See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Dihedral Angle in Gauche Conformation of n-Butane **Understanding n-Butane Structure:** n-Butane is $CH_3-CH_2-CH_2-CH_3$. The two methyl groups are at C1 and C4, attached to the central C2-C3 bond. **Gauche Conformation Definition:** Gauche is a staggered conformation where two groups are positioned at a dihedral angle of **60°** relative to each other (not directly opposite as in anti, which is 180°). **Newman Projection Analysis:** When viewing n-butane along the C2-C3 bond in gauche conformation: - One methyl group (on C1) is positioned at 60° relative to the other methyl group (on C4) - This is a staggered arrangement with minimal steric hindrance compared to eclipsed forms - The three staggered positions are at 60°, 180°, and 300° (equivalent to −60°) **Why Other Options Are Wrong:** - **(A) 109°28'**: This is the tetrahedral angle, not relevant to dihedral angles - **(B) 120°**: Not a standard dihedral angle in butane conformations - **(C) 180°**: This would be anti-conformation (opposite position), not gauche **Answer: (D) 60°** ✓ The gauche conformation specifically refers to the 60° dihedral arrangement, making it distinct from the anti (180°) arrangement.