Structural isomers, also known as constitutional isomers, are molecules that have the same chemical formula but a different physical arrangement of the atoms and/or bonds. They are a common type of isomer.
Understanding Structural Isomers
According to the provided reference, isomers in general are molecules sharing the identical chemical formula but differing in the physical arrangement of their atoms or bonds. Structural isomers fit this definition precisely; they possess the same number and types of atoms as another molecule (hence the same chemical formula), but the atoms are connected or bonded in a different order.
This difference in connectivity is key. Imagine having the same set of building blocks (atoms) but assembling them into different shapes (molecules). Each shape represents a distinct structural isomer.
Key Characteristics
Structural isomers are defined by:
- Identical Chemical Formula: They must contain the exact same number of atoms of each element.
- Different Connectivity: The atoms are linked together in a fundamentally different order or pattern. This means the sequence of atoms and the positions of bonds differ.
This difference in structure leads to distinct physical and chemical properties, even though the molecular formula is the same.
Examples of Structural Isomers
Understanding structural isomers is easiest with examples.
Butane and Isobutane (C₄H₁₀)
| Molecule | Chemical Formula | Structure | Description |
|---|---|---|---|
| n-Butane | C₄H₁₀ |  |
A straight chain of four carbon atoms. |
| Isobutane | C₄H₁₀ |  |
A branched structure with a central carbon. |
Both molecules have the formula C₄H₁₀, but the arrangement of carbons and hydrogens is different. n-Butane has a continuous chain, while isobutane (also called 2-methylpropane) has a branching point.
Propanol and Isopropanol (C₃H₈O)
- Propan-1-ol (n-Propanol): C₃H₈O. The hydroxyl (-OH) group is attached to an end carbon atom.
- Propan-2-ol (Isopropanol): C₃H₈O. The hydroxyl (-OH) group is attached to the middle carbon atom.
Again, the formula is the same, but the position of the functional group (-OH) on the carbon chain creates two different structural isomers with different properties (e.g., boiling points).
Why Connectivity Matters
The way atoms are bonded together dictates a molecule's shape, polarity, and how it interacts with other molecules. Therefore, even a slight difference in connectivity between structural isomers results in them behaving as distinct substances with varying melting points, boiling points, reactivity, and biological activity.
