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H
NAMING ISOMERS
H
An isomer naming system is needed because there are just too
many isomers to keep track of. The system of naming the
branched-chain alkanes is described by rules agreed upon by a
professional organization, the International Union of Pure and
Applied Chemistry, or IUPAC. Here are the steps in naming
the alkane isomers.
6
C
H
H
5
C
H
H
4
H
H
3
C
C
C
H
H
H
H
2
C
H
H
1
C
H
Step 1: The longest continuous chain of carbon atoms
H
determines the base name of the molecule. The longest
continuous chain is not necessarily straight and can
take any number of right-angle turns as long as the
continuity is not broken. The base name corresponds
to the number of carbon atoms in this chain as in
Table 12.1. For example, the structure has six carbon
atoms in the longest chain, so the base name is hexane.
Step 2: The locations of other groups of atoms attached to the
base chain are identified by counting carbon atoms
from either the left or the right. The direction selected
is the one that results in the smallest numbers for
attachment locations. For example, the hexane chain
TABLE 12.1
The first 10 straight-chain alkanes
Name
Molecular
Formula
Structural Formula
Molecular
Formula
Name
Structural Formula
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
H
H
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
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
Methane
CH4
H
C
H
Hexane
H
C6H14
H
Ethane
Propane
Butane
C2H6
C3H8
C4H10
H
H
H
H
H
C
C
H
H
Heptane
C7H16
H
H
H
H
H
C
C
C
H
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
Octane
H
C5H12
H
H
H
H
Nonane
C9H20
H
H
Decane
C10H22
H
12-1
H
C8H18
H
Pentane
H
CHAPTER 12
Organic Chemistry
H
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has a CH3 attached to the third or the fourth carbon
atom, depending on which way you count. The third
atom direction is chosen since it results in a smaller
number.
Step 3: The hydrocarbon groups attached to the base chain are
named from the number of carbons in the group by
changing the alkane suffix -ane to -yl. Thus, a hydrocarbon group attached to a base chain that has one carbon
atom is called methyl. Note that the -yl hydrocarbon
groups have one less hydrogen than the corresponding
alkane. Therefore, methane is CH4, and a methyl group
is CH3. The first 10 alkanes and their corresponding
hydrocarbon group names are listed in Table 12.2. In
the example, a methyl group is attached to the third
carbon atom of the base hexane chain. The name and
address of this hydrocarbon group is 3-methyl. The
compound is named 3-methylhexane.
Step 4: The prefixes di-, tri-, and so on are used to indicate
if a particular hydrocarbon group appears on the main
chain more than once. For example,
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H
H
H
H
H
C
C
H
H
C
H
H
C
C
H
H
H
C
H
H
(or)
CH3
H
H
H
C
C
H
H
H
C
C
H
H
CH3
is 2,2-dimethylbutane and
H
TABLE 12.2
H
Alkane hydrocarbons and corresponding hydrocarbon groups
Alkane
Name
Molecular
Formula
Hydrocarbon
Group
Molecular
Formula
Methane
CH4
Methyl
–CH3
Ethane
C2H6
Ethyl
–C2H5
Propane
C3H8
Propyl
–C3H7
Butane
C4H10
Butyl
–C4H9
Pentane
C5H12
Amyl
–C5H11
Hexane
C6H14
Hexyl
–C6H13
Heptane
C7H16
Heptyl
–C7H15
Octane
C8H18
Octyl
–C8H17
Nonane
C9H20
Nonyl
–C9H19
Decane
C10H22
Decyl
–C10H21
H
C
H where * denotes unattached. The attachment takes
H
place on a base chain or functional group.
2
CHAPTER 12 Organic Chemistry
H
C
C
H
H
C
H
H
C
C
H
H
H
H
H
(or)
CH3
H
H
H
C
C
H
H
Note: —CH3 means
H
C
is 2,3-dimethylbutane.
H
C
C
H
H
H
CH3
If hydrocarbon groups with different numbers of carbon
atoms are on a main chain, they are listed in alphabetic order.
For example,
12-2
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H
H
C
H
C
C
H
H
EXAMPLE 12.2
H
H
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H
H
H
C
C
C
H
H
H
C
H
H
C
H
Write the structural formula for 2,2-dichloro-3-methyloctane.
Answer:
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
C
H
Cl
H
C
C
C
H
Cl
H
H
(or)
CH3
H
H
C
C
H
H
H
H
H
C
C
C
H
H
H
C2H5
is named 3-ethyl-2-methylpentane. Note how numbers are
separated from names by hyphens.
EXAMPLE 12.1
What is the name of an alkane with the following formula?
H
H
H
H
C
H
H
C
H
H
H
H
C
C
C
C
H
H
H
H
H
C
C
C
H
H
H
H
C
C
H
H
H
H
H
SOLUTION
The longest continuous chain has seven carbon atoms, so the base
name is heptane. The smallest numbers are obtained by counting
from right to left and counting the carbons on this chain; there is a
methyl group in carbon atom 2, a second methyl group on atom 4,
and an ethyl group on atom 5. There are two methyl groups, so the
prefix di- is needed, and the “e” of the ethyl group comes first in the
alphabet, so ethyl is listed first. The name of the compound is
5-ethyl-2,4-dimethylheptane.
