4
Alkanes Chapter 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
__________________.
___________ projections are often used to draw the various conformations of a
compound.
_____________ conformations are lower in energy, while ____________
conformations are higher in energy.
The difference in energy between staggered and eclipsed conformations of ethane
is referred to as _____________ strain.
________ strain occurs in cycloalkanes when bond angles deviate from the
preferred _____.
The _______ conformation of cyclohexane has no torsional strain and very little
angle strain.
The term ring flip is used to describe the conversion of one ____________
conformation into the other. When a ring has one substituentthe equilibrium
will favor the chair conformation with the substituent in the _____________
position.
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.
58
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 4.3 Identifying and Naming Complex Substituents
PROVIDE A NAME FOR THE FOLLOWING
COMPLEX SUBSTITUENT (HIGHLIGHTED)
SkillBuilder 4.4 Assembling the Systematic Name of an Alkane
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
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
SkillBuilder 4.6 Identifying Constitutional Isomers
DETERMINE IF THESE TWO
COMPOUNDS ARE THE SAME
BY ASSIGNING A SYSTEMATIC
NAME TO EACH AND THEN
COMPARING THEM.
SkillBuilder 4.7 Drawing Newman Projections
STEP 1 - IDENTIFY THE THREE GROUPS
CONNECTED TO THE FRONT CARBON ATOM
H Br
STEP 2 - IDENTIFY THE THREE GROUPS
CONNECTED TO THE BACK CARBON ATOM
Br
CH3
H 3C
Br
Br
H
STEP 3 - ASSEMBLE THE
NEWMAN PROJECTION
FROM THE TWO PIECES
OBTAINED IN THE
PREVIOUS STEPS
CHAPTER 4
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
SkillBuilder 4.9
Drawing a Chair Conformation
DRAW A CHAIR CONFORMATION
STEP 3 - DRAW ALL THREE ECLIPSED
CONFORMATIONS AND DETERMINE WHICH ONE
HAS THE HIGHEST ENERGY INTERACTIONS
SkillBuilder 4.10
Drawing Axial and Equatorial Positions
DRAW A CHAIR CONFORMATION SHOWING ALL SIX AXIAL
POSITIONS AND ALL SIX EQUATORIAL POSITIONS
SkillBuilder 4.11 Drawing Both Chair Conformations of a Monosubstituted Cyclohexane
DRAW BOTH CHAIR CONFORMATIONS OF BROMOCYCLOHEXANE
SkillBuilder 4.12 Drawing Both Chair Conformations of Disubstituted Cyclohexanes
DRAW BOTH CHAIR
CONFORMATIONS OF THE
FOLLOWING COMPOUND
Et
Me
SkillBuilder 4.13 Drawing the More Stable Chair Conformation of Polysubstituted Cyclohexanes
DRAW BOTH CHAIR
CONFORMATIONS OF
THE FOLLOWING
COMPOUND AND
DETERMINE WHICH ONE
IS MORE STABLE
Et
Me
Cl
59
60
CHAPTER 4
Solutions
4.1.
a) parent = hexane
c) parent = heptanes
e) parent = octane
g) parent = cyclopentane
i) parent = cyclopropane
b) parent = heptane
d) parent = nonane
f) parent = heptane
h) parent = cycloheptene
4.2.
4.3.
parent = hexane
parent = pentane
parent = butane
parent = butane
parent = pentane
4.4.
Only three of the isomers will have a parent name of heptane:
4.5.
methyl
methyl
ethyl
a)
All groups are
methyl groups
ethyl
b)
methyl
methyl
methyl
c)
ethyl
d)
methyl
CHAPTER 4
methyl
propyl
e)
f)
cyclobutyl
methyl
ethyl
methyl
ethyl
g)
methyl
4.6.
a)
b)
4.7.
Systematic = (1,1-dimethylethyl)
Common = tert-butyl
a)
Systematic = (1-methylethyl)
Common = isopropyl
b)
Systematic = methyl
Common = methyl
Systematic = (2,2-dimethylpropyl)
Common = neopentyl
c)
Systematic = (1-methylethyl)
Common = isopropyl
d)
Systematic = (2-methylpropyl)
Common = isobutyl
61
62
CHAPTER 4
Systematic = (2-methylpropyl)
Common = isobutyl
Systematic = (1-methylethyl)
Common = isopropyl
Systematic = (1-methylpropyl)
Common = sec-butyl
Systematic = (1,1-dimethylethyl)
Common = tert-butyl
e)
4.8.
phenyl
ethyl
(4-ethylphenyl)
(2-methylcyclobutyl)
4.9.
