See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Conformational stability is determined by torsional strain and steric interactions, but for molecules bearing polar substituents, intramolecular hydrogen bonding or dipole-dipole interactions can also influence the preferred conformation. Step 1 - Identify the substituents on C2 and C3. C2 bears the -OH group (and two H's), and C3 bears the -F group (and two H's). Both OH and F are polar groups capable of participating in intramolecular interactions. Step 2 - Consider the staggered conformations. For the C2-C3 bond, there are three staggered conformations: anti (OH and F at 180°), gauche+ (OH and F at +60°), and gauche- (OH and F at -60°). In the anti conformation, the large groups are maximally separated, which normally favors stability. However, in this molecule, the gauche conformation places OH and F in proximity (dihedral ~60°), allowing the formation of an intramolecular O-H···F hydrogen bond. Fluorine, being highly electronegative, can act as a weak hydrogen bond acceptor, and the OH is a good hydrogen bond donor. Step 3 - Evaluate the gauche conformer. The intramolecular O-H···F hydrogen bond in the gauche conformer provides extra stabilization that compensates for the gauche strain. This makes the gauche conformer more stable than the anti (fully staggered) conformer for this particular molecule. Step 4 - Why other options fail. (a) Staggered (anti): Although it has minimum torsional strain and maximum separation of groups, it lacks the stabilizing intramolecular H-bond between OH and F, making it less stable than gauche for this compound. (b) Eclipsed (partially): Eclipsed conformations have higher torsional strain than staggered ones and are not favored. (d) Fully eclipsed: This has maximum torsional strain and is the least stable conformer. Step 5 - Conclusion. The intramolecular O-H···F hydrogen bond in the gauche conformation provides extra stabilization, making it the most stable conformer of HO-CH2-CH2-F across the C2-C3 bond. Therefore, the correct answer is C.