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.
