🔬 Positional Isomer Nomenclature
Brief Overview
This note covers positional isomer chemistry and was created from a 5-page PDF. It
introduces how identical carbon skeletons can differ by functional group placement and
walks through systematic naming for alkenes and alcohols.
Key Points
Identify the longest carbon chain containing the functional group.
Number the chain from the end nearest the functional group to determine
locants.
Apply IUPAC suffixes: “-ene” for alkenes, “-anol” for alcohols.
Review common examples: butene, pentene, hexene, propanol, butanol,
pentanol.
Definition of Positional Isomers
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Positional isomers are molecules that have the same molecular formula, the same
carbon skeleton, but differ in the position that the functional group occupies.
Naming Positional Isomers of Alcohols and Alkenes
Step-by-Step Naming Process:
1. Find the longest continuous carbon chain containing the functional group.
This determines the base name with suffixes 'ene' (for alkenes) or 'anol' (for
alcohols).
2. Number the chain from the end closer to the functional group. This identifies
the locant - the number indicating the functional group's position.
Positional Isomerism in Alkenes
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Starting Point: Butene
The simplest alkene demonstrating positional isomerism is butene (C H ), which exists
as:
4
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8
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but-1-ene (or 1-butene): double bond between carbons 1 and 2
but-2-ene (or 2-butene): double bond between carbons 2 and 3
Unbranched Isomers of Pentene and Hexene
The image above displays the systematic arrangement of pentene and hexene isomers,
showing how the double bond position affects the molecule's structure while maintaining
the same carbon skeleton.
Key Examples:
Pentene isomers (C H ):
5
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10
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pent-1-ene: double bond at position 1
pent-2-ene: double bond at position 2
Hexene isomers (C H ):
6
​
12
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hex-1-ene: double bond at position 1
hex-2-ene: double bond at position 2
hex-3-ene: double bond at position 3
Positional Isomerism in Alcohols
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Starting Point: Propanol
Alcohols begin showing positional isomerism with propanol (C H O):
3
propan-1-ol (1-propanol): OH group on carbon 1
propan-2-ol (2-propanol): OH group on carbon 2
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8
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Extended Examples:
Butanol isomers (C H O):
4
10
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butan-1-ol: OH group on carbon 1
butan-2-ol: OH group on carbon 2
Pentanol isomers (C H O):
pentan-1-ol: OH group on carbon 1
pentan-2-ol: OH group on carbon 2
pentan-3-ol: OH group on carbon 3
5
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12
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The image above illustrates the systematic naming approach for alcohols, showing how
the position of the hydroxyl group determines the compound's name while maintaining
identical molecular formulas.