**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

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

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