English: Newman projections of butane conformations & their relative energy differences (not total energies). Conformations form when butane rotates about one of its single covalent bond. Torsional/dihedral angle is shown on x-axis.
Conformations (according to IUPAC):
A: antiperiplanar, anti or trans
B: synclinal or gauche
C: anticlinal or eclipsed
D: synperiplanar or cis
Valleys of the pink graph are conformations lowest in energy (shown as A & B). Peaks are conformations highest in energy (shown as C & D). Energies are A<B<C<D with D highest & A lowest in energy. A is thus the most stable conformation &, of all the other conformations, occurs most often in room temperature.
Valleys A & B are local energy minima & A is global minima. A & B can thus be classified as rotamers (a class of conformers). Peaks are not rotamers, & are caused by repulsive forces of the hydrogens & methyls (-CH3).
Source for conformation names & conformer classification:
- Pure & Appl. Chem., Vol. 68, No. 12, pp. 2193-2222, 1996
Source for the relative energy differences & other info:
- McMurry JE (2003). Organic Chemistry. (8 ed.). Brooks Cole. p. 98.
ISBN
9780840054449.