A similar observation can be made for 3-methyloxirane-2-carboxylic acid but here both the cis and trans isomers are chiral compounds each of which exists as a pair of enantiomers (Figure 7).

Figure 7

The term chiral centre used so far is actually a subset of the term stereogenic centre. A stereogenic centre is defined as an element where the interchange of two substituents will lead to a stereoisomer. Not all stereogenic centres are chiral centres and even achiral molecules may have stereogenic centres.
The actual arrangement of the atoms or groups in a molecule about a stereocentre is called absolute configuration. It is mostly determined by X-ray crystallography or by inference based on chemical reactions of specific stereochemistry involving a compound whose absolute configuration is known, whereas the relative configuration is defined as the correlation between the different stereogenic centres within the molecule.

Though, here, the enantiomers are represented in the flying wedge form where two of the groups around the chiral centre are depicted in the plane of the paper and groups towards us in bold bonds and groups away from us in broken bonds, there are other forms of depictions of chiral compounds. These are discussed below.

• Fischer projection formula. It is a representation of a 3D molecule as a flat structure where a tetrahedral carbon is represented as two crossed lines. The two vertical bonds about the stereocentre are above the plane of paper (towards the viewer) while the horizontal bonds are below the plane of the paper (away from the viewer) (Figure 8).

Figure 8

A few examples of depiction of molecules in Fischer projection formula is given below. It must be noted that when bonds are rotated by 180°, they result in the same identical molecule (Figure 9).

Figure 9