See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Conformational Stability of Ethylene Glycol **Step 1: Identify the three conformations** For ethylene glycol ($HOCH_2CH_2OH$), rotating around the central $C-C$ bond gives: - **Eclipse**: $OH$ groups directly opposite each other (dihedral angle = 0°) - **Gauche**: $OH$ groups at 60° dihedral angle - **Anti**: $OH$ groups at 180° dihedral angle **Step 2: Analyze steric and electronic effects** - **Eclipse**: Maximum steric repulsion between $OH$ groups; least stable - **Gauche**: Moderate steric strain; BUT intramolecular hydrogen bonding is possible between the two $OH$ groups at 60°, which provides stabilization - **Anti**: $OH$ groups far apart (minimizes steric repulsion) but NO hydrogen bonding possible **Step 3: Compare stability** Gauche configuration benefits from **intramolecular H-bonding** ($O-H···O$), which significantly stabilizes this conformation despite some steric clash. Anti has no H-bonding but less steric repulsion than eclipse. $$\text{Eclipse} < \text{Anti} < \text{Gauche}$$ **Increasing order of stability: Eclipse, Anti, Gauche** **Answer: (C)**