27
Chapter 2 Answer Key
P2.1
Convert the following Lewis Structures to skeletal pictures
H H H H
H C C C C H
H H H H
H
H H
H C C
H H
H
C
C
H
H H H H H H
H C C C C C C H
H H H H H H
H
H
H H
H C C
H H
H
H
C H
H
CH3CH2CH2CH2CH3
H
H
H H
C C H
H H
CH3CH(CH3)CH2CH2CH3
CH3C(CH3)2CH2CH3
P2.2
H
C
C
C
H
CH3CH(CH3)CH2CH2CH2C(CH3)2CH2CH3
Provide Newman projections for the following
a
b
Newman projection
along C1 –C2
H 3C CH3
H
H
H
H
Newman projection
along C1 –C2
H
H
c
H
H
CH3
H
H
H
d
Newman projection
along C2 –C3
CH3
H
H
H
CH3
F
H
e
F
Newman projection
along C1 –C2
H
H
F
F
H
H
f
Cl
Cl
Cl
Newman projection
along C1 –C2
H
Cl
Cl
Cl
H
H
Newman projection H 3C
along C2 –C3
H 3C
H
H
CH3
CH3
28
P2.3
Draw the conformers for the following Newman projection.
H
H
H
Cl
H
H 3C
P2.4.
F
F
H
H
H
H
H
H
H
H
Cl
Cl
CH3
CH3
CH3
H
H
H
H
F
F
H
H
F
H
H
Cl
Cl
Cl
CH3
H
CH3
CH3
Convert the following Newman projects into skeletal structures
H
CH2CH2CH3
H
H
H
H
CH3
H 3C
H
H 3C
H
H
H
H
H
F
Cl
H
H
Cl
Cl
H
H 3C
H
H
CH3
F
Cl
H
H
CH3
Cl
F
F
Cl
F
Cl
P2.5
Which two structures represent the same compound?
H
H 3C
CH3
CH3
CH3
CH2CH3
H 3C
H
H
CH3
H
CH3
CH2CH3
H 3C
H
CH3
CH3
CH3
CH2CH3
29
P2.6.
Provide the correct IUPAC name the following compounds:
a. The longest chain has 4 carbons so it is a butane,
1
3
2
4
2-methylbutane to be exact (also called isobutane)
b. The longest chain has 5 carbons so it is a pentane,
5
3
4
1
2
2-methylpentane to be exact.
c. The longest chain has 6 carbons so it is named as a hexane,
3
2
1
5
4
6
2,5-dimethylhexane. Don’t forget to include di- (a common mistake). Interestingly, this could be
numbered from either end.
d. Don’t be fooled. If you look at the horizontal chain, you might be tempted to name this as a hexane, but
you mustn’t. The longest chain has 8 carbons. Can you find it?
no
6
5
4
3
6
7
6
8
5
4
no
1
1
3
2
7
2
2
1
5
3
yes!
4
Be sure to number from the end that gives the ethyl group the lower number: 4-ethyloctane.
e.
Here the longest chain happens to be on the horizontal axis and has 8 carbons.
1
2
3
4
5
6
7
8
There are two substituents on the chain: methyl and isopropyl. Note this is not the ordinary propyl, but
rather the isopropyl because of the branching pattern. If you number from left to right the substituents
come at positions 2 & 5, but if you had numbered from right to left, the substituents would be at 4 & 7.
Rules require us to number is such a way that the substituents come out with the lowest numbers. Don’t
forget the substituents must be alphabetized. Therefore the correct name is 5-isopropyl-2methyloctane.
f.
Here’s an interesting case. There are actually 2 distinct chains that 8 carbons. Can you find them? One
of them would be named as 4-isobutyloctane and the other is 2-methyl-4-propyloctane.
1
1
2
3
4
5
6
7
no
8
2
3
4
5
6
7
yes
8
30
IUPAC has a special rule for this: choose the name that give the most number of substituents. 4Isobutyloctane is a monosubstituted octane, whilst 2-methyl-4-propyloctane is a disubstituted octane
therefore the correct name is 2-methyl-4-propyloctane.
g. Here again the longest chain has eight carbons.
