Alkanes - Isomerism
Number of
Carbons
6
Molecular
Formula
C6H14
Number of
Isomers
5
7
C7H16
9
12
C12H26
75
15
C15H32
4347
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Chapter 2
1
Nomenclature

Organic nomenclature is based on a
class of compounds known as alkanes


Contain only carbon and hydrogen
All carbon atoms are sp3 hybridized
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Chapter 2
2
Nomenclature




International Union of Pure and Applied Chemistry
(IUPAC) has established the system
We will refer to names derived from this system
as systematic nomenclature or IUPAC
nomenclature (shown in blue)
Additional names such as isopentane and
neopentane are common names (shown in red)
A compound may have more than one name BUT
a name must specify only one compound
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Chapter 2
3
Nomenclature

Typically any unspecified alkyl group is
represented as R© Prentice Hall 2001
Chapter 2
4
Nomenclature

Alkyl substituent group names are combined as
needed with the alkane name for the longest
continuous straight chain to constitute a
compound name
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Note that these compounds
all have the formula C7H16;
they are isomers
Chapter 2
5
IUPAC Systematic
Nomenclature - Alkanes
1. Determine longest continuous chain (i.e.
parent hydrocarbon)
2. Cite the name of substituent before the
name of the parent hydrocarbon along
with the number of the carbon to which it
is attached
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Chapter 2
6
IUPAC Systematic
Nomenclature - Alkanes
3. Number in the direction that gives the
lower number for the lowest-numbered
substituent. Substituents are listed in
alphabetical order – neglecting
prefixes such as di- tri- tert- etc.
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Chapter 2
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IUPAC Systematic
Nomenclature - Alkanes
4. When both directions
yield the same lower
number for the
lowest numbered
substituent, select
the direction that
yields the lower
number for the next
lowest numbered
substituent
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Chapter 2
8
IUPAC Systematic
Nomenclature - Alkanes
5. If same substituent numbers are
obtained in either direction, number in
direction giving lowest number to the
first named substituent
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Chapter 2
9
IUPAC Systematic
Nomenclature - Alkanes
6. If compound has two or more chains of
the same length, parent hydrocarbon is
chain with greatest number of
substituents
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Chapter 2
10
IUPAC Systematic
Nomenclature - Alkanes
7.
Names such as sec-butyl and tert-butyl are
acceptable, but systematic substituent names are
preferable
Numbering of the substituent begins with the carbon
attached to the parent hydrocarbon
This number together with the substituent name is
placed inside parentheses
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Chapter 2
11
Alkyl Nomenclature


A primary carbon is one that is bonded to
only one other carbon, frequently
represented by 1
A primary hydrogen is attached to a
primary carbon
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Chapter 2
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Alkyl Nomenclature


A secondary carbon is one that is bonded to
two other carbons, represented by 2, also by
sec- or sA secondary hydrogen is attached to a
secondary carbon
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Chapter 2
13
Alkyl Nomenclature


A tertiary carbon is one that is bonded to
three other carbons, represented by 3,
also by tert- or tA tertiary hydrogen is attached to a
tertiary carbon
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Chapter 2
14
Alkyl Nomenclature

Reactivity often depends on whether a
carbon or hydrogen is 1, 2, or 3
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Chapter 2
15
Nomenclature of Cycloalkanes

Cycloalkanes generally are shown as skeletal
structures
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Chapter 2
16
Nomenclature of Cycloalkanes
1. Ring is the parent hydrocarbon unless the
alkyl substituent has more carbons; in that
case the substituent becomes the parent
hydrocarbon
If only one substituent, no need to give it a
number
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Chapter 2
17
Nomenclature of Cycloalkanes
2. If the ring has 2 substituents, list in
alphabetical order and give number 1 to
first named group
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Chapter 2
18
Nomenclature of Cycloalkanes
3. If there is more than one substituent, list in
alphabetical order; one substituent is given
the position number 1; number either
clockwise or counterclockwise - lowest
numbers
© Prentice Hall 2001
Chapter 2
19