropyl group in an equatorial
position.
Notice that the equatorial isopropyl group is down on the carbon atom to which it is attached.
Add a methyl group to C-3 so that it is also down.
Both substituents are equatorial in the most stable conformation of cis-1-isopropyl-3-methyl-
cyclohexane.
(d) One substituent is up and the other is down in the most stable conformation of trans-1-
isopropyl-3-methylcyclohexane. Begin as in part (c) by placing an isopropyl group in an equa-
torial orientation on a chair conformation of cyclohexane.
To be trans to the C-1 isopropyl group, the C-3 methyl group must be up.
The bulkier isopropyl group is equatorial and the methyl group axial in the most stable
conformation.
(e) To be cis to each other, one substituent must be axial and the other equatorial when they are
located at positions 1 and 4 on a cyclohexane ring.
H
H
1
4
H
H
CH(CH
3
)
2
CH
3
H
CH(CH
3
)
2
1
3
H
H
3
C
H
CH(CH
3
)
2
H
CH(CH
3
)
2
1
3
2,2,5,5-Tetramethylhexane
CONFORMATIONS OF ALKANES AND CYCLOALKANES 57
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
Place the larger substituent (the tert-butyl group) at the equatorial site and the smaller sub-
stituent (the ethyl group) at the axial one.
( f ) First write a chair conformation of cyclohexane, then add two methyl groups at C-1, and draw
in the axial and equatorial bonds at C-3 and C-4. Next, add methyl groups to C-3 and C-4
so that they are cis to each other. There are two different ways that this can be accomplished:
either the C-3 and C-4 methyl groups are both up or they are both down.
(g) Draw the projection formula as a chair conformation.
Check to see if this is the most stable conformation by writing its ring-flipped form.
The ring-flipped form, with two equatorial methyl groups and one axial methyl group, is more
stable than the originally drawn conformation, with two axial ethyl groups and one equatorial
methyl group.
3.29 Begin by writing each of the compounds in its most stable conformation. Compare them by exam-
ining their conformations for sources of strain, particularly van der Waals strain arising from groups
located too close together in space.
(a) Its axial methyl group makes the cis stereoisomer of 1-isopropyl-2-methylcyclohexane less
stable than the trans.
cis-1-Isopropyl-2-methylcyclohexane
(less stable stereoisomer)
trans-1-Isopropyl-2-methylcyclohexane
(more stable stereoisomer)
H
H
CH
3
CH(CH
3
)
2
4
6
1
2
CH
3
CH(CH
3
)
2
H
H
H
H
3
C
CH
3
CH
3
1
24
Less stable conformation: two
axial methyl groups
More stable conformation: one
axial methyl group
2
4
H
H
CH
3
H
3
C
1
H
CH
3
H
H
H
CH
3
H
3
C
CH
3
1
24
H
H
3
C H
H
CH
3
CH
3
1
24
More stable chair conformation: C-3 methyl
group is equatorial; no van der Waals strain between
axial C-1 methyl group and C-3 methyl
Less stable chair conformation: C-3 methyl
group is axial; strong van der Waals strain
between axial C-1 and C-3 methyl groups
H
H
CH
3
CH
3
CH
3
CH
3
1
4
CH
3
H
3
C
H
H
CH
3
CH
3
1
4
H
H
C(CH
3
)
3
CH
2
CH
3
58 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
The axial methyl group in the cis stereoisomer is involved in unfavorable repulsions with the
C-4 and C-6 axial hydrogens indicated in the drawing.
(b) Both groups are equatorial in the cis stereoisomer of 1-isopropyl-3-methylcyclohexane; cis is
more stable than trans in 1,3-disubstituted cyclohexanes.
(c) The more stable stereoisomer of 1,4-disubstituted cyclohexanes is the trans; both alkyl groups
are equatorial in trans-1-isopropyl-4-methylcyclohexane.
(d) The first stereoisomer of 1,2,4-trimethylcyclohexane is the more stable one. All its methyl
groups are equatorial in its most stable conformation. The most stable conformation of the
second stereoisomer has one axial and two equatorial methyl groups.
