Chapter 4
Alkanes and Cycloalkanes
Review of Concepts
Fill in the blanks below. To verify that your answers are correct, look in your textbook at
the end of Chapter 4. Each of the sentences below appears verbatim in the section
entitled Review of Concepts and Vocabulary.
• Hydrocarbons that lack ____________ are called saturated hydrocarbons, or
___________.
• _________________ provide a systematic way for naming compounds.
• Rotation about C-C single bonds allows a compound to adopt a variety of
__________________.
Review of Skills
Fill in the blanks and empty boxes below. To verify that your answers are correct, look
in your textbook at the end of Chapter 4. The answers appear in the section entitled
SkillBuilder Review.
SkillBuilder 4.1 Identifying the Parent
IDENTIFY THE
PARENT IN EACH OF
THE FOLLOWING
COMPOUNDS.
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CHAPTER 4
SkillBuilder 4.2 Identifying and Naming Substituents
STEP 1 – IDENTIFY THE PARENT
IN THE FOLLOWING COMPOUND
STEPS 2 AND 3 – CIRCLE AND NAME ALL ALKYL SUBSTITUENTS
CONNECTED TO THE PARENT
SkillBuilder 4.5 Assembling the Name of a Bicyclic Compound
PROVIDE A SYSTEMATIC NAME FOR THE FOLLOWING COMPOUND
1) IDENTIFY THE PARENT
2) IDENTIFY AND NAME SUBSTITUENTS
3) ASSIGN LOCANTS TO EACH SUBSTITUENT
4) ALPHABETIZE
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59
SkillBuilder 4.8 Identifying Relative Energy of Conformations
STEP 1 – DRAW A
NEWMAN
PROJECTION
LOOKING DOWN THE
BOND INDICATED
STEP 2 – DRAW ALL THREE STAGGERED
CONFORMATIONS AND DETERMINE WHICH
ONE HAS THE FEWEST OR LEAST SEVERE
GAUCHE INTERACTIONS
STEP 3 – DRAW ALL THREE ECLIPSED
CONFORMATIONS AND DETERMINE WHICH ONE
HAS THE HIGHEST ENERGY INTERACTIONS
SkillBuilder 4.12 Drawing Both Chair Conformations of Disubstituted Cyclohexanes
Et
Me
DRAW BOTH CHAIR
CONFORMATIONS OF THE
FOLLOWING COMPOUND
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CHAPTER 4
Solutions
4.1.
a) parent = hexane b) parent = heptane
4.2.
4.5.
All groups are
methyl
methyl
ethyl
ethyl
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61
4.6.
a)
b)
4.7.
a)
Systematic = (1,1-dimethylethyl)
Common = tert-butyl
d)
Systematic = (2-methylpropyl)
Common = isobutyl
Systematic = (1-methylethyl)
Common = isopropyl
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CHAPTER 4
4.8.
phenyl
ethyl
4.9.
pentyl (1-methylbutyl) (2-methylbutyl) (3-methylbutyl)
4.10.
a) 3,4,6-trimethyloctane
b) sec-butylcyclohexane
c) 3-ethyl-2-methylheptane
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63
4.11.
a) b) c)
4.12.
a) 4-ethyl-1-methylbicyclo[3.2.1]octane
b) 2,2,5,7-tetramethylbicyclo[4.2.0]octane
4.13.
a) b) c)
4.14.
a) same compound
4.15.
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CHAPTER 4
4.16.
a)
CH
3
CH
3
H
CH
3
CH
3
H
b)
CH
3
CH
3
H
ClH
Cl
c)
CH
2
CH
3
CH
2
CH
3
H
CH
3
H
H
4.17.
a) b) c)
4.18. The compounds are not constitutional isomers. They are just two different
representations of the same compound. They are both 2,3-dimethylbutane.
4.19.
a) The energy barrier is expected
to be approximately 18 kJ / mol
b) The energy barrier is expected
to be approximately 16 kJ / mol
4.20.
CH
3
H
Et
H
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65
4.21. The gauche conformations are capable of intramolecular hydrogen bonding, as
shown below. The anti conformation lacks this stabilizing effect.
4.22.
4.24.
4.25.
4.27. There are eight hydrogen atoms in axial positions and seven hydrogen atoms in
equatorial positions.
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CHAPTER 4
b)
NH
2
NH
2
4.29.
a) The bromine atom occupies an equatorial position.
b)
Br
c)
B
r
4.30. Although the OH group is in an axial position, nevertheless, this conformation is
capable of intramolecular hydrogen bonding, which is a stabilizing effect:
O
O
O
H
4.31.
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67
c)
Me
Me
Br
Br
h)
Me
Me
Me
Me
4.32.
Cl
C
l
Cl
Cl
Cl
Cl
Cl Cl
Cl
Cl
Cl Cl
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CHAPTER 4
4.34. The two chair conformations of lindane are degenerate. There is no difference in
energy between them.