1
3
2
5
4
7
6
8
The substituent has 4 carbons, but is highly branched and is in fact the tert-butyl group. The correct
name is 4-tert-butyloctane.
h. Don’t be fooled. The longest chain is not the obvious horizontal hexane chain. The longest chain has 7
carbons and is names as a heptane.
1
1
2
3
4
2
no
5
3
6
4
5
7
6
yes
The correct name is 2,2,3-trimethylheptane.
i.
The longest chain makes this a hexane. Numbering from right to left gives substituents at 2, 3, and 5
(numbering left to right gives 2, 4, & 5).
1
2
3
4
5
6
no
6
5
4
3
2
1
yes
The correct name is 2,3,5-trimethylhexane.
j.
Again hexane with methyl groups at 2 & 4 (not 3 & 5):
3
2
1
5
4
6
The correct name is 2,4-dimethylhexane.
k. This molecule has two distinct chains of 8 carbons and both are disubstituted and both are substituted at
4 & 5. Therefore we chose a numbering scheme that is consistent with alphabetization: 4-ethyl-5propyloctane.
1
l.
3
2
5
4
7
6
8
Depending on how you number this heptane, it could be either 2,2,6-trimethyl or 2,6,6-trimethyl,
1
2
3
4
5
6
7
but you must choose 2,2,6 because the numbers come out smaller: 2,2,6-trimethylheptane.
31
P2.7.
Draw skeletal structures and provide systematic names for all the isomers of each of the following (L2.6)
a) C4H10
butane
2-methylpropane
b) C5H12
pentane
2-methylbutane
hexane
2-methylpentane
2,2-dimethylbutane
3-methylpentane
2,3-dimethylbutane
2,2-dimethylpropane
c) C6H14
d) C7H16
heptane
2-methylhexane
2,2-dimethylpentane
2,2,3-dimethylbutane
3-methylhexane
3,3-dimethylpentane
2,3-dimethylpentane
2,4-dimethylpentane
3-ethylpentane
32
e) C8H18
octane
2-m3thylheptane
2,2-dimethylhexane
2,2,3-trimethylpentane
3-m3thylheptane
4-m3thylheptane
3,3-dimethylhexane
2,2-dimethylhexane
2,2,4-trimethylpentane
2,3,3-trimethylpentane
2,4-dimethylhexane
2,5-dimethylhexane
2,3,4-trimethylpentane
3-ethyl-3-methylpentane
2,2,3,3-tetramethylbutane
P2.8.
Provide the correct IUPAC name the following compounds (L2.7)
P2.9.
a
b
5-ethyl-4-methylnonane
4-ethyl-5,5,6-trimethylnonane
Provide the correct IUPAC name the following compound (L2.8)
2-ethyl-3-propylheptane
P2.10. Provide the correct IUPAC name the following compound (L2.29)
a
4-ethyl-5,5-dimethyloctane
b
4-ethyl-5-methyloctane
3,4-dimethylhexane
3-ethylhexane
3-ethyl-2-methylpentane
33
P2.10 (cont.)
d
c
e
2,3-dimethylnonane
4-isopropyloctane
1,3-diethyl-2,2-dimethylcyclopentane
P2.11. Draw the structure of 4-isopropyl-2,4,5-trimethylheptane (L2.9)
P2.12. Draw structures that correspond to the following name (L2.30)
(a) 4-isobutyl-2,5-dimethylheptane
(b) 2,3,5-trimethyl-4-propylheptane
(c) 5-sec-butyl-6-tert-butyl-2,2-dimethylnonane
P2.13. Label each carbon in the following molecules as 1˚, 2˚, 3˚, and 4˚. (L2.27)
a
1˚
2˚
2˚
4˚
2˚
2˚
2˚
3˚
1˚
b
2˚
1˚
3˚
2˚
P2.14. For the structure 4-isopropyl-2,4,5-trimethylheptane (L2.10)
1˚
4˚
2˚
1˚
1˚
34
a) Identify the 1˚, 2˚, 3˚, and 4˚ carbons
1˚
1˚
3˚
1˚
1˚
3˚
2˚
4˚
2˚
3˚
1˚
1˚
1˚
b) Circle one example each of a methyl group, an ethyl group, and isopropyl group, a sec-butyl group, and
tert-butyl group.
