4.1 : Chiralité

Chirality refers to a special type of asymmetry whereby an object and its mirror image are not identical. For example, our left hand possesses chirality, as its mirror image, the right hand, is not superposable on the left hand.

Such objects are classified as chiral, as they possess the property of chirality. Achiral objects, such as a mug and its mirror image, can be superposed on each other.

For example, 2-butanol is chiral, as the mirror image of 2-butanol is not superposable on the molecule. In contrast, ethanol is achiral, as the mirror image of ethanol is the same as the molecule.

Here, 2-butanol has a tetrahedral carbon atom with four different groups attached to it, referred to as a chiral center. Molecules with only one chiral center are always chiral.

In general, chiral objects are identified by the asymmetry in their structure. An object or a molecule is usually chiral if it lacks both a plane of symmetry and a center of symmetry.

A plane of symmetry is an imaginary plane that divides the object into two equivalent parts, each of which is a mirror image of the other. For example, the achiral ethanol molecule has a plane of symmetry passing through the methyl and hydroxyl groups.

A center of symmetry is a point located in the object such that any two lines from this point in opposite directions contact identical components at equal distances. For example, the achiral molecule trans-2-butene has a center of symmetry located at the center of the π bond.

While the cis isomer of 1,3-dimethylcyclohexane has a plane of symmetry, the trans isomer lacks both a plane of symmetry and a center of symmetry. Accordingly, the cis isomer is achiral and the trans isomer is chiral.

Compounds such as 2,3-pentadiene lack a plane of symmetry and a center of symmetry due to hindered rotation around the π bond; as such, 2,3-pentadiene is chiral.