pentyl
(1,1-dimethylpropyl)
(1-methylbutyl)
(1,2-dimethylpropyl)
(2-methylbutyl)
(2,2-dimethylpropyl)
4.10.
a) 3,4,6-trimethyloctane
b) sec-butylcyclohexane
c) 3-ethyl-2-methylheptane
d) 3-isopropyl-2,4-dimethylpentane
e) 3-ethyl-2,2-dimethylhexane
f) 2-cyclohexyl-4-ethyl-5,6-dimethyloctane
g) 3-ethyl-2,5-dimethyl-4-propylheptane
h) 5-sec-butyl-4-ethyl-2-methyldecane
i) 2,2,6,6,7,7-hexamethylnonane
j) 4,5-dimethylnonane
(3-methylbutyl)
(1-ethylpropyl)
CHAPTER 4
k) 2,4,4,6-tetramethylheptane
l) 2,2,5-trimethylpentane
m) 4-tert-butylheptane
n) 3-ethyl-6-isopropyl-2,4-dimethyldecane
o) 3,5-diethyl-2-methyloctane
p) 1,3-diisopropylcyclopentane
q) 3-ethyl-2,5-dimethylheptane
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
c) 2,7,7-trimethylbicyclo[4.2.2]decane
d) 3-sec-butyl-2-methylbicyclo[3.1.0]hexane
e) 2,2-dimethylbicyclo[2.2.2]octane
f) 2,7-dimethylbicyclo[3.3.0]octane
g) bicyclo[1.1.0]butane
h) 5,5-dimethylbicyclo[2.1.1]hexane
i) 3-(3-methylbutyl)bicyclo[4.4.0]decane
4.13.
a)
b)
4.14.
a) same compound
b) same compound
c) same compound
d) constitutional isomers
4.15.
c)
63
64
CHAPTER 4
4.16.
H
CH3
H
CH3
H
CH3
c)
CH3
Cl
H
H
H
CH3
d)
H
b)
CH3
Cl
H
H
Cl
H
a)
Cl
CH3
Cl
CH3
CH3
Cl
CH3
e)
H
Br
f)
CH2CH3
CH3
H
CH2CH3
CH3
Cl
H
CH3
4.17.
a)
4.18.
b)
c)
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
(calculation below):
b) The energy barrier is expected
to be approximately 16 kJ / mol
(calculation below):
6 kJ / mol
6 kJ / mol
H3C
H
H
6 kJ / mol
H3C
CH3
H
H
4 kJ / mol
H
H3C H
H3C H
6 kJ / m ol
6 kJ / m ol
4.20.
CH3
CH3
CH3
H
H
H3C
H
Lowest Energy
Me
H
H
Me
c)
Lowest Energy
Me
H
Highest Energy
Me
H
H
H
H3C H
CH3
a)
Et
H
CH3
H
H
b)
Lowest Energy
Et
Me
H
H
H
Me Me
Et
d)
H
Me
Et Et
Et
H
Me
Highest Energy
H
H
Lowest Energy
Highest Energy
Me
H
H
H
Et Et
Highest Energy
CHAPTER 4
4.21. The gauche conformations are capable of intramolecular hydrogen bonding, as
shown below. The anti conformation lacks this stabilizing effect.
H
H
O
H
H
OH
H
H
O
H
H
H
H
H
O
H
H
O
H
OH
H
H
Anti
Gauche
Gauche
4.22.
4.23.
N
a)
O
H
b)
O
4.24.
4.25.
4.26.
4.27. There are eight hydrogen atoms in axial positions and seven hydrogen atoms in
equatorial positions.
4.28.
OH
a)
OH
65
66
CHAPTER 4
NH2
NH2
b)
Cl
Cl
c)
CH3
CH3
d)
e)
4.29.
a) The bromine atom occupies an equatorial position.
Br
b)
c)
Br
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:
H
O
O
O
4.31.
Me
Me
a)
Et
Et
Me
b)
Et
Et
Me
67
CHAPTER 4
Me
Br
Br
Me
Me
c)
Br
Br
Me
d)
Me
Me
e)
Me
Me
Me
Me
f)
g)
Me
Me
h) Me
Me
4.32.
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
4.33.
Me
Cl
Me
Me
a)
b)
c)
Me
Cl
Cl
d)
Cl
Me
e)
Me
f)
Cl
Me
68
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-tertbutylcyclohexane cannot have both of its substituents in equatorial positions. Each chair
conformation has one of the substituents in an axial position, which is too high in energy.
The compound can achieve a lower energy state by adopting a twist boat conformation.
