Symmetry criteria for optical activity

 

Symmetry criteria for optical activity :- 

• Non-superimposable mirror image
• Symmetry criteria for optical activity
• Lacks a plane of symmetry
• No Centre of symmetry
• S1 = σ
• S2 = i
• Neither σ nor i but has S n-optically inactive
• Condition- Does not have Sn axis (must be dissymmetric or asymmetric)
•  S4
• Not dissymmetric
• Asymmetric- lack all symmetry elements (Cn;
• n=1)
• Dissymmetric – Cn axis and no other element

Point groups-optical active :- 

• All Cn and Dn molecules (lack Sn axis)
• Must be dissymmetric
• Symmetry restrictions on dipole moments

Lacking dipole moment :- 

• Inversion
• Contain more than one Cn axis

Why meso compounds are achiral ? 

An achiral compound with chiral centers is referred to as a meso compound. A meso compound has two or more stereocenters but is optically inactive because it has an internal plane of symmetry that allows it to be superimposed on its mirror counterpart.