GATE 2025 BT Q.24: solution and answer

Official answer: B, DSource: Official answer key, GATE 2025 BT (IIT Roorkee)

1. Optical activity (rotation of plane-polarised light) needs chiral molecules present in a net excess of one enantiomer. An achiral compound, or a racemic mixture, gives no net rotation.

2. (A) NaCl dissociates into ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$. These are simple spherical ions with no chirality, so there is no optical activity. Wrong.

3. (B) D-Glucose has several chiral carbons (C2 to C5) and is a single enantiomer. Its solution rotates plane-polarised light (dextrorotatory, with the rotation settling at a fixed value after mutarotation). Correct.

4. (C) Glycine, ${\text{H}}_2{\text{N-CH}}_2\text{-COOH}$, has two hydrogens on the alpha carbon. The alpha carbon is therefore not a stereocentre and the molecule is achiral, so there is no optical activity. Wrong.

5. (D) L-Proline has a chiral alpha carbon bearing four different groups (H, COO-, the ring nitrogen and the ring CH2), and it exists as a single enantiomer. Its solution is optically active (levorotatory). Correct.

6. Hence the optically active solutions are D-glucose and L-proline: options B and D.