A chiral molecule is a molecule that is not superimposable on its mirror image.
eg. 1:
Molecule 1 is not superimposable on its mirror image and, therefore, is chiral.
eg. 2:
Molecule 2 is not superimposable on its mirror image and, therefore, is chiral.
An achiral molecule is a molecule that is superimposable on its mirror image.
eg. 1:
Molecule 3 is superimposable on its mirror image and, therefore, is achiral.
eg. 2:
Molecule 4 is superimposable on its mirror image and, therefore, is achiral.
Alternatively, an achiral molecule is a molecule that has at least one plane of symmetry.
eg. 1:
The vertical plane that bisects the bromine atom and the methyl group, which is the plane of the screen, is a plane of symmetry. Thus, 3 is achiral.
eg. 2:
The vertical plane that bisects the molecule perpendicular to the plane of the screen is a plane of symmetry. Thus, 4 is achiral.
A chiral molecule has no plane of symmetry.
eg. 1
1 is chiral and has no plane of symmetry.
eg. 2:
2 is chiral and has no plane of symmetry.
Although relatively rare, molecules do exist that have no plane of symmetry but is achiral.
eg:
Thus, presence of a plane of symmetry is not a foolproof method to determine whether a molecule is chiral or achiral.
