See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Conformational analysis of butane involves rotating around the C2-C3 bond. The stability of conformations depends on torsional strain and steric interactions between substituents. Step 1: Identify the conformations shown. - Option (a): A gauche conformation where the two CH3 groups are at approximately 60° to each other. This has moderate steric strain. - Option (b): An eclipsed conformation where the two CH3 groups are eclipsed (0° dihedral angle). This is the highest energy conformation due to maximum torsional strain and steric clash between the two methyl groups. - Option (c): An anti conformation where the two CH3 groups are at 180° to each other (fully staggered, anti). In the Newman projection, CH3 is at top on the front carbon and CH3 is at bottom on the back carbon, with H atoms staggered. This arrangement maximizes the distance between the bulky methyl groups. - Option (d): Another gauche conformation similar to (a) but with a different arrangement; still has CH3 groups at ~60° dihedral. Step 2: Apply the principle of conformational stability. For butane, the order of stability (lowest to highest energy) is: Anti > Gauche > Eclipsed (with H/CH3) > Fully eclipsed (CH3/CH3) The anti conformation (dihedral angle = 180° between the two CH3 groups) is the most stable because the two large methyl groups are as far apart as possible, minimizing both torsional strain and steric (van der Waals) repulsion. Step 3: Identify option (c) as the anti conformation. In option (c), the Newman projection shows CH3 at the top of the front carbon and CH3 at the bottom of the back carbon — a 180° dihedral angle — which is the anti conformation. Why other options fail: - (a) and (d) are gauche conformations (~60° between methyls), which are less stable than anti due to steric repulsion between the methyl groups. - (b) is a fully eclipsed conformation (both CH3 groups eclipsed), which is the highest energy conformation of butane. Therefore, the correct answer is C.