4.36. cis-1,3-dimethylcyclohexane is expected to be more stable than trans-1,3dimethylcyclohexane because the former can adopt a chair conformation in which both
substituents are in equatorial positions (highlighted below):
cis-1,3-dimethylcyclohexane
CH3
H
CH3
trans-1,3-dimethylcyclohexane
CH3
H
CH3
H
CH3
H
CH3
CH3
CH3
CH3
H3C
H3C
CH3
4.37. trans-1,4-dimethylcyclohexane is expected to be more stable than cis-1,4dimethylcyclohexane 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
CH3
CH3
H
CH3
H
H
CH3
H
CH3
H3C
CH3
CH3
H3 C
CH3
CH3
CH3
CHAPTER cis-1,3-di-tert-butylcyclohexane 4
69
4.38. can adopt a chair conformation in which both
tert-butyl groups occupy equatorial positions (highlighted below), and as a result, it is
expected to exist primarily in that conformation. In contrast, trans-1,3-di-tertbutylcyclohexane cannot adopt a chair conformation in which both tert-butyl groups
occupy equatorial positions. In either chair conformation, one of the tert-butyl groups
occupies an axial position. This compound can achieve a lower energy state by adopting
a twist-boat conformation.
cis-1,3-di-tert-butylcyclohexane
R
H
R
H
R
trans-1,3-di-tert-butylcyclohexane
R
H
R
H
R
R
R
R
R
R
where R = tert-butyl group
4.39.
a) parent = octane
b) parent = nonane
c) parent = octane
d) parent = heptane
4.40.
a)
methyl
ethyl
b) isopropyl or (1-methylethyl)
c)
methyl
propyl
d) tert-butyl or (1,1-dimethylethyl)
R
70
CHAPTER 4
4.41.
a) 2,3,5-trimethyl-4-propylheptane
b) 1,2,4,5-tetramethyl-3-propylcyclohexane
c) 2,3,5,9-tetramethylbicyclo[4.4.0]decane
d) 1,4-dimethylbicyclo[2.2.2]octane
4.42.
a) same compound
b) constitutional isomers
c) same compound
4.43.
Me
Me
H
Et
H
H
4.44.
4.45.
b)
a)
c)
4.46. The energy diagram more closely resembles the shape of the energy diagram for the
conformational analysis of ethane.
Potential
Energy
180
120
60
0
Dihedral Angle
60
120
180
CHAPTER 4
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:
Me
Me
Me
Me
H
Me
Me
Me
H
Me
H
H
Potential
Energy
Me
H
Me
Me
Me
H
4.48.
OH
Cl
OH
a)
Cl
Cl
OH
OH
Cl
b)
Cl
Cl
OH
OH
c)
4.49.
a)
has more CH2 groups.
b)
cannot adopt a chair conformation in which both groups occupy equatorial
positions.
c)
positions.
d)
positions.
cannot adopt a chair conformation in which both groups occupy equatorial
cannot adopt a chair conformation in which both groups occupy equatorial
72
CHAPTER 4
4.50.
H
H
H
H
H
Cl
H
H
Cl
H
Cl Cl
H Cl
H
H
H Cl
Potential
Energy
H
Cl
H
H
H
Cl
Cl
120
Cl
H
H
Cl
Cl
180
H
H
H
H
H
H
H
Cl
60
0
60
H
H
H
Cl
120
180
Dihedral Angle
4.51.
a) hexane
b) methylcyclohexane
c) methylcyclopentane
d) trans-1,2-dimethylcyclopentane
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.
a)
b)
more stable
more stable
CHAPTER 4
73
more stable
c)
more stable
d)
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.
b) The first compound can adopt a chair conformation in which all three
substituents occupy equatorial positions. Therefore, the first compound is
expected to be more stable.
c) The first compound can adopt a chair conformation in which both substituents
occupy equatorial positions. Therefore, the first compound is expected to be more
stable.
d) The first compound can adopt a chair conformation in which all four
substituents occupy equatorial positions. Therefore, the first compound is
expected to be more stable.
4.56.
Me
Br
Cl
Cl
Br
Me
4.57. All groups are in equatorial positions.
HO
HO
HO
O
OH
OH
4.58.
Me
Me
Me
Me
Me
Me
2,2,4,4-tetramethylbutane
All staggered conformations are degenerate, and the same is true for all eclipsed
conformations. The energy diagram has a shape that is similar to the energy diagram for
the conformational analysis of ethane:
74
CHAPTER 4
Potential
Energy
180
120
60
0
60
120
180
Dihedral Angle
The staggered conformations have six gauche interactions, each of which has an energy
cost of 3.8 kJ / mol. Therefore, each staggered conformation has an energy cost of 22.8
kJ / mol. The eclipsed conformations have three methyl-methyl eclipsing interactions,
each of which has an energy cost of 11 kJ / mol. Therefore, each eclipsed conformation
has an energy cost of 33 kJ / mol. The difference in energy between staggered and
eclipsed conformations is therefore expected to be approximately 10.2 kJ / mol.
