3. Conformères

The same molecule can adopt different spatial arrangements. These arrangements are different conformations of the molecule. Some are energetically favored, and are referred to as conformers. All it takes is a small amount of energy to induce rotations within the molecule, transforming it from one conformer to another, without breaking any bonds. In the case of ethane, less than 12 kJ · mol ^–1 are required (4 of the total binding energy), which is very little compared with the thermal energy available at 25°C, so that interconversion occurs rapidly and continuously at ordinary temperature. This energy is called the rotation barrier. These movements can be slowed down by lowering the temperature. By comparison, it takes 410 kJ · mol ^–1 to break a C bond—H. Nevertheless, an instantaneous "snapshot"...