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Chiral Molecule

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.

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