gauche

Meaning: Gauche describes a staggered conformation about a C–C single bond in which two substituents on adjacent carbons have a 60° dihedral angle. In butane, rotation about C2–C3 gives anti (CH₃ groups 180° apart), gauche (60°), and eclipsed forms. Gauche has no eclipsing torsional strain, but the two methyl groups crowd each other (steric strain), so it sits slightly above anti in energy. It is a local energy minimum, not a transition state.

On the MCAT: Expect a Newman projection to draw or read, or an energy versus dihedral angle plot. For butane, rank stability from most to least stable: anti, gauche, eclipsed (CH₃ over H), fully eclipsed (CH₃ over CH₃). On the plot, gauche appears as shallow minima at 60° and 300°, above the anti minimum at 180°. The idea also explains ring chemistry: an axial methyl on cyclohexane has gauche interactions with ring carbons (1,3-diaxial strain), so the equatorial chair is usually favored.

Common confusion: Gauche is not eclipsed. Gauche and anti are both staggered, and they differ only in the angle between the large groups. Gauche is also not the less stable form in every molecule; intramolecular hydrogen bonding, as in ethylene glycol, can favor it.