isopropyl
methyl
ethyl
sec-butyl
P2.15. How many carbons are in an alkane of n carbons containing
(a) two rings? CnH2n–2
(b) three rings? CnH2n–4
(c) m rings? CnH2n+2–2m
P2.16. How many rings does an alkane have if its formula is
(a) C8H10? 4 rings
(b) C7H12? 2 rings
P2.17. Name the following compounds (L2.13)
2,4-diethyl-3-methyl-1,1-dipropylcyclobutane
1-ethyl-2,4-dimethylcyclopentane
P2.18. Match each of the following isomers with the appropriate melting and boiling points (L2.16)
More highly branched molecules are more spherical. Spherical object has less contact with neighbors than
linear objects. Thus more linear molecules interact more with each other, thereby increasing the
intermolecular forces between them. Linear molecules should have higher mp and bp.
octane: mp = +100.1˚C, bp = +125.7˚C
2,2,3,3-tetramethylbutane: mp = –56.8˚C, bp = +106.5˚C
35
P2.19. Consider benzene and methylbenzene. Which has the greater boiling point? Which has the greater melting
point? (L2.17)
H
H
CH3
H
H
H
H
H
benzene
H
H
H
H
methylbenzene
Methylbenzene has more atoms, a higher molecular mass, and therefore higher mp and bp.
P2.20. Give the general balanced reaction for
(a) the complete combustion of an alkane (MF CnH2n+2)
CnH2n+2 + 3n+1 O2 → n CO2 + n+1 H2O
(b) the complete combustion of a cycloalkane containing one ring (MF CnH2n)
CnH2n + 3n O2 → n CO2 + n H2O
P2.21. Optional. Calculate the number of pounds of CO2 released into the atmosphere when a 15-gallon tank of
gasoline is burned in an automobile engine. Assume complete combustion. Also assume that gasoline is a
mixture of octane isomers and that the density of gasoline is 0.692 g/mL. Useful conversion factors: 1 gallon
= 3.785 L; 1 kg = 2.204 lb.
15-gallons gasoline = 15 gal *
P2.22. Optional. What is the carbon “footprint” (pounds of CO2 released into the atmosphere) for an automobile
which averages 25 miles to the gallon and travels 12,000 miles per year? Use the information in the
previous problem.
(L2.22)
36
#2.23
#2.24
37
#2.25
#2.26
Mass Spectrum
Rel. Abundance
100
80
60
40
20
0
0
10
20
30
40
50
60
70
80
Transmittance
Infrared Spectrum
0.8
0.6
0.4
0.2
3000
2000
1000 cm-1
90
38
#2.27
#2.28
39
#2.29
#2.30
40
#2.31
#2.32
41
#2.33
#2.34
#3
#2
#1
0.0
0.4
3000
2000
1000
cm-1
#7
0.0
0.2
0.4
0.6
0.8
0.2
0.4
0.6
0.8
0.2
0.8
0.4
0.2
#6
0.6
#5
0.2
0.4
1000 cm-1
1000 cm–1
0.6
2000
2000
0.8
3000
3000
0.4
0.6
0.8
0.8
0.2
0.4
0.6
0.8
#4
OH
O
OH
3000
OH
3000
O
3000
3000
OH
2000
2000
2000
2000
1000
cm–1
1000 cm-1
1000 cm-1
1000
cm-1
IR Pattern Recognition
#11
#10
#9
0.2
0.4
0.6
0.8
0.2
0.4
0.6
0.8
0.0
4000
0.5
1.0
0.0
0.2
0.4
0.6
0.8
#8
NH2
NH2
O
O
3000
3000
3000
3000
2000
2000
2000
2000
1000
cm-1
1000 cm-1
1000 cm-1
1000 cm-1
42