See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Most Stable Conformation of 2,3-Butanediol **Structure Analysis:** 2,3-butanediol has the structure: $CH_3-CHOH-CHOH-CH_3$ The two chiral centers (C2 and C3) are adjacent, making this a problem of **gauche/anti relationships** between bulky groups. **Stability Criteria:** 1. **Option (1)**: Both $OH$ groups are equatorial, but they are oriented **gauche** to each other (120° dihedral angle). This creates steric clash between the two hydroxyl groups. 2. **Option (2)**: Both $OH$ groups equatorial and **anti** to each other (180° dihedral angle). Good spacing, but the two methyl groups are also anti—less optimal packing. 3. **Option (3)**: One $OH$ is equatorial (C2), one is axial (C3), with the methyl groups positioned **anti** to each other. The hydroxyl groups are **gauche** on the ring, reducing 1,3-diaxial interactions while maintaining reasonable $OH$ separation. **Minimizes steric strain.** 4. **Option (4)**: Both $CH_3$ groups equatorial creates significant **1,3-diaxial interactions** with axial hydrogens, destabilizing the conformation. **Answer: Option (3)** In the most stable conformation, the two bulky hydroxyl groups avoid the **gauche-pentane effect** (mutual steric repulsion) by adopting a staggered arrangement with optimized axial/equatorial positioning. This configuration balances minimizing 1,3-diaxial strain while avoiding OH-OH repulsion.