eclipsed

Meaning: An eclipsed conformation is one in which substituents on adjacent carbons line up, giving a dihedral angle of 0° in a Newman projection. The electron clouds of the aligned bonds repel (torsional strain), and bulky groups that line up add steric strain. Eclipsed forms are energy maxima for rotation about a C–C single bond; staggered forms (offset by 60°) are minima. In ethane, the eclipsed form is about 12 kJ/mol above the staggered form.

On the MCAT: Expect a Newman projection and a request to rank conformers by energy, or an energy diagram for rotation about the C2–C3 bond of butane. The highest peak is the fully eclipsed form (two methyls at 0°). The lower peaks are eclipsed forms with a methyl against a hydrogen. The lowest valley is anti, with gauche slightly higher. Chair cyclohexane is fully staggered, while the boat has eclipsed interactions, so the chair is more stable.

Common confusion: Eclipsed and staggered forms are conformations. They interconvert by rotation without breaking bonds, so they are not different isomers you can isolate at room temperature. Gauche is a staggered form (60°), not an eclipsed one, even though its methyl groups crowd each other. An eclipsed form is a peak on the energy diagram, not a stable intermediate.