Axis of symmetry
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Topic: Stereochemistry : Axis of symmetry

Axis of symmetry
Symmetry axis Cn, also called n-fold axis, is an axis which rotates the object (molecule) around by 360/n, such that the new position of an object is superimposable with the original one. For example, 1,3-substituted cyclobutane has a twofold axis C2 and the cis-tetrasubstituted cyclobutane has a C4 axis. The presence of the Cn does not preclude chirality.
n-Fold Rotation Cn : The n-fold rotation (the symmetry operation) about a n-fold axis of symmetry (the corresponding symmetry element) produces molecular orientations indistinguishable from the initial for each rotation of 360/n (clockwise and counter-clockwise). A water molecule has a single C2 axis bisecting the H-O-H bond angle, and benzene has one C6 axis (amongst one C3 axis and seven C2 axes of which the C3 and one C2 axis coincide with the C6 axis).If a molecule has one (or more) rotation axes Cn , the axis with the greatest n is called the principal axis.

Example 1 : Methene : It has both C2 and C3 axis of Symmetry.
Example 2 : Ethene : It has only C2 axis of Symmetry.
Example 3 : Methylene Chloride: CH2Cl2It has only C2 axis of Symmetry.
Example 4 : Benzene : It has C2 , C3 and C6 axis of Symmetry.
Example 5 : Cyclo Butane : Planar form has C2 , and C4 axis of Symmetry.
Example 6 : Allene : It has C2 axis of Symmetry.
Example 7 : Lactic acid : It has no axis of Symmetry.