27

Chapter 2 Answer Key

P2.1 Convert the following Lewis Structures to skeletal pictures

P2.2 Provide Newman projections for the following

a b

c d

e f

H C C C CH H H H

H H H HH C C C C C C

H H H H H

H H H H H

H C C C C HH H C H

H H H HH C C C C C H

H H C H H

H H H H H

H HH

H

HHH C

H

HHHH

CH3CH2CH2CH2CH3 CH3CH(CH3)CH2CH2CH3

CH3C(CH3)2CH2CH3 CH3CH(CH3)CH2CH2CH2C(CH3)2CH2CH3

H3C CH3Newman projection along C1–C2

H

H HH

HHH

H HCH3

HH

Newman projection along C1–C2

CH3

H HCH3

HHNewman projection along C2–C3

FF

F

H HF

HH

Newman projection along C1–C2

ClCl

ClCl

H HCl

HCl

Newman projection along C1–C2

H

H3C CH3

H

CH3H3C

Newman projection along C2–C3

28

P2.3 Draw the conformers for the following Newman projection.

P2.4. Convert the following Newman projects into skeletal structures

P2.5 Which two structures represent the same compound?

H

H HH

CH3H3C

H

H HCH2CH2CH3

HH3CH

Cl ClF

FH

CH3

H CH3

CH3

CH2CH3H3CCH3

H H

CH2CH3H

CH3H3C

CH2CH3

H3C CH3

CH3

CH3H

F

H HH

FHF

H HH

HFF

H HF

HH

Cl

H HCl

ClHCl

H HH

ClClCl

H HCl

HCl

CH3

H HCH3

CH3HCH3

H HH

CH3H3CCH3

H CH3

CH3

HH3C

ClF

Cl

F

29

P2.6. Provide the correct IUPAC name the following compounds:

a. The longest chain has 4 carbons so it is a butane,

2-methylbutane to be exact (also called isobutane)

b. The longest chain has 5 carbons so it is a pentane,

2-methylpentane to be exact.

c. The longest chain has 6 carbons so it is named as a hexane,

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?

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.

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-2-methyloctane.

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 23

4

123

45

12

34

5 6

12

34

56

1

23

456

7

12

34

5

67

8

no no yes!

1 23 4

56

7 8

1 23

45 6 7

8

1

23

45 6 7

8

no yes

30

IUPAC has a special rule for this: choose the name that give the most number of substituents. 4-Isobutyloctane 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.

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.

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).

The correct name is 2,3,5-trimethylhexane.

j. Again hexane with methyl groups at 2 & 4 (not 3 & 5):

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-5-propyloctane.

l. Depending on how you number this heptane, it could be either 2,2,6-trimethyl or 2,6,6-trimethyl,

but you must choose 2,2,6 because the numbers come out smaller: 2,2,6-trimethylheptane.

1 23

45 6 12

3456no yes

1 23

45

67

8

1 23 4 5 6

12

3 4 5 6 7no yes

1 2 34

56

7 8

1 23 4

5 6 7

12

34

5 6

31

P2.7. Draw skeletal structures and provide systematic names for all the isomers of each of the following (L2.6)

a) C4H10

b) C5H12

c) C6H14

d) C7H16

butane 2-methylpropane

pentane 2-methylbutane 2,2-dimethylpropane

hexane 2-methylpentane 2,2-dimethylbutane

3-methylpentane 2,3-dimethylbutane

heptane

2-methylhexane

2,2-dimethylpentane 3,3-dimethylpentane 2,3-dimethylpentane 2,4-dimethylpentane

2,2,3-dimethylbutane

3-methylhexane

3-ethylpentane

32

e) C8H18

P2.8. Provide the correct IUPAC name the following compounds (L2.7)

a b

5-ethyl-4-methylnonane 4-ethyl-5,5,6-trimethylnonane

P2.9. 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)

4-ethyl-5,5-dimethyloctane 4-ethyl-5-methyloctane

octane

2-m3thylheptane 3-m3thylheptane 4-m3thylheptane

2,2-dimethylhexane 3,3-dimethylhexane 2,2-dimethylhexane 2,4-dimethylhexane 2,5-dimethylhexane 3,4-dimethylhexane 3-ethylhexane

2,2,3-trimethylpentane 2,2,4-trimethylpentane 2,3,3-trimethylpentane 2,3,4-trimethylpentane 3-ethyl-3-methylpentane 3-ethyl-2-methylpentane

2,2,3,3-tetramethylbutane

a b

33

P2.10 (cont.)

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)

P2.14. For the structure 4-isopropyl-2,4,5-trimethylheptane (L2.10)

c d e

a b1˚

2˚

3˚

4˚

1˚2˚

3˚4˚

2˚

2˚2˚

2˚

2˚

2˚

1˚

1˚1˚

1˚

34

a) Identify the 1˚, 2˚, 3˚, and 4˚ carbons

b) Circle one example each of a methyl group, an ethyl group, and isopropyl group, a sec-butyl group, and tert-butyl group.

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

1˚

2˚3˚

4˚

1˚1˚

1˚

1˚1˚ 1˚

2˚

3˚

3˚

isopropyl

ethyl

sec-butyl

methyl

35

P2.19. Consider benzene and methylbenzene. Which has the greater boiling point? Which has the greater melting

point? (L2.17)

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)

HH

HH

H

HCH3

H

HH

H

H

benzene methylbenzene

36

#2.23

#2.24

37

#2.25

#2.26

Infrared Spectrum

cm-1

Tran

smitt

ance

100020003000

0.2

0.4

0.6

0.8

Mass Spectrum

Rel.

Abun

danc

e

30 60 900

20

40

60

80

100

0 10 20 40 50 70 80

38

#2.27

#2.28

39

#2.29

#2.30

�

40

#2.31

#2.32

41

#2.33

#2.34

42

cm–1

1000

2000

3000

0.2

0.4

0.6

0.8

cm-1

1000

2000

3000

0.2

0.4

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0.8

cm-1

1000

2000

3000

0.0

0.4

0.8

1000

cm

-120

0030

00

0.0

0.2

0.4

0.6

0.8

1000

cm

-120

0030

0040

00

0.51.0 0.0

1000

cm

-120

0030

00

0.2

0.4

0.6

0.8

1000

cm

-120

0030

00

0.2

0.4

0.6

0.8

1000

cm

-120

0030

00

0.2

0.4

0.6

0.8

cm–1

1000

2000

3000

0.0

0.2

0.4

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0.8

cm-1

1000

2000

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0.2

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cm-1

1000

2000

3000

0.2

0.4

0.6

0.8

IR P

atte

rn R

ecog

nitio

n

OH

OH

OH

O

OH

O

O O NH

2

NH

2

#1 #2 #3

#4 #5

#6

#7

#8 #9 #10

#11