OPTICAL ISOMERISM

Chiral and achiral molecules
The essential difference between the two examples we've looked at lies in the symmetry of the molecules.

If there are two groups the same attached to the central carbon atom, the molecule has a plane of symmetry. If you imagine slicing through the molecule, the left-hand side is an exact reflection of the right-hand side.
Where there are four groups attached, there is no symmetry anywhere in the molecule.

A molecule which has no plane of symmetry is described as chiral. The carbon atom with the four different groups attached which causes this lack of symmetry is described as a chiral centre or as an asymmetric carbon atom.
The molecule on the left above (with a plane of symmetry) is described as achiral.
Only chiral molecules have optical isomers.

The relationship between the enantiomers
One of the enantiomers is simply a non-superimposable mirror image of the other one.
In other words, if one isomer looked in a mirror, what it would see is the other one. The two isomers (the original one and its mirror image) have a different spatial arrangement, and so can't be superimposed on each other.

If an achiral molecule (one with a plane of symmetry) looked in a mirror, you would always find that by rotating the image in space, you could make the two look identical. It would be possible to superimpose the original molecule and its mirror image.

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