Which of the following is 'not' true about enantiomers?
Which of the following is 'not' true about enantiomers?
Options
Correct option: (B) They have the same specific rotation.
This statement is NOT true. Enantiomers rotate plane-polarised light by equal magnitude but in opposite directions (one +, one −), so their specific rotations are equal in value but opposite in sign — not the same.
All other listed physical properties (m.p., b.p., refractive index) and chemical reactivity towards achiral reagents are identical.
Marking Scheme
- 11 mark: correct option (B).
- 2Award full mark only for identifying the specific-rotation statement as the false one.
Hint
Enantiomers share all physical properties EXCEPT the sign of optical rotation — equal magnitude, opposite direction.
Quick Oral Answer
Enantiomers are identical in all physical properties except that they rotate plane-polarised light by equal amounts in opposite directions, so saying they have the same specific rotation is not true.
Analysis & Explanation
Concept
Enantiomers are non-superimposable mirror images. Because their atoms have identical connectivity and bonding environments, all scalar physical properties — melting point, boiling point, density, refractive index, solubility — are identical, and their reactivity with achiral reagents is the same.
The one difference
They differ in the way they interact with plane-polarised light: one is dextrorotatory (+), the other laevorotatory (−). The magnitude of specific rotation is equal but the sign is opposite, so 'same specific rotation' is false.
Exam trap
Watch the word 'not'. The property that distinguishes enantiomers is optical rotation (and behaviour toward chiral reagents/enzymes), so any statement claiming identical rotation is the false one.
Why the distractors are true (hence wrong choices)
- (A) Same chemical reactivity toward achiral reagents — true.
- (C) Same melting/boiling point — true.
- (D) Same refractive index — true.
Common Mistakes
- 1Overlooking the word 'not' and selecting a true statement.
- 2Believing enantiomers differ in melting/boiling point — those are identical; only optical rotation sign differs.
- 3Confusing enantiomers with diastereomers, which do differ in physical properties.
Interesting Facts
A 1:1 mixture of enantiomers (racemic mixture) is optically inactive because the equal and opposite rotations cancel exactly.
Enantiomers can smell different: (R)-carvone smells of spearmint while (S)-carvone smells of caraway, because our chiral smell receptors distinguish them.
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Frequently Asked Questions
Do enantiomers differ in any physical property besides optical rotation?
No. Enantiomers have identical melting point, boiling point, density, refractive index and solubility in achiral solvents. The only difference is the sign (direction) of the plane-polarised light rotation and their behaviour toward other chiral substances such as enzymes.
How do enantiomers differ from diastereomers?
Enantiomers are mirror images with identical physical properties (except rotation sign). Diastereomers are stereoisomers that are not mirror images; they have different physical properties like melting point and solubility and can be separated by ordinary methods.