Question 3:
Pairs of stereoisomers that are not mirror images of each other are called:
Title: Pairs of Stereoisomers Not Mirror Images: Diastereomers Explained
The correct answer to the question “Pairs of stereoisomers that are not mirror images of each other are called” is (D) Diastereomers. This concept is fundamental in organic chemistry, especially for exams like NEET or JEE, distinguishing stereoisomer types.
Correct Answer Explanation
Diastereomers are stereoisomers with the same molecular formula and connectivity but differing spatial arrangements at some—not all—chiral centers, making them non-mirror images and non-superimposable. Unlike enantiomers, they exhibit distinct physical properties like melting points or solubility due to this asymmetry. For molecules with two or more stereocenters, diastereomers arise when configurations differ at one or more (but not every) position, as seen in cis-trans alkenes or sugar pairs like D-erythrose and D-threose.
Option Breakdown
| Option | Description | Why Incorrect/Correct |
|---|---|---|
| (A) Enantiomers | Stereoisomers that are non-superimposable mirror images of each other, with opposite configurations at all chiral centers . | Incorrect: They are defined as mirror images, rotating plane-polarized light oppositely. |
| (B) Chiral centres | sp³-hybridized carbon atoms bonded to four different groups, enabling stereoisomerism . | Incorrect: This refers to structural features, not pairs of stereoisomers. |
| (C) Racemic mixture | 1:1 mixture of enantiomers, optically inactive due to equal rotation cancellation . | Incorrect: Describes a mixture, not stereoisomer pairs. |
| (D) Diastereomers | Stereoisomers that are not mirror images, often with multiple stereocenters differing at some sites . | Correct: Matches the query precisely. |
Key Differences Table
| Feature | Enantiomers | Diastereomers |
|---|---|---|
| Mirror images? | Yes, non-superimposable | No |
| Chiral centers needed | ≥1, opposite at all | ≥2, differ at some |
| Physical properties | Identical (except rotation) | Different (e.g., solubility) |
| Example | D- and L-lactic acid | Cis/trans-2-butene |