H
3
C CH
3
CH
3
1
24
More stable stereoisomer All methyl groups equatorial in
most stable conformation
CH
3
H
3
C
CH
3
1
2
4
Less stable stereoisomer
One axial methyl group in most
stable conformation
CH
3
CH
3
H
3
C
1
2
4
CH
3
CH
3
H
3
C
12
4
CH
3
CH
3
CH
3
1
2
4
cis-1-Isopropyl-4-methylcyclohexane
(less stable stereoisomer; methyl
group is axial and involved in
repulsions with axial
hydrogens at C-2 and C-6)
trans-1-Isopropyl-4-methylcyclohexane
(more stable stereoisomer; both
groups are equatorial)
H
3
C
CH(CH
3
)
2
CH
3
H
H
CH(CH
3
)
2
1
2
4
6
cis-1-Isopropyl-3-methylcyclohexane
(more stable stereoisomer; both
groups are equatorial)
trans-1-Isopropyl-3-methylcyclohexane
(less stable stereoisomer; methyl group
is axial and involved in repulsions
with axial hydrogens at C-1 and C-5)
CH
3
CH(CH
3
)
2
CH
3
CH(CH
3
)
2
H H
1
5
3
CONFORMATIONS OF ALKANES AND CYCLOALKANES 59
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
(e) The first stereoisomer of 1,2,4-trimethylcyclohexane is the more stable one here, as it was in
part (d). All its methyl groups are equatorial, but one of the methyl groups is axial in the most
stable conformation of the second stereoisomer.
( f ) Each stereoisomer of 2,3-dimethylbicyclo[3.2.1]octane has one axial and one equatorial
methyl group. The first one, however, has a close contact between its axial methyl group and
both methylene groups of the two-carbon bridge. The second stereoisomer has repulsions with
only one axial methylene group; it is more stable.
3.30 First write structural formulas showing the relative stereochemistries and the preferred conforma-
tions of the two stereoisomers of 1,1,3,5-tetramethylcyclohexane.
The cis stereoisomer is more stable than the trans. It exists in a conformation with only one axial
methyl group, while the trans stereoisomer has two axial methyl groups in close contact with each
other. The trans stereoisomer is destabilized by van der Waals strain.
CH
3
CH
3
CH
3
H
3
C
CH
3
CH
3
CH
3
H
3
C
H
3
C
CH
3
CH
3
H
3
C
H
3
C
written in its most stable
conformation as
cis-1,1,3,5-Tetramethylcyclohexane
trans-1,1,3,5-Tetramethylcyclohexane
written in its most stable
conformation as
CH
3
CH
3
CH
3
Less stable stereoisomer
(more van der Waals strain)
More stable stereoisomer
(less van der Waals strain)
CH
3
H
3
C
CH
3
CH
3
H
H
H
CH
3
CH
3
H
3
C
1
24
CH
3
H
3
C
CH
3
1
24
More stable stereoisomer
Less stable stereoisomer
All methyl groups equatorial in
most stable conformation
CH
3
H
3
C
1
CH
3
2
4
One axial methyl group in
most stable conformation
CH
3
H
3
C
1
H
3
C
2
4
60 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
3.31 Both structures have approximately the same degree of angle strain and of torsional strain. Structure B
has more van der Waals strain than A because two pairs of hydrogens (shown here) approach each
other at distances that are rather close.
3.32 Five bond cleavages are required to convert cubane to a noncyclic skeleton; cubane is pentacyclic.
3.33 Conformational representations of the two different forms of glucose are drawn in the usual way. An
oxygen atom is present in the six-membered ring, and we are told in the problem that the ring exists
in a chair conformation.
The two structures are not interconvertible by ring flipping; therefore they are not different confor-
mations of the same molecule. Remember, ring flipping transforms all axial substituents to equato-
rial ones and vice versa. The two structures differ with respect to only one substituent; they are
stereoisomers of each other.
3.34 This problem is primarily an exercise in correctly locating equatorial and axial positions in cyclo-
hexane rings that are joined together into a steroid skeleton. Parts (a) through (e) are concerned with
positions 1, 4, 7, 11, and 12 in that order. The following diagram shows the orientation of axial and
equatorial bonds at each of those positions.
(a) At C-1 the bond that is cis to the methyl groups is equatorial (up).
(b) At C-4 the bond that is cis to the methyl groups is axial (up).
Both methyl groups are up.
