Structural Isomers: Molecules with the Same Formula, Different Structures
Types of Structural Isomers
Structural isomers can be grouped into three main types based on how their structures differ:
Chain Isomers: Rearranging the Carbon Backbone
Chain isomers
Chain isomers differ in the arrangement of the carbon skeleton.
The molecular formula remains the same, but the carbon atoms are connected in different ways, forming either straight chains or branched chains.Example
Butane ($C_4H_{10}$) has two chain isomers:
- n-Butane: A straight chain of four carbon atoms.
- 2-Methylpropane: A branched chain where one carbon atom forms a side group (methyl group) attached to a three-carbon chain.
Think of chain isomers like rearranging train cars: the same number of cars (atoms) can be connected in a straight line or with one car branching off to the side.
Positional Isomers: Shifting Functional Groups
Positional isomers
Positional isomers occur when the functional group or substituent is attached to different positions on the same carbon chain.
This subtle shift in position can alter the compound’s reactivity and physical properties.Example
Bromopentane ($C_5H_{11}Br$) has three positional isomers:
- 1-Bromopentane: The bromine atom is attached to the first carbon.
- 2-Bromopentane: The bromine atom is attached to the second carbon.
- 3-Bromopentane: The bromine atom is attached to the third carbon.
- Do not confuse positional isomers with chain isomers.
- Positional isomers have the same carbon backbone, while chain isomers differ in the carbon skeleton.
Functional Group Isomers: Different Functional Groups
Functional group isomers
Functional group isomers have the same molecular formula but belong to entirely different functional group families.
This occurs when the atoms are rearranged to form different functional groups.Example
- $C_2H_6O$: Ethanol (an alcohol) and methoxymethane (an ether).
- $C_3H_6O$: Propanal (an aldehyde) and propanone (a ketone).
Functional group isomers often exhibit dramatically different chemical and physical properties because they belong to distinct compound classes.
Recognizing and Naming Isomers
To identify and name structural isomers:
- Count the carbon atoms in the longest continuous chain to determine the parent name.