4.35. trans-1,4-di-tert-butylcyclohexane exists predominantly in a chair conformation,
because both substituents can occupy equatorial positions. In contrast, cis-1,4-di-tert–
4.36. cis-1,3-dimethylcyclohexane is expected to be more stable than trans-1,3-
dimethylcyclohexane because the former can adopt a chair conformation in which both
substituents are in equatorial positions (highlighted below):
CH
3
CH
3
CH
3
H
H
CH
cis-1,3-dimethylcyclohexane trans-1,3-dimethylcyclohexane
4.37. trans-1,4-dimethylcyclohexane is expected to be more stable than cis-1,4-
dimethylcyclohexane because the latter can adopt a chair conformation in which both
substituents are in equatorial positions (highlighted below):
cis-1,4-dimethylcyclohexane trans-1,4-dimethylcyclohexane
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69
4.38. cis-1,3-di-tert-butylcyclohexane can adopt a chair conformation in which both
tert-butyl groups occupy equatorial positions (highlighted below), and as a result, it is
R
H
R
H
R
R
R
R
R
H
R
R
H
R
RR
cis-1,3-di-tert-butylcyclohexane trans-1,3-di-tert-butylcyclohexane
where R = tert-butyl group
4.39.
a) parent = octane
4.40.
a)
methyl
ethyl
b) isopropyl or (1-methylethyl)
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CHAPTER 4
4.41.
a) 2,3,5-trimethyl-4-propylheptane
4.42.
4.43.
H
Me
Et
H
Me
H
4.44.
4.46. The energy diagram more closely resembles the shape of the energy diagram for the
conformational analysis of ethane.
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71
4.47. Two of the staggered conformations are degenerate. The remaining staggered
conformation is lower in energy than the other two, as shown below:
H
Me
Me
Me
Me
H
H
Me
H
Me
Me
Me
4.48.
a)
OH
C
l
OH
Cl
4.49.
a) has more CH
2
groups.
b) cannot adopt a chair conformation in which both groups occupy equatorial
positions.
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CHAPTER 4
4.50.
H
Cl
HCl
H
H
H
H
Cl Cl
H
H
H
H
HCl
H
Cl
4.51.
a) hexane
4.52. Each H-H eclipsing interaction is 4 kJ / mol, and there are two of them (for a total
of 8 kJ / mol). The remaining energy cost is associated with the Br-H eclipsing
interaction: 15 – 8 = 7 kJ / mol.
4.53.
OH
HO
more stable
(all groups are equatorial)
4.54.
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73
4.55.
a) The second compound can adopt a chair conformation in which all three
substituents occupy equatorial positions. Therefore, the second compound is
expected to be more stable.
4.56.
Me
M
e
Cl
Cl
Br
Br
4.57. All groups are in equatorial positions.
O
OH
OH
HO
HO
HO
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CHAPTER 4
Potential
Energy
4.59.
H
H
Br
Br
H
HHH
Br
H
H
Br
Br
Br
HH
H
H
Br
H
HH
H
Br
Increasing energy
4.60.
a) This conformation has three gauche interactions, each of which has an energy cost of
3.8 kJ / mol. Therefore, this conformation has a total energy cost of 11.4 kJ / mol
associated with torsional strain and steric strain.
4.61.
OH
OH
OH
HO
HO
OH
4.62.
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75
4.63.
cyclopropane
4.64. As mentioned in Section 4.9, cyclobutene adopts a slightly puckered conformation
in order to alleviate some of the torsional strain associated with the eclipsing hydrogen
4.65. Cyclohexene cannot adopt a chair conformation because two of the carbon atoms
are sp
2
hybridized and trigonal planar. A chair conformation can only be achieved when
all six carbon atoms are sp
3
hybridized and tetrahedral (with bond angles of 109.5º).
4.66.
a) identical compounds b) constitutional isomers
4.67.
a) the trans isomer s expected to be more stable, because the cis isomer has a very
high energy methyl-methyl eclipsing interaction (11 kJ / mol). See calculation below.
b) We calculate the energy cost associated with all eclipsing interactions in both
compounds. Let’s begin with the trans isomer. It has the following eclipsing
interactions, below the ring and above the ring, giving a total of 32 kJ / mol:
Eclipsing Interactions Below the Ring Eclipsing Interactions Above the Ring
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CHAPTER 4
Now let’s focus on the cis isomer. It has the following eclipsing interactions, below the
ring and above the ring, giving a total of 35 kJ / mol:
HH
H – H
HH
CH
3
– H
CH
– H
Eclipsing Interactions Below the Ring Eclipsing Interactions Above the Ring
4.68. With increasing halogen size, the bond length also increases. That is, the C-I bond
is longer than the C-Br bond, which is longer than the C-Cl bond. So, although iodine is
4.69.
a)
m
o
r
e
s
t
a
b
l
e
OH
Et
Cl Et
OH
Cl
b) Comparison of these chair conformations requires a comparison of the energy costs
associated with all axial substituents (see Table 4.8). The first chair conformation has
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77
4.70.
a) cis-Decalin has three gauche interactions, while trans-decalin has only two gauche
interactions.
cis-decalin
H
H
H
H
trans-decalin
H
H
H
H