Correct Answer: 4. Mirror image of each other

Enantiomers are nonsuperimposable mirror-image stereoisomers, making option 4 the precise definition.

Option Analysis

• 1. Isobars of each other: Incorrect. Isobars refer to atoms of different elements with the same mass number, unrelated to molecular stereochemistry.​

• 2. Isotopes of each other: Incorrect. Isotopes are atoms of the same element differing only in neutron count, not applicable to stereoisomers.​

• 3. Not the mirror image of each other: Incorrect. This describes diastereomers, which are stereoisomers lacking mirror-image symmetry.

• 4. Mirror image of each other: Correct. Enantiomers are defined as non-superimposable mirror images with identical connectivity but opposite spatial arrangements.

Enantiomers are a special type of stereoisomers that are mirror images of each other, playing a key role in molecular biology, genetics, and chirality studies for competitive exams.

Core Definition

Enantiomers exhibit identical physical properties like melting point but rotate plane-polarized light oppositely—one clockwise (dextrorotatory, +) and the other counterclockwise (levorotatory, -). They arise from chiral centers, typically a carbon with four different substituents, ensuring nonsuperimposability.

Key Properties

• Identical connectivity and formula but opposite 3D configurations.

• Optically active; form racemic mixtures when equal.​

• Differ in biological interactions, vital for drug design in biotechnology.​

Examples in Biology

Lactic acid enantiomers: D- and L-forms, where one mirrors the other nonsuperimposably. In plants, amino acids like alanine show enantiomeric forms affecting enzyme specificity.

Exam Relevance

This MCQ tests stereoisomer classification, common in GATE Life Sciences for molecular biology sections. Diastereomers (non-mirror images) contrast with enantiomers.