12-3
CHAPTER 12
Organic Chemistry
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PARALLEL EXERCISES
Group B
Group A
1. Draw the structural formulas for (a) n-pentane and (b) an isomer
of pentane with the maximum possible branching. (c) Give the
IUPAC name of this isomer.
2. Write structural formulas for all the hexane isomers you can
identify. Write the IUPAC name for each isomer.
3. Write structural formulas for
a. 3,3,4-trimethyloctane.
b. 2-methyl-1-pentene.
c. 5,5-dimethyl-3-heptyne.
4. Write the IUPAC name for each of the following:
1. Write structural formulas for (a) n-octane and (b) an isomer of
octane with the maximum possible branching. (c) Give the
IUPAC name of this isomer.
2. Write the structural formulas for all the heptane isomers you can
identify. Write the IUPAC name for each isomer.
3. Write structural formulas for
a. 2,3-dimethylpentane.
b. 1-butene.
c. 3-ethyl-2-methyl-3-hexene.
4. Write the IUPAC name for each of the following:
H
H
A
C
H
A
B
B
H
H
H
H
H
C
C
C
C
H
H
Br
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
C
H
H
H
H
C
C
D
H
Br
C
C
H
Cl
H
H
H
H
C
C
C
C
H
5. Which would have the higher octane rating, 2,2,3-trimethylbutane or 2,2-dimethylpentane? Explain with an illustration.
4
CHAPTER 12 Organic Chemistry
C
H
C
C
C
H
H
H
H
H
H
5. Which would have the higher octane rating, 2-methyl-butane or
dimethylpropane? Explain with an illustration.
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Group A—Continued
6. Use the information in Table 12.5 to classify each of the following
as an alcohol, ether, organic acid, ester, or amide.
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Group B—Continued
6. Classify each of the following as an alcohol, ether, organic acid,
ester, or amide.
A
A
B
B
C
C
D
E
12-5
D
E
CHAPTER 12
Organic Chemistry
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SOLUTIONS TO ADDITIONAL GROUP A & B PARALLEL EXERCISES FOR CHAPTER 12
Group A
Group B
1. (a)
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
(b)
1. (a) n-octane
H
H
H
C
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
CH 3 CH 3
H
C
H
(b) maximum possible branching
H
H
H
C
C
H
CH 3
H
C
C
CH 3
CH 3 CH 3
H
H
H
C
H
(c)
(c) 2,2,3,3 tetramethylbutane
2,2-dimethylpropane
2. n-hexane
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
2. The structures can be deduced from the names. The names are
as follows: heptane; 2-methylhexane; 3-methylhexane;
2-propylbutane; 2,3-dimethylpentane.
H
3-methylpentane
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
C
H
2-methylpentane
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
C
H
H
H
2,2-dimethylbutane
H
6
H
CH3 H
H
C
C
C
C
H
CH3 H
H
H
CHAPTER 12 Organic Chemistry
12-6
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Group B—Continued
Group A—Continued
3. (a)
H
H
H
CH3 CH3 H
H
H
H
C
C
C
C
C
C
C
C
H
CH3 H
H
H
H
H
H
(b)
H
(c)
H
H
CH3 H
H
H
C
C
C
C
C
H
H
H
H
H
C
C
H
H
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C
C
3. (a) 2,3-dimethylpentane
H
H
H
CH 3 CH 3 H
H
C
C
C
C
C
H
H
H
H
H
H
(b) 1-butene
H
H
CH3 H
H
C
C
C
CH3 H
H
H
H
H
H
C
C
C
C
H
H
H
(c) 2-methyl-3-ethyl-3-hexene
H
H
H
H
H
C
C
C
CH 2 H
H
H
H
CH 3
C
C
C
H
H
H
CH 3
4. (a) 2-chloro-4-methylpentane
(b) 2-methyl-l-pentene
(c) 3-ethyl-4-methyl-2-pentene
4. (a)
(b)
(c)
(d)
5. The 2,2,3-trimethylbutane is more highly branched, so it will
have the higher octane rating.
5. Dimethylpropane would have a higher octane because it has
more branches than 2-methyl-butane:
2,2,3-trimethylbutane
H
H
H
CH3 CH3 H
C
C
C
C
H
CH3 H
H
H
H
6. (a)
(b)
(c)
(d)
(e)
12-7
CH3 H
H
H
C
C
C
C
C
H
CH3 H
H
H
alcohol
amide
ether
ester
organic acid
H
CH 3 H
C
C
H
CH 3 H
C
H
dimethylpropane
H
2,2,-dimethylpentane
H
2-bromo-3-methylbutane
5-methyl-2-hexene
2-chloro-2,4-hexadiene
1-bromo-2-butene
H
CH 3 H
H
C
C
C
C
H
H
H
H
H
2-methyl-butane
H
6. (a)
(b)
(c)
(d)
(e)
alcohol
organic acid
ether
amine
ester
CHAPTER 12
Organic Chemistry
7