Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. They have identical physical and chemical properties but differ in their interaction with plane-polarized light (optical activity) and in chiral environments.

Key characteristics of enantiomers:

They must be mirror images.
They must not be superimposable.
Each chiral center must have opposite configurations (R vs S).

To identify which pair is not enantiomers, analyze each option:

Step 1: Check if the structures are mirror images. If not, they cannot be enantiomers.

Step 2: If they are mirror images, check if they are non-superimposable. If superimposable, they are identical (meso or same compound), not enantiomers.

Step 3: Assign R/S configurations to chiral centers. Enantiomers must have opposite configurations at every chiral center.

Without the exact images, general guidance: Look for pairs where molecules are identical (e.g., meso compounds with internal symmetry) or where mirror images are superimposable. Also, pairs with different connectivity (structural isomers) or with more than one chiral center but not exact mirror images (diastereomers) are not enantiomers.

Common trick: Meso compounds (with plane of symmetry) are achiral and their mirror images are superimposable, so they are not enantiomers. Also, pairs with identical R/S configurations are the same molecule, not enantiomers.

Final Answer: The pair that is either identical, meso, or not mirror images is not a pair of enantiomers. Based on typical questions, the option with meso forms or identical structures is often the correct choice for "not enantiomers".

Related Topics & Formulae

Chirality: A molecule is chiral if it lacks an internal plane of symmetry and is not superimposable on its mirror image.

Meso Compounds: Compounds with chiral centers but an internal plane of symmetry, making them achiral (optically inactive). Their mirror images are superimposable.

R/S Nomenclature: Assign priority to substituents around a chiral center using Cahn-Ingold-Prelog rules. Clockwise is R; counterclockwise is S.

Diastereomers: Stereoisomers that are not mirror images, often with multiple chiral centers where some configurations are same and some are different.