See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Analysis of D-(+)-Tartaric Acid Nomenclature **Understanding the "D-(+)" designation:** The "D-(+)" notation in tartaric acid has two distinct meanings that students often confuse: - **"D"** = Refers to the **absolute configuration** (Fischer convention) based on the structure's relationship to D-glyceraldehyde - **(+)** = Refers to the **direction of optical rotation** (dextrorotatory, rotates plane of polarized light to the right) **Why option (B) is correct:** The pH in organic solvent is the distinguishing factor. In aqueous solution, the optical rotation of tartaric acid can vary due to: - Ionization of carboxylic acid groups (affected by water and pH) - Changes in molecular conformation and association In a non-aqueous organic solvent (aprotic solvent), the carboxylic acid groups remain unionized, eliminating pH effects. This allows **consistent and reproducible measurement of intrinsic optical rotation**, which definitively establishes the dextrorotatory (+) character independent of solvent pH variations. **Key Point:** The "+" designation specifically requires measurement conditions where the optical rotation remains unaffected by ionization effects—hence, organic solvent conditions are essential for properly characterizing D-(+)-tartaric acid. **Answer: (B)** ✓