Elements of Symmetry and Chiral Molecules Explained

Elements of Symmetry and Chiral Molecules and a chiral molecules Explained very Easy way .

1. Elements of Symmetry

Definition:

• Elements of symmetry are features in a molecule that divide it into identical halves.

Main types:

1. Plane of symmetry (σ):
   • A plane that divides the molecule into two identical halves.
   • Example: CH2Cl2 has a plane of symmetry.
2. Center of symmetry (i):
   • A point in the molecule where every part has an identical part directly opposite.
   • Example: BrCH2CH2Br has a center of symmetry.
3. Axis of symmetry (Cn):
   • An axis around which the molecule can be rotated n times to return to the original position.
   • Example: Benzene has a C6 axis.
4. Rotary-reflection axis (Sn):
   • Combination of rotation followed by reflection in a plane perpendicular to the rotation axis.

Important Exam Note:

• Chirality Rule:
  • Molecules with plane or center of symmetry → achiral
  • Molecules without symmetry → chiral

2. Chiral Molecules

Definition:

• Molecules that cannot be superimposed on their mirror images and have at least one chiral center are called chiral molecules.
• Chiral molecules are optically active.

Criteria for Chirality:

1. At least one chiral center (carbon with 4 different groups)
2. No plane of symmetry
3. No center of symmetry

Examples:

• Lactic acid: CH3-CHOH-COOH → chiral
• 2-Butanol: CH3-CH(OH)-CH2-CH3 → chiral
• Most amino acids (except glycine) → chiral

3. Achiral Molecules

Definition:

• Molecules whose mirror image is superimposable or have plane or center of symmetry are achiral.
• Achiral molecules are optically inactive.

Examples:

• CH2Cl2 → plane of symmetry → achiral
• (R,S)-Tartaric acid → symmetric → achiral (meso compound)

Key Points:

• Chiral molecules: optically active
• Achiral molecules: optically inactive
• Symmetry elements are used to determine chirality

Exam-Oriented Summary Table:

Feature	Chiral Molecule	Achiral Molecule	Example
Plane of symmetry	No	Yes	CH2Cl2 (achiral)
Center of symmetry	No	Yes	BrCH2CH2Br (achiral)
Chirality centers	≥ 1	0 or symmetric	Lactic acid (chiral), Glycine (achiral)
Optical activity	Yes	No	2-Butanol (chiral)

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