CH
3
CH
3
e
c
a
b
d
4
1
11
12
7
O
HOCH
2
OHHO
OHHO
One axial OH substituent
written in its most
stable conformation as
O
HOCH
2
OHHO
HO OH
written in its most
stable conformation as
All substituents equatorial
O
CH
2
OH
OH
HO
HO
HO
OH
OH
O
CH
2
OH
HO
HO
A:
More stable stereoisomer Van der Waals strain
destabilizes B
H
H
H
H
CONFORMATIONS OF ALKANES AND CYCLOALKANES 61
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
(c) At C-7 the bond that is trans to the methyl groups is axial (down).
(d) At C-11 the bond that is trans to the methyl groups is equatorial (down).
(e) At C-12 the bond that is cis to the methyl groups is equatorial (up).
3.35 Analyze this problem in exactly the same way as the preceding one by locating the axial and equa-
torial bonds at each position. It will be seen that the only differences are those at C-1 and C-4.
(a) At C-1 the bond that is cis to the methyl groups is axial (up).
(b) At C-4 the bond that is cis to the methyl groups is equatorial (up).
(c) At C-7 the bond that is trans to the methyl groups is axial (down).
(d) At C-11 the bond that is trans to the methyl groups is equatorial (down).
(e) At C-12 the bond that is cis to the methyl groups is equatorial (up).
3.36 (a) The torsion angle between chlorine substituents is 60° in the gauche conformation and 180° in
the anti conformation of ClCH
2
CH
2
Cl.
(b) All the individual bond dipole moments cancel in the anti conformation of ClCH
2
CH
2
Cl, and
this conformation has no dipole moment. Since ClCH
2
CH
2
Cl has a dipole moment of 1.12 D,
it can exist entirely in the gauche conformation or it can be a mixture of anti and gauche con-
formations, but it cannot exist entirely in the anti conformation. Statement 1 is false.
3.37–3.40 Solutions to molecular modeling exercises are not provided in this Study Guide and Solutions Man-
ual. You should use Learning By Modeling for these exercises.
SELF-TEST
PART A
A-1. Draw Newman projections for both the gauche and the anti conformations of 1-chloro-
propane, CH
3
CH
2
CH
2
Cl. Sight along the C-1, C-2 bond (the chlorine is attached to C-1).
A-2. Write Newman projection formulas for
(a) The least stable conformation of butane
(b) Two different staggered conformations of CHCl
2
CHCl
2
Gauche
(can have a dipole moment)
Anti
(cannot have a dipole moment)
H
Cl
H
H
H
Cl
Cl
Cl
H
H
H
H
Both methyl groups are up.
CH
3
CH
3
d
e
c
a
b
11
12
7
1
4
H
62 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
A-3. Give the correct IUPAC name for the compound represented by the following Newman
projection.
A-4. Write the structure of the most stable conformation of the less stable stereoisomer of 1-tert-
butyl-3-methylcyclohexane.
A-5. Draw the most stable conformation of the following substance:
Which substituents are axial and which equatorial?
A-6. A wedge-and-dash representation of a form of ribose (called H9252-D-ribopyranose) is shown
here. Draw the most stable chair conformation of this substance.
A-7. Consider compounds A, B, C, and D.
(a) Which one is a constitutional isomer of two others?
(b) Which two are stereoisomers of one another?
(c) Which one has the highest heat of combustion?
(d) Which one has the stereochemical descriptor trans in its name?
A-8. Draw clear depictions of two nonequivalent chair conformations of cis-1-isopropyl-
4-methylcyclohexane, and indicate which is more stable.
A-9. Which has the lower heat of combustion, cis-1-ethyl-3-methylcyclohexane or cis-1-ethyl-
4-methylcyclohexane?
A-10. The hydrocarbon shown is called twistane. Classify twistane as monocyclic, bicyclic, etc.
What is the molecular formula of twistane?
AB
CH
3
H
3
C CH
3
H
3
C
CD
CH
3
CH
3
CH
3
CH
3
H
3
C
O
OH
OHHO
HO
CH
3
H
3
C
C(CH
3
)
3
C(CH
3
)
3
CH
3
H
H
3
CCH
3
H
CONFORMATIONS OF ALKANES AND CYCLOALKANES 63
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
A-11. Sketch an approximate potential energy diagram similar to those shown in the text (Fig-
ures 3.4 and 3.7) for rotation about a carbon–carbon bond in 2-methylpropane. Does the
form of the potential energy curve more closely resemble that of ethane or that of butane?
A-12. Draw the structure of the sulfur-containing heterocyclic compound that has a structure anal-
ogous to that of tetrahydrofuran.
