See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Conformational stability in molecules with polar substituents capable of intramolecular hydrogen bonding. Step 1: Identify the structure. Ethylene glycol is HOCH2-CH2OH. Rotating about the C-C bond gives different conformations: anti (dihedral angle ~180°), gauche (~60°), partially eclipsed (~120°), and fully eclipsed (0°). Step 2: Apply steric considerations alone. For a simple molecule like ethane, the anti conformation would be most stable because it minimizes steric repulsion between the two sets of substituents. By this logic alone, the anti conformation of ethylene glycol (OH groups 180° apart) should be most stable. Step 3: Consider intramolecular hydrogen bonding. Ethylene glycol has two hydroxyl (-OH) groups. In the gauche conformation (dihedral angle ~60°), the two OH groups are close enough in space to form an intramolecular hydrogen bond (O-H···O). This stabilizing interaction lowers the energy of the gauche conformer significantly. Step 4: Compare stability. Although the anti conformation avoids steric repulsion between the OH groups, it does not allow intramolecular H-bonding. The gauche conformation, while having some gauche strain, is stabilized by the intramolecular hydrogen bond between the two hydroxyl groups, making it the net most stable conformation in the gas phase and in nonpolar environments. Step 5: Eliminate other options. Partially eclipsed and fully eclipsed conformations have torsional (eclipsing) strain, making them significantly less stable than staggered conformations (anti and gauche). Conclusion: The intramolecular hydrogen bonding in the gauche conformation of ethylene glycol overcomes the gauche strain, making gauche more stable than anti. Therefore, the correct answer is B.