Molecules are not rigid drawings. Rotation about many single bonds changes the three-dimensional arrangement of atoms without changing which atoms are connected. Different spatial arrangements connected by such rotations are called conformations.

Consider ethane, $\mathrm{CH_3CH_3}$. Rotation around its carbon-carbon single bond changes the relative positions of the C-H bonds. Two limiting arrangements are commonly distinguished:

• staggered: bonds on the rear carbon lie between bonds on the front carbon when viewed along the C-C axis;
• eclipsed: bonds on the rear carbon align with bonds on the front carbon.

A Newman projection represents this view directly along a bond. The front carbon is drawn as a point and the rear carbon as a circle; three bonds radiate from each.

Conformations can differ in energy even though they have identical connectivity. Eclipsed ethane is higher in energy than staggered ethane because nearby bonding electron regions interact less favorably when aligned. In larger molecules, bulky groups also prefer arrangements that keep them farther apart; this spatial crowding is called steric interaction or steric strain.

At ordinary temperatures, rotation about many carbon-carbon single bonds occurs rapidly, so a sample usually contains an ensemble of conformations rather than one permanently fixed arrangement.

Example: butane

Looking down the central C-C bond of butane,

$$\mathrm{CH_3CH_2CH_2CH_3},$$

the two terminal methyl groups can be opposite one another in an anti staggered conformation or about $60^\circ$ apart in a gauche staggered conformation. The anti arrangement is generally lower in energy because the bulky methyl groups are farther apart.

A change of conformation does not create a constitutional isomer and normally does not require breaking a covalent bond. It is also different from changing configuration, where groups have a distinct spatial arrangement that cannot be interconverted merely by free rotation about an ordinary single bond.

Conformational flexibility is central to large molecules: polymer chains, proteins and many biological ligands can adopt many conformations while retaining the same covalent connectivity.