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: CH
2Cl
2It 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.