Chiral came from the Greek word cheir, meaning “hand”. An object is said to be chiral if it does not have a plane of symmetry. It means that if you cut a line through the object, the resulting half is not a mirror image of the other. A hand, for example, does not have a plane of symmetry. When one cuts a line through it, the resulting halves are not mirror images.
A flask, without its marks on the surface, is not chiral. When one cuts a line through the flask, the two halves are mirror images (Figure 1). Objects which are not chiral are known as achiral.
Figure 1 Achiral object
Chirality can be extended towards smaller molecules. The molecule in Figure 2 is chiral. It lacks a plane of symmetry. The red line drawn at the centre cuts the molecule into half. Each side is not a mirror image of the other.
Figure 2 Example of a chiral molecule
On the other hand, Figure 3 shows a molecule which is achiral. When the red line cuts through the molecule, each side is a mirror image of the other. This molecule contains a plane of symmetry.
Figure 3 Example of an achiral molecule
To easily identify chiral molecules, aside from the absence of plane of symmetry, is to determine its chiral centers. Figure 1 identifies the carbon atom as the chiral center of the molecule. Four different substituents are attached to it.
More examples of chiral molecules are shown in Figure 4. Carbon atoms with asterisk are their respective chiral centers.
Figure 4 Other examples of chiral molecules
Light consists of electromagnetic waves, oscillating in different planes perpendicular to the direction of propagation. When it passes through a polarizer, only light oscillating in a single plane can get through. It is now called a plane-polarized light. The rest of the light oscillating in other planes is blocked by the polarizer (see Figure 5).
Figure 5 (A) Light oscillating in different planes; (B) Plane-polarized light
When a plane-polarized light passed through a chiral compound with a known concentration, the light will rotate either towards the right or left. Hence, chiral compounds are said to be optically active.
A polarimeter is a device that measures the degree of rotation of the plane-polarized light. The light from an ordinary source passes through a polarizer. The plane-polarized light passes through the sample and this light further enters into the analyzer. If the sample is chiral, optical rotation takes place.
Optical rotation refers to the extent of rotation of the plane-polarized light. This is dependent on the concentration of the sample and the sample path length. Optical rotation is expressed in terms of specific rotation, [α], where its formula is:
Specific rotation is dependent on the temperature and wavelength of the light source. Usually, this information is collected at 25ºC and at 589.6 nm (the wavelength of the yellow light from sodium lamp, usually referred as the D line).
Enantiomers are two chiral compounds which are mirror images of each other. They are known as optical isomers, just like one’s right hand is a reflection of the left hand. Figure 6 shows an example of an enantiomeric pair of 2-hydroxypropanoic acid, also known as lactic acid.
Figure 6 Examples of enantiomers
Enantiomers are not identical compounds. They are not superimposable on each other. That is, if one enantiomer is placed mentally over the top of the other, two substituents will not coincide with each other.
Though different, enantiomers have the same physical properties such as melting point, solubility, and density. However, they differ on the direction of the rotation of plane-polarized light.