Final answer:
The pair of disubstituted cyclopentane molecules are classified as diastereomers if they are stereoisomers with different configurations at some but not all stereocenters, and they are not mirror images of each other. Meso structures have a plane of symmetry and therefore do not have enantiomers. so, option 3 is the correct answer.
Step-by-step explanation:
To classify the pair of two disubstituted cyclopentane molecules, we must consider the arrangement of their substituents. If they have different configurations at one or more (but not all) stereocenters, and they are stereoisomers but not mirror images, they are considered diastereomers. An example is the relationship between trans-1,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane, where they are stereoisomers with different configurations around the carbon-carbon bond, but not mirror images, making them diastereomers. Additionally, cis-structures like cis-1,2-dimethylcyclopropane which do not have enantiomers due to a plane of symmetry are referred to as meso structures.
Molecules with the same molecular formula but different arrangements of atoms are known as isomers, and the number of possible isomers can be determined from the number of chiral centers using the formula 2n, where n represents the number of chiral centers. It's essential to understand that to be enantiomers, the molecules must be non-superimposable mirror images of each other, which requires the central carbon to have four different substituents.