Symmetry and Stereochemistry There are three elements of symmetry that a molecule may possess …
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Symmetry and Stereochemistry
There are three elements of symmetry that a molecule may possess…
C C
O H
H
HH
H
H
C C
O H
H
HH
H
Hσ
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.
The blue plane is a plane of symmetry of A.
The operation of reflection (σ) involves projection of each atom onto the plane, followed by movement through the plane to a distance equal to the projection distance.
A C C
O H
H
HH
H
H
Animations of the reflection process
Reflection through the plane is a symmetry operation on A. Although the red and magenta H’s have changed places, the molecule looks the same. We say that A possesses reflection symmetry. C C
O H
H
HH
H
H
C C
O H
H
HH
H
Hσ
Symmetry Operations: Rotation
The red axis is an axis of symmetry of A.
The operation of rotation (Cn) involves rotation of the molecule 360/n degrees about an axis. The axis shown is a “C2” axis.
Animations of the rotation process
Rotation about the axis is a symmetry operation on A. Although the red and magenta H’s have changed places, the molecule looks the same. We say that A possesses 2-fold rotational symmetry.
O
H H
H H H H
180°
O
H H
H H H H
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.
A O
H H
H H H H
H
H
CH3
Br
H3C
Br
Symmetry Operations: Inversion
The blue point is an inversion center of G.
The operation of inversion (i) involves the projection of each atom onto a point at the center of the molecule, followed by movement through the point to a distance equal to the projection distance.
G
Animations of the inversion process
Inversion through the point is a symmetry operation on G. Although inversion exchanges all three groups attached to the central carbon, the molecule looks the same. We say that G possesses inversion symmetry.
H
H
CH3
Br
H3C
Br
H
H
CH3
Br
H3C
Br
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.
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