Which of the following represents a gauche conformation for butane?

• A. The dihedral angle between the two methyl groups is $180°$
• B. The dihedral angle between the two methyl groups is $0°$
• C. The dihedral angle between the two methyl groups is $120°$
• D. The dihedral angle between the two methyl groups is $60°$

Answer: (D)

Verified step by step guidance

1. Understand that the term 'gauche conformation' refers to a specific spatial arrangement in staggered conformations of molecules like butane, where two substituents (in this case, methyl groups) are separated by a dihedral angle of approximately 60°.
2. Recall that in butane, rotation around the central C-C bond leads to different conformations characterized by the dihedral angle between the two methyl groups attached to the ends of the molecule.
3. Identify the key dihedral angles for butane conformations: 0° (eclipsed), 60° (gauche), 120° (partially eclipsed), and 180° (anti).
4. Recognize that the anti conformation corresponds to a dihedral angle of 180°, where the two methyl groups are opposite each other, minimizing steric hindrance.
5. Conclude that the gauche conformation is specifically the staggered conformation where the dihedral angle between the two methyl groups is 60°, indicating they are close but not eclipsed.