CHAPTER 8: ANSWERS TO SELECTED PROBLEMS
SAMPLE PROBLEMS (“Try it yourself”)
8.1
H
H
H
H
H
H
C
C
C
C
C
H
H
This carbon atom forms five bonds.
This carbon atom only forms three bonds.
8.2
H
8.3
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H
CH3–CH2–CH2–CH2–CH2–CH3
8.4
8.5
a) This molecule contains only five carbon atoms and twelve hydrogen atoms, so it is not
an isomer of hexane.
b) This molecule has the same molecular formula as hexane (C6H14), but it does not have
the same structure (it is branched while hexane is not). Therefore, this molecule is an isomer of
hexane.
8.6
cycloheptane
8.7
4-propyloctane
8.8
4,4-diethyl-3-methylheptane
8.9
CH3
CH3 CH2
CH3 CH2 CH
CH CH2 CH2 CH2 CH3
8.10 Only molecule c (propanoic acid) has the same functional group as acetic acid, so
propanoic acid should show similar chemical behavior to acetic acid.
O
C
OH
The functional group in acetic
acid and propanoic acid.
8.11
F
F
C
C
F
8.12
F
2-heptene
8.13
H
H
H
C
C
H
H
C
C
H
H
C
C
H
H
CH3 CH2 C
H
(full structural formula)
8.14
C
CH2 CH3
(condensed structural formula)
cyclooctene
8.15 2-heptene does have cis and trans forms, because the double bond is not at the #1
position.
8.16
cis-2-nonene
8.17
CH3 CH2
CH2 CH2 CH3
C
C
H
H
8.18 These two molecules are structural isomers. To convert 2-hexene (either cis or trans) into
3-hexene (either cis or trans), you must move a hydrogen atom from carbon #4 to carbon #2 (try
it!).
8.19 The second molecule contains an aromatic ring. The right-hand ring in this molecule
contains six carbon atoms linked by alternating single and double bonds.
8.20
alkene
group
CH3
OH
CH
CH2
alkene
group
O
8.21 Propylene is a hydrocarbon that contains only three carbon atoms, so it is a gas at room
temperature.
8.22 Cyclohexane has the higher boiling point, because it is a larger molecule than
cyclopentane.
8.23
2 C3H6 + 9 O2 → 6 CO2 + 6 H2O
SECTION PROBLEMS
Section 8.1
8.1
A carbon atom has four valence electrons, so it needs four more electrons to satisfy the
octet rule. The atom gets one additional electron when it shares an electron pair with another
atom, so it must share four electron pairs.
8.2
In a tetrahedral arrangement, there is an atom at each corner of the three-sided pyramid,
and there is one atom at the center of the pyramid. Each corner atom is bonded to the central
atom. CH4 contains a tetrahedral arrangement, as does any other molecule that contains four
atoms bonded to carbon (CF4, CCl4, etc.).
8.3
The atoms that do not form the correct number of bonds (four for carbon, one for
hydrogen) are circled.
H
H
H
H
H
H
C
C
C
C
C
H
H
H
C
C
C
H
H
H
H
H
H
C
C
C
H
H
H
H
H
H
C
H
H
H
Section 8.2
8.4
Only the first molecule is an alkane. Alkanes contain only carbon and hydrogen atoms,
and they do not contain double or triple bonds.
8.5
CH3–CH2–CH2–CH2–CH2–CH3
8.6
H
8.7
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
(full structural formula)
H
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH3
(Condensed structural formula)
8.8
8.9
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
H
H
H
H
H
H
H
CH3 CH2 CH2 CH2 CH2 CH2 CH3
H
(full structural formula)
(Condensed structural formula)
8.10 The central carbon atom makes five bonds in this structure, giving it ten electrons in its
valence shell. Carbon can only share four electron pairs, never five.
Section 8.3
8.11
a) linear alkane
d) cycloalkane
8.12
b) branched alkane
e) branched alkane
CH3
c) linear alkane
CH3
CH3 CH
CH2 CH
8.13
CH3
H
H
H
H
C
C
H
H
H
C
H
H
C
H
H
H
C
C
C
H
H
H
H
8.14
CH3
8.15
CH3
a) CH3 CH CH2 C
CH3
CH3
CH CH2 CH2 CH3
CH2 CH2
CH3
8.16
b) CH2
CH2
This carbon atom makes five bonds.