PART B
B-1. Which of the listed terms best describes the relationship between the methyl groups in the
chair conformation of the substance shown?
(a) Eclipsed (c) Anti
(b) Trans (d) Gauche
B-2. Rank the following substances in order of decreasing heat of combustion (largestBsmallest).
(a)1 H11022 2 H11022 4 H11022 3(c)3 H11022 4 H11022 2 H11022 1
(b)2 H11022 4 H11022 1 H11022 3(d)1 H11022 3 H11022 2 H11022 4
B-3. Which of the following statements best describes the most stable conformation of trans-
1, 3-dimethylcyclohexane?
(a) Both methyl groups are axial.
(b) Both methyl groups are equatorial.
(c) One methyl group is axial, the other equatorial.
(d) The molecule is severely strained and cannot exist.
B-4. Compare the stability of the following two compounds:
A: cis-1-Ethyl-3-methylcyclohexane
B: trans-1-Ethyl-3-methylcyclohexane
(a) A is more stable.
(b) B is more stable.
(c) A and B are of equal stability.
(d) No comparison can be made.
B-5. What, if anything, can be said about the magnitude of the equilibrium constant K for the
following process?
(a) K H11005 1(c) K H11021 1
(b) K H11022 1(d) No estimate of K can be made.
H
CH(CH
3
)
2
H
CH
3
H
H
CH(CH
3
)
2
H
3
C
CH
3
1
CH
3
CH
3
2 3
CH
3
CH
3
4
CH
3
CH
3
64 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
B-6. What is the relationship between the two structures shown?
(a) Constitutional isomers
(b) Stereoisomers
(c) Different drawings of the same conformation of the same compound
(d) Different conformations of the same compound
B-7. The two structures shown here are each other.
(a) identical with (c) constitutional isomers of
(b) conformations of (d) stereoisomers of
B-8. The most stable conformation of the following compound has
(a) An axial methyl group and an axial ethyl group
(b) An axial methyl group and an equatorial ethyl group
(c) An axial tert-buytl group
(d) An equatorial methyl group and an equatorial ethyl group
(e) An equatorial methyl group and an axial ethyl group
B-9. Which of the following statements is not true concerning the chair–chair interconversion of
trans-1,2-diethylcyclohexane?
(a) An axial group will be changed into the equatorial position.
(b) The energy of repulsions present in the molecule will be changed.
(c) Formation of the cis substance will result.
(d) One chair conformation is more stable than the other.
B-10. The most stable conformation of the compound
(in which all methyl groups are cis to one another) has:
(a) All methyl groups axial
(b) All methyl groups equatorial
(c) Equatorial methyl groups at C-1 and C-2
(d) Equatorial methyl groups at C-1 and C-4
(e) Equatorial methyl groups at C-2 and C-4
CH
3
CH
3
H
3
C
1
24
C(CH
3
)
3
CH
3
CH
2
CH
3
CH
3
H
3
C
H
H
H
CH
3
CH
3
CH
3
CH
3
H
H
H
ClCH
3
Cl
CH
3
CONFORMATIONS OF ALKANES AND CYCLOALKANES 65
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
B-11. Which point on the potential energy diagram is represented by the Newman projection
shown?
B-12. Which of the following statements is true?
(a) Van der Waals strain in cis-1,2-dimethylcyclopropane is the principal reason for its
decreased stability relative to the trans isomer.
(b) Cyclohexane gives off more heat per CH
2
group on being burned in air than any other
cycloalkane.
(c) The principal source of strain in the boat conformation of cyclohexane is angle strain.
(d) The principal source of strain in the gauche conformation of butane is torsional strain.
B-13. Which one of the following has an equatorial methyl group in its most stable conformation?
B-14. The structure shown is the carbon skeleton of adamantane, a symmetrical hydrocarbon
having a structure that is a section of the diamond lattice.
Adamantane is:
(a) Bicyclic (c) Tetracyclic
(b) Tricyclic (d) Pentacyclic
C(CH
3
)
3
H
3
C
(a)
C(CH
3
)
3
CH
3
(b)
C(CH
3
)
3
CH
3
(c)
C(CH
3
)
3
CH
3
(d)
C(CH
3
)
3
CH
3
(e)
H
H
HH
H
3
C
CH
3
(a)
(b)
(c)
(d)
66 CONFORMATIONS OF ALKANES AND CYCLOALKANES
Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website