Molecules that form nonsuperimposable mirror images, and thus exist as enantiomers, are said to be chiral molecules. For a molecule to be chiral, it cannot contain a plane of symmetry. A plane of symmetry is a plane that bisects an object (a molecule, in this case) in such a way that the two halves are identical mirror images. An example of a structure that has a plane of symmetry is a cylinder. Cutting a cylinder in half lengthwise generates two halves that are exact mirror images of each other. A molecule that possesses a plane of symmetry in any of its conformations is identical to its own mirror image. Such molecules are achiral, or nonchiral. Butane is an achiral molecule, while 2‐bromobutane is chiral.
The most common cause of chirality in an organic molecule is a carbon atom with four different atoms or groups bonded to it. This carbon atom is called a stereogenic, chiral, or asymmetric center. (Such centers are often designated with an asterisk in formulas and projections.)
The van't Hoff rule predicts the maximum number of enantiomers an optically active molecule can possess. This rule states that the maximum number of enantiomers a molecule can have is equal to 2 raised to the nth power, where n equals the number of stereogenic centers. The molecule 2‐chlorobutane has one stereogenic center, so two enantiomers are possible.