Lecture on Special Cases of Chiral and Achiral Molecules

Introduction to Chirality

Definition of Chirality:
- Requires left and right-handed forms that are mirror images.
- These forms must be non-superimposable.
Key Points:
- Chiral centers alone do not determine chirality.
- Absence of chiral centers does not mean a molecule is achiral.

Conformational Mobility:
- Organic molecules can change conformation.
- A molecule is achiral if it equilibrates with an achiral conformation.
Determining Achirality:
- Look for a plane of symmetry in the most symmetric conformation.

Structure:
- Most symmetric conformation is a flat hexagonal ring with a mirror plane of symmetry.
- Chair conformation shows non-superimposable images.
Chirality Assessment:
- Achiral as it can interconvert through a ring flip making images superimposable.
- Example of a molecule with chiral centers but is achiral.

Conformational Enantiomers:
- Mirror image isomers that cannot interconvert due to steric or ring strain.
- Example: Substituted biphenyl rings with bulky groups preventing specific conformations.
- Non-superimposable mirror images due to inability to rotate around bonds.
Allenes:
- Structures containing consecutive carbon-carbon double bonds.
- Central carbon is sp-hybridized, end carbons are sp2-hybridized.
- Planes of trigonal planar carbons are perpendicular to each other.
- Example: Propadiene is achiral, but more complex allenes like penta-2,3-diene are chiral.

Chirality does not solely depend on the presence of chiral centers.
Conformational mobility and geometric arrangements can influence chirality.
Special cases like conformational enantiomers and allenes highlight unusual scenarios in chirality.