Constitutional Isomerism

Definition: Constitutional isomers are compounds that share the same molecular formula but differ in the connectivity of their atoms.
Properties: Due to different connectivity, constitutional isomers have distinct names and different chemical and physical properties.
Example ( $C_2H_6O$ ):
- Dimethyl ether: Boiling point of $-23\,^{\circ}\text{C}$ ; gas at room temperature; used as an aerosol spray propellant.
- Ethanol: Boiling point of $78.4\,^{\circ}\text{C}$ ; liquid at room temperature; found in alcoholic beverages.

Step 1: Check Molecular Formula:
- Count all atoms to ensure compounds have identical molecular formulas.
- Structures with different atom counts (such as $C_4H_7Br$ vs. $C_4H_9Br$ ) are not constitutional isomers.
Step 2: Determine Atom Connectivity:
- Mentally simplify structures using carbon skeletal backbones and disregarding hydrogen atoms.
- Number the carbon atoms in the longest continuous carbon chain ( $C_1$ , $C_2$ , etc.), consistently starting from the end closest to the attached group (e.g., bromine).
Step 3: Compare Structures:
- Evaluate the carbon chain structure (e.g., straight four-carbon chain vs. branched three-carbon chain).
- Determine the exact carbon position of substituents to identify matching compounds or unique constitutional isomers.