CH3 CH2 CH2 CH2 CH2 CH3
8.17
a) These two molecules are isomers.
b) These two molecules are actually the same compound (pentane) drawn in two different
ways, so they are not isomers.
c) These two molecules have different molecular formulas (hexane is C6H14 and
cyclohexane is C6H12), so they are not isomers.
Section 8.4
8.18
a propyl group
8.19
a) 3-methylpentane
c) 3-ethyl-2-methylheptane
e) 2,2,3-trimethylpentane
g) cyclohexane
b) 2-methyloctane
d) 2,3-dimethylhexane
f) 5,5-diethyl-2-methylheptane
h) methylcyclopentane
CH2 CH2 CH3
8.20
CH3
a) CH3 CH2 CH2 CH CH2 CH2 CH3
b) CH3 C
CH2 CH2 CH3
CH3
CH3 CH2 CH2 CH2 CH3
c) CH3 CH2 CH CH CH2 CH2 CH2 CH2 CH3
CH3 CH3 CH3 CH2 CH2 CH2 CH3
d) CH3 CH CH CH CH CH2 CH2 CH2 CH2 CH3
CH2
e) CH2
CH2
or
CH2 CH2
f)
CH2 CH CH2 CH2 CH3
CH2 CH2 CH3
or
CH2 CH2
8.21 a) This name corresponds to the molecule shown below, but the principal chain has been
numbered in the wrong direction. The correct name is 2-methylhexane.
CH3
CH3 CH2 CH2 CH2 CH CH3
b) The alkyl groups must be listed in alphabetical order, so the correct name is 3-ethyl-2methylpentane.
Section 8.5
8.22 The second and third molecules contain functional groups. (A cycloalkane is not a
functional group; it is just a special kind of alkane.)
8.23 Molecule c should behave similarly to 1-butene, because it contains the same functional
group (two carbon atoms connected by a double bond). Molecules a and b also contain double
bonds, but these bonds do not connect two carbon atoms.
Section 8.6
8.24
a) Alkenes contain a carbon-carbon double bond (C=C).
b) Alkynes contain a carbon-carbon triple bond (C≡C).
8.25 The atoms on each side of the double bond adopt the trigonal planar arrangement, as
shown below.
Cl
Cl
C
C
Cl
8.26
The atoms attached to a triple bond line up, as shown below.
Cl
8.27
8.28
Cl
a) 1-hexene
d) propyne
a)
or
c) HC
C
CH3
C
C
Cl
b) cyclohexene
e) 6-methyl-3-heptene
HC
CH
H2C
CH2
c) 2-heptyne
f) 3-ethyl-2-methyl-2-pentene
b) CH3 CH2 CH CH CH2 CH2 CH2 CH3
CH2 CH2 CH2 CH3
CH2
e) CH2 CH CH CH2 CH2 CH3
d) HC
CH3
f) CH3 C
CH3
8.29
CH3
8.30
CH3 C
CH3
CH CH CH2 CH2 CH3
CH
C
C
CH2 CH2 CH2 CH3
8.31 a) The second carbon from the left is making five bonds, which is not possible. The
correct structure is:
CH3
CH3 C
CH CH3
b) Both of the first two carbon atoms are making five bonds, which is not possible. The
correct structure is:
CH
C
CH3
Section 8.7
8.32 Only 2-pentene has cis and trans forms. Alkenes that have the double bond at the end of
the chain (1-alkenes) do not have cis and trans forms.
8.33
a) trans-3-heptene
CH3
8.34
CH2 CH2 CH2 CH3
C
H
b) cis-4-octene
C
H
8.35 a) These are geometric isomers, because you can interconvert them without breaking any
bonds.
b) These are structural isomers. Whenever you move a double bond from one position to
another (the 3 position to the 2 position in this case), you must also move a hydrogen atom from
one carbon atom to another. This requires breaking a covalent bond.
c) These are structural isomers. This is similar to part b.
d) These molecules are not isomers, because they have different molecular formulas. cis3-hexene is C6H12, but cyclohexene is C6H10.
Section 8.8
8.36
methylbenzene
8.37
ethylbenzene
CH2 CH2 CH3
8.38
8.39 Only the middle compound contains an aromatic ring. An aromatic ring must contain
three double bonds, alternating around the ring.
8.40
This molecule contains two alkene groups, an alkyne group, and an aromatic ring.
