**Elements of Symmetry**

__Session Objectives__

**By the end of this
session, students will be able to:**

• What are elements of symmetry?

• Types of symmetry

__Symmetry
and Stereochemistry__

__Symmetry and Stereochemistry__

• There are three **elements of symmetry** that a
molecule may __possess__…

1. Rotation

2. Reflection

3. Inversion

__Symmetry Operations: Reflection__

__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 (М) **involves projection
of each atom onto the plane, followed by movement through the plane to a
distance equal to the projection distance.

• 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.

• **Animations **of the reflection process

**Elements
of symmetry**

__Symmetry Operations: Rotation__

__Symmetry Operations: Rotation__

• 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 red axis is an **axis of symmetry **of **A**.

• The operation of **rotation (C _{n}) **involves
rotation of the molecule 360/n degrees about an axis.

• The axis shown is a “C_{2}” axis.

• 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.

**Elements
of symmetry**

__Symmetry Operations: Inversion__

__Symmetry Operations: Inversion__

• 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 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.

• 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.

__Alternating
axis of symmetry (Sn) or Rotation reflection axis of Symmetry or improper axis
of Symmetry__

__Alternating axis of symmetry (Sn) or Rotation reflection axis of Symmetry or improper axis of Symmetry__

• A molecular has an alternating axis of symmetry of order (n) if rotation about the axis by 360/n degree following by reflaction in a plane perpendicular to this axis produces an equivalent structure.

__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

## 0 Comments