Learning Objective: To learn about positional isomerism, a subtype of structural isomerism.
Skill Level – Intermediate
Prerequisites:
IUPAC writing guidelines
Related –
Chapter: Structural Isomerism
Author's Note: Positional isomerism is when two structures have the same arrangement of carbon atoms in a chain; however, their functional group is placed at different locations. Whether the functional group is internal or external dramatically affects the chemical reactivity.
For instance, look at the first example of positional isomers shown below - propan-1-ol and propan-2-ol, where the position of the alcohol functional group is at carbon 1 and 2 of the three-carbon chain. If you want to know its impact, simply subject these two compounds, which belong to the functional group alcohol, to an oxidation reaction; we will obtain two different products containing different functional groups – aldehyde and ketone, only because one was primary alcohol, and the other was secondary. But, if you subject propan-1-ol and propan2-ol to a dehydration reaction, both would give propene. So, the utility of positional isomers is very subjective to the desired reaction and product. Specific reactions favor one positional isomer over the other based on its stability. Therefore, knowledge of these isomers is of great importance.
This section will cover common types of positional isomerism seen in organic chemistry with examples.
Positional Isomerism
Compounds with the same structural arrangement of the carbon chain but differ only in the position of multiple bonds (double or triple) or functional groups are called position isomers. The phenomenon is called positional isomerism.
Positional isomers are commonly encountered in organic compounds, particularly in alkanes, alkenes, alcohols, and aromatic compounds.
A few examples are-
Position isomers – functional groups
The propan-1-ol and propan-2-ol are positional isomers where the position of the alcohol (-OH) functional group changes, keeping the carbon framework intact.
The two positional isomers of C4H9Br are shown below. In these isomers, only the position of the -Br substituent group changes.
Position isomers – double bond
In these isomers, only the position of the double bond changes. For example, C5H10 has a double bond in two different positions.
Positional isomers of the benzene ring
Unsubstituted and monosubstituted benzene rings show no isomerism. However, disubstituted benzenes can show positional isomerism where the substituents can be 1,2 (ortho), 1,3 (meta-one carbon separation), and 1,4 (para-two carbon separation) substituted.
For example, ortho, meta, and para-xylene are positional isomers.
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