4.59.
Increasing energy
Br
H
Br
H
H
H
H
H
Br
Br
Br
H
Br
H
H
H
Br
Br
H
H
HH
HH
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.
b) This conformation has two methyl-H eclipsing interactions, each of which has an
energy cost of 6 kJ / mol. In addition, it also has one methyl-methyl eclipsing interaction,
which has an energy cost of 11 kJ / mol. Therefore, this conformation has a total energy
cost of 23 kJ / mol associated with torsional strain and steric strain.
4.61.
OH
HO
HO
4.62.
a) equatorial
d) equatorial
OH
OH
OH
b) equatorial
e) equatorial
c) axial
f) axial
CHAPTER 4
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
atoms:
Cl
H
H
H
H
H
Cl
H
In this non-planar conformation, the individual dipole moments of the C-Cl bonds in
trans-1,3-dichlorocyclobutane do not fully cancel each other, giving rise to a small
molecular dipole moment.
4.65. Cyclohexene cannot adopt a chair conformation because two of the carbon atoms
are sp2 hybridized and trigonal planar. A chair conformation can only be achieved when
all six carbon atoms are sp3 hybridized and tetrahedral (with bond angles of 109.5).
4.66.
a) identical compounds
c) identical compounds
e) identical compounds
g) stereoisomers
i) constitutional isomers
k) stereoisomers
b) constitutional isomers
d) constitutional isomers
f) stereoisomers
h) stereoisomers
j) different conformations of the same compound
l) 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. Lets 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
H-H
eclipsing
interaction
(4 kJ / mol)
H
H3C
H
H
H
CH3
CH3 - H eclipsing
interaction
(6 kJ / mol)
CH3 - H
eclipsing
interaction
(6 kJ / mol)
Eclipsing Interactions Above the Ring
CH3 - H
eclipsing
interaction
(6 kJ / mol)
H
H3C
H
H
H
CH3
CH3 - H eclipsing
interaction
(6 kJ / mol)
H-H
eclipsing
interaction
(4 kJ / mol)
76
CHAPTER 4
Now lets 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:
Eclipsing Interactions Below the Ring
H-H
eclipsing
interaction
(4 kJ / mol)
H
H-H
eclipsing
interaction
(4 kJ / mol)
H
H3C
CH3
H
Eclipsing Interactions Above the Ring
CH3 - H
eclipsing
interaction
(6 kJ / mol)
H
H
H3C
CH3
H
H - H eclipsing
interaction
(4 kJ / mol)
H
CH3 - H
eclipsing
interaction
(6 kJ / mol)
H
CH3 - CH3 eclipsing
interaction
(11 kJ / mol)
The difference between these two isomers is therefore predicted to be (35 kJ / mol) (32
kJ / mol) = 3 kJ / mol.
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
much larger than the other halogens, the longer bond length helps to accommodate the
additional steric bulk. These two factors (increased steric bulk and increased bond
length) mostly offset each other.
4.69.
a)
OH
OH
Et
Cl
Cl
Et
more stable
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
two axial substituents: an OH group (energy cost = 4.2 kJ / mol) and a Cl group (energy
cost = 2.0 kJ / mol), giving a total of 6.2 kJ / mol. The second chair conformation has
two axial substituents: an isopropyl group (energy cost = 9.2 kJ / mol) and an ethyl
group (energy cost = 8.0 kJ / mol), giving a total of 17.2 kJ / mol. The first chair
conformation has a lower energy cost, and is therefore more stable.
c) Using the numbers calculated in part b, the difference in energy between the these two
chair conformations is expected to be (17.2 kJ / mol) (6.2 kJ / mol) = 11 kJ / mol.
Using the numbers in Table 4.8, we see that a difference of 9 kJ / mol corresponds with a
ratio of 97:3 for the two conformations. In this case, the difference in energy is more
CHAPTER 4
77
than 9 kJ / mol, so the ratio should be even higher (more than 97%). Therefore, we do
expect the compound to spend more than 95% of its time in the more stable chair
conformation.
4.70.
a) cis-Decalin has three gauche interactions, while trans-decalin has only two gauche
interactions.
H
H
H
H
H
trans-decalin
H
H
H
cis-decalin
b) trans-Decalin is incapable of ring flipping, because a ring flip of one ring would cause
its two alkyl substituents (which comprise the second ring) to be too far apart to
accommodate the second ring.
hypothetical ring flip
cannot accomodate
a six membered ring
connecting these
two substituents.