Newman projection

Meaning: A Newman projection looks down a single C–C bond. The front carbon is drawn as the point where three bonds meet. The back carbon is a circle, with its three bonds starting at the edge. Rotating about the bond changes the dihedral angle between front and back groups, which produces different conformations. Staggered conformations (groups 60° apart) have the least repulsion. Eclipsed conformations (groups aligned) have the most torsional strain. In butane viewed down C2–C3, anti (methyls 180° apart) is lowest in energy. Gauche (60°) is higher because of steric strain, and fully eclipsed (methyls 0°) is highest.

On the MCAT: Expect to rank conformations by stability, read an energy versus dihedral angle graph (peaks are eclipsed, valleys are staggered), or tell gauche from anti. You may also convert between a Newman projection, a sawhorse drawing and a wedge-dash structure. Newman projections are also used for chair cyclohexane substituents.

Common confusion: Gauche is a staggered arrangement, not an eclipsed one. Staggered dihedral angles are 60°, 180° and 300°. Eclipsed angles are 0°, 120° and 240°. Conformations also interconvert by rotation about a single bond, with no bonds broken, so they are not different compounds.