Organic Chemistry MCAT Practice Exam

What distinguishes constitutional isomers from one another?

1. Same molecular formula, different connectivity

2. Same connectivity, different molecular formula

3. Different spatial arrangement

4. Identical properties

The correct answer is: Same molecular formula, different connectivity

Constitutional isomers are entities that share the same molecular formula but differ in how their atoms are connected or arranged. This variation in connectivity leads to different chemical structures, which can result in distinct physical and chemical properties. For example, butane (C4H10) can exist as two structural forms: n-butane, which is a straight-chain structure, and isobutane, which is branched. Both compounds have the same chemical formula but exhibit different connectivity of their carbon atoms. The focus on connectivity is crucial; while molecular formula indicates the types and numbers of atoms present, it does not reveal how these atoms are arranged. Therefore, the determination that constitutional isomers must have the same molecular formula yet different connectivity accurately captures the essence of what distinguishes them from one another.