CH
alkene
CH
C
CH
alkyne
alkene
aromatic ring
Section 8.9
8.41 The first molecule (benzene) is a liquid, and the second is a solid. In general, the larger
the hydrocarbon is, the higher its melting and boiling points, because the dispersion force is
stronger for larger molecules.
8.42 Pentane does not contain any oxygen or nitrogen atoms, so it cannot form hydrogen
bonds. Therefore, there is no significant attraction between water molecules and pentane
molecules.
8.43
2 C2H2 + 5 O2 → 4 CO2 + 2 H2O
8.44 These two molecules are both hydrocarbons and are similar sizes, so they should have
similar boiling points.
CUMULATIVE PROBLEMS (Odd-numbered problems only)
8.45 Sulfur can only form two covalent bonds, because it has six valence electrons. Therefore,
chains of sulfur atoms cannot be connected to other atoms such as hydrogen, oxygen, nitrogen,
or carbon (except at the ends). As a result, sulfur cannot form as great a variety of compounds as
can carbon.
8.47 Carbon has four valence electrons, so it needs four more electrons to satisfy the octet
rule. As a result, carbon shares four pairs of electrons in virtually all of its compounds. The
carbon atom in CH3 does not satisfy the octet rule, so this molecule is unstable.
8.49 The four chlorine atoms are arranged around the carbon atom in a tetrahedral
arrangement. See Figure 8.1 for an illustration.
8.51 The first and third molecules (CH4 and C2H6) contain tetrahedral arrangements.
Whenever there are four atoms bonded to a central carbon, the four atoms form a tetrahedral
arrangement.
8.53 The second structure is correct. Carbon makes four bonds, while hydrogen and chlorine
only form one bond. Therefore, the carbon atom must be in the center of this molecule, with the
other four atoms bonded to it.
8.55
The first molecule is an alkane, the second is an alkyne, and the third is an alkene.
CH2
8.57
CH3 CH2 C
CH2 CH3
CH2 CH C
CH
8.59 Alkenes contain a carbon-carbon double bond (C=C), so alkenes must contain at least
two carbon atoms. An alkane need not contain any carbon-carbon bonds, so methane (CH4) is
classified as an alkane.
8.61
The molecular formula is C13H28. The condensed structural formula of this molecule is:
CH3
CH3 CH2 CH2 CH2 CH2 CH CH2 CH2 CH2 CH2 CH2 CH3
CH3
8.63
a) CH3 CH2 CH2 CH2 CH3
b) CH3 CH CH2 CH2 CH2 CH3
CH3 CH2
CH3
c) CH3 CH2 C CH2 CH2 CH2 CH3
d) CH3 CH CH2 CH CH2 CH2 CH2 CH3
CH3 CH2
CH3 CH2
CH2
e)
or
f)
CH2 CH2 CH3
CH2 CH2
or
CH2
CH2
CH CH2 CH2 CH3
CH2 CH2
8.65
a)
b)
c)
e)
d)
f)
8.67
a) CH3 C
CH3
b)
CH2 CH2 CH2 CH3
C
C
H
C
H
CH2 CH2 CH3
CH2
c) CH2 CH2
d)
or CH2
CH2
CH CH
e)
CH3 CH2 CH2
CH2
C
CH2 CH2 CH3
f)
or
CH
CH
CH
CH
CH
CH
CH3
CH3
g) CH3 C
C
C
CH2 CH3
h)
CH3
8.69
CH3
C
H
a)
or
b)
c)
CH CH2 CH3
C
H
8.71
a) pentane
b) 3-ethylhexane
c) 2,2,7-trimethyloctane
d) 4-ethyl-2,2-dimethylheptane
e) cyclopentane
f) methylcycloheptane
8.73
a) 2-pentyne
b) trans-2-octene
c) 4-methyl-1-pentene
d) 2,5-dimethyl-3-heptyne
e) cyclopentene
f) 6-ethyl-7-methyl-cis-3-octene
g) ethylbenzene
8.75 a) These two molecules are isomers. Both have the molecular formula C6H14, but they
have different structures.
b) These two molecules are not isomers, because they have different molecular formulas.
Pentane is C5H12, but 2-methylpentane is C6H14.
c) These two molecules are not isomers, because they have different molecular formulas.
