Lecture No. 07
Elements of symmetry
Session Objectives
By the end of this session, students will be able to:
➢What are elements of symmetry
➢Types of symmetry
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Symmetry and Stereochemistry
There are three elements of symmetry
that a molecule may possess…
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Symmetry Operations: Reflection
Symmetry operations are spatial transformations (rotations,
reflections, inversions). A molecule is said to possess a symmetry
element if the molecule is unchanged in appearance after applying the
symmetry operation corresponding to the symmetry element.
H H The blue plane is a plane of symmetry of A.
A O
H H
C The operation of reflection (М) involves
C projection of each atom onto the plane, followed
by movement through the plane to a distance
H H equal to the projection distance.
H H H H
O М O
H H H H
C C
C C
H H H H
Reflection through the plane is a symmetry
operation on A. Although the red and magenta
М H’s have changed places, the molecule looks
H O H H O H the same. We say that A possesses reflection
H H H H symmetry.
C C
C C
H H H H Animations of the reflection process
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Symmetry Operations: Rotation
Symmetry operations are spatial transformations (rotations,
reflections, inversions). A molecule is said to possess a symmetry
element if the molecule is unchanged in appearance after applying the
symmetry operation corresponding to the symmetry element.
Animations of the rotation process The red axis is an axis of symmetry of A.
O The operation of rotation (Cn) involves rotation of
A H H the molecule 360/n degrees about an axis.
The axis shown is a “C2” axis.
H H H H
Rotation about the axis is a symmetry
O O operation on A. Although the red and magenta
H’s have changed places, the molecule looks
H H H H the same. We say that A possesses 2-fold
rotational symmetry.
H H H H H H H H
180°
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Elements of symmetry
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Symmetry Operations: Inversion
Symmetry operations are spatial transformations (rotations,
reflections, inversions). A molecule is said to possess a symmetry
element if the molecule is unchanged in appearance after applying the
symmetry operation corresponding to the symmetry element.
H3C H The blue point is an inversion center of G.
Br The operation of inversion (i) involves the
G projection of each atom onto a point at the center
of the molecule, followed by movement through the
Br point to a distance equal to the projection distance.
H CH3
H3C H H3CH Inversion through the point is a symmetry
Br Br operation on G. Although inversion exchanges all
three groups attached to the central carbon, the
Br Br molecule looks the same. We say that G
H CH3 H CH3 possesses inversion symmetry.
Animations of the inversion process
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Summary
• Chirality is only the criteria for optical isomerism
• The molecule should lack these elements of symmetry to possess optical isomerism
• Different types of symmetry
Plane of symmetry – reflection
Axis of symmetry – rotation
Centre of symmetry – inversion centre
Alternating axis of symmetry
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Thank You
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