Q.Which of the following is ‘not’ true about enantiomers ? (A) They have the same chemical reactivity. (B) They have the same specific rotation. (C) They have the same melting or boiling point. (D) They have the same refractive index.

Concept: Enantiomers are non-superimposable mirror images that differ only in their interaction with plane-polarized light and chiral environments.

Enantiomers are stereoisomers with identical physical properties in achiral environments—same melting point, boiling point, refractive index, density, and solubility. Their chemical reactivity is also identical when reacting with achiral reagents.

The critical distinction lies in optical activity. Enantiomers rotate plane-polarized light by equal magnitudes but in opposite directions: one is dextrorotatory $(+)$, the other levorotatory $(-)$. Their specific rotations have the same absolute value but opposite signs, e.g., $+25°$ and $-25°$. …

Enantiomers are stereoisomers that stand as mirror images of each other, like your left and right hands. Because they have identical connectivity and the same functional groups in the same spatial arrangement (just reflected), most of their physical and chemical properties are identical. The one property that must differ is their interaction with plane-polarized light.

When plane-polarized light passes through a solution of a chiral molecule, the molecule rotates the plane of polarization. Enantiomers rotate light by the same magnitude but in opposite directions: one clockwise (dextrorotatory, $+$), the other counterclockwise (levorotatory, $-$). This is the defining optical difference.

Let's examine each statement:

1. Chemical reactivity (A): Enantiomers have identical chemical reactivity in an achiral environment. They react at the same rate with achiral reagents because the transition states formed are also mirror images with equal energy. In biological systems or with chiral reagents, reactivity can differ — but the statement as written (without qualification) is generally considered true for standard achiral conditions.

2. Specific rotation (B): Specific rotation $[\alpha]$ is defined as the observed rotation per unit concentration and path length. For a pair of enantiomers, if one has $[\alpha] = +50°$, the other has $[\alpha] = -50°$. They have equal magnitude but opposite sign. The statement claims they have "the same" specific rotation, which is false — they have opposite specific rotations. …