1-hexene is C6H12, but cyclohexene is C6H10.
d) These two molecules are isomers. Both have the molecular formula C6H12, but they
have different structures.
e) These two molecules are isomers. Both have the molecular formula C7H14, but they
have different structures.
f) These two molecules are isomers. Both have the molecular formula C7H16, but they
have different structures.
8.77 There are seven possible molecules. Here are the structures of five of them. The first
one is the easiest to find: simply move the methyl group to a different carbon atom. The others
have a shorter principal chain and more (or longer) branches.
CH3
CH3
CH3 CH2 CH CH2 CH2 CH3
CH3
C
CH3 CH3
CH2 CH2 CH3
CH3 CH CH CH2 CH3
CH3
3-methylhexane
2,3-dimethylpentane
2,2-dimethylpentane
CH2 CH3
CH3 CH3
CH3 CH2 CH CH2 CH3
CH3
3-ethylpentane
C
CH CH3
CH3
2,2,3-trimethylbutane
8.79
The only possible answer is cis-3-octene.
8.81 Only 3-hexene (choice c) has cis and trans forms. Alkenes have two forms (geometric
isomers) if the double bond is not at the #1 position. Alkynes and alkanes never have cis and
trans forms.
8.83 The molecular formula of 2-pentyne is C5H8. (CH3–C≡C–CH2–CH3) Molecules b and d
also have this molecular formula, so they are isomers of 2-pentyne. The other molecules have
different molecular formulas (molecules b and c are C5H6, molecule e is C6H10).
8.85
Acetaminophen contains an aromatic ring, but fumigatin does not.
8.87
HO CH2 CH2 CH2 CH2
alkyne
alkyne
C
C
C
C
alkene
CH
CH
alkene
CH
alkene
CH
CH
CH
CH
Cicutoxin
(a toxic compound found in water hemlock)
CH3 CH2 CH2
OH
O
CH3
O
alkene
CH2 CH CH2
O
CH2
C
N
CH2
CH3
CH2 CH3
aromatic ring
Estil
(an anesthetic)
8.89 Pentane has a higher boiling point than propane. Both compounds are alkanes, but
pentane is a larger molecule than propane, so the dispersion force that attracts pentane molecules
to one another is stronger than the dispersion force between propane molecules.
8.91 Melting points can be affected dramatically by the shape of the molecule, whereas boiling
points of molecules that are the same size are rather similar.
8.93
2 C6H14 + 19 O2 → 12 CO2 + 14 H2O
8.95 1.00 gram of methane produces more heat. The first equation tells you that you get 213
kcal of heat when you burn 16.042 g of methane (the formula weight of CH4 is 16.042). The
second equation tells you that you get 531 kcal of heat when you burn 44.094 g of propane (the
molecular weight of C3H8 is 44.094). Use both of these as conversion factors:
1.00 g CH 4 ×
1.00 g C 3H 8 ×
€
213 kcal
16.042 g CH 4
531 kcal
44.094 g C 3H 8
= 13.3 kcal of heat (when you burn 1.00 g CH 4 )
= 12.0 kcal of heat (when you burn 1.00 g C3H 8 )
8.97 You have 0.0347 moles of pentane (the calculator answer is 0.03465196 moles). Pentane
has the molecular formula C5H12 and a formula weight of 72.146, so one mole of pentane weighs
72.146 g. Use this as a conversion factor to convert 2.50 g into moles. (Review Section 5.6 if
you have forgotten how to do this.)
8.99 Any carbon atom that makes four single bonds will be the center of a tetrahedral
arrangement. In this molecule, there is only one carbon atom that forms four single bonds.
CH2 CH CH2
C
CH
8.101 a) The density of toluene is 0.867 g/mL (the calculator answer is 0.86666667 g/mL), and
the specific gravity is 0.867. Density and specific gravity are covered in Section 1.7.
b) The toluene will float, because its density is lower than the density of water. (The
density of water is about 1 g/mL.)
8.103 5 ppb equals 5 ng/mL (see page 5-32). A nanogram is a billionth of a gram, so the
solubility of benzene in water is 600,000,000 ng/L, or 600,000 ng/mL. The solubility of benzene
in water is much higher than the maximum level in drinking water. Therefore, benzene in
drinking water can be a serious concern, because the concentration of benzene in drinking water
could be far higher than the safe limit.
8.105 Combustion reactions supply most of the energy that we use, either directly (in
automobiles, airplanes, buses, etc.) or indirectly (in power plants that convert the heat of
combustion into electrical energy).