Optical Isomerism & Chirality – Comprehensive Bullet Notes

Isomerism: Same molecular formula, different structural formulae or different 3-D orientations of atoms/groups.
Major classes (contextual background):
- Structural isomerism (chain, position, functional, etc.)
- Stereoisomerism (geometric/cis–trans & optical).
  • Optical isomerism is the focus of this transcript.

Occurs when a molecule lacks a plane of symmetry.
Such molecules can exist in two distinct 3-D forms that are non-superimposable mirror images.
Key vocabulary:
- Optically active: Molecule able to rotate plane-polarised light.
- Enantiomers: The two non-superimposable mirror images.
Visual mnemonic suggested: Picture your left and right hands—they are mirror images but cannot be perfectly overlaid.

Chiral carbon (stereogenic centre): A carbon atom bonded to four different atoms or groups.
• Presence of at least one chiral carbon → molecule lacks an internal plane of symmetry → optical activity possible.
Practical identification tip: Scan every carbon; if any attached substituent repeats, that carbon is achiral.
Ring systems:
• Ring carbons bearing substituents and ring-junction carbons are frequently chiral because the cyclic path in each direction counts as a different group.

Plane-polarised light is obtained by passing light through a polariser so all waves oscillate in a single plane.
Each enantiomer rotates this plane by an equal magnitude but in opposite directions:
- Dextrorotatory (d or +): rotates light to the right (clockwise).
- Laevorotatory (l or –): rotates light to the left (anticlockwise).
The magnitude of rotation (
$\alpha$ ) is measured with a polarimeter.

Racemic mixture (racemate): $50\%$ of the $d$ enantiomer + $50\%$ of the $l$ enantiomer.
• Net optical rotation $=0$ because opposite rotations cancel.
Optical excess (enantiomeric excess): Any mixture other than exactly 50-50.
• Net rotation observed.
• Percentage composition can be determined by comparing measured rotation $\alpha{\text{mix}}$ with the rotation of the pure enantiomer $\alpha{\text{pure}}$ :
$\text{ee}\,(\%) = \left(\frac{|\alpha{\text{mix}}|}{|\alpha{\text{pure}}|}\right) \times 100$

If a molecule has $n$ independent chiral centres and no other symmetry elements, the maximum number of optical isomers (stereoisomers) is:
$2^n$
Examples stressed in the transcript:
• $n=2 \Rightarrow 2^2 = 4$ optical isomers possible.
• $n=3 \Rightarrow 2^3 = 8$ optical isomers possible.
Caveat: The presence of an internal mirror plane or meso forms can reduce the actual count.

Identify chiral carbon(s) in any given structure.
Decide whether the molecule is optically active.
Draw correct 3-D (wedge-dash or perspective) structures of both enantiomers.
State that enantiomers are non-superimposable mirror images.
Given a molecular structure, predict the total number of optical isomers.
Define a racemic mixture clearly.
Solve practice questions such as:
• “Identify if the following compounds exhibit optical isomerism.”
• “If the compound contains two chiral carbons, how many optical isomers can exist?”

In pharmaceutical chemistry, one enantiomer may be therapeutic while the other is inactive or harmful (e.g., thalidomide tragedy).
Regulatory agencies require enantiomeric purity data—reinforcing the importance of polarimetry & chiral synthesis.

Polarimeter output: angle $\alpha$ read directly, sign indicates direction (+ or –).
Accuracy improved by using sodium D-line (589 nm) as standard wavelength.

Isomerism → Stereoisomerism → Optical isomerism.
Chiral carbon = four different groups.
No plane of symmetry → optically active.
Two enantiomers: $d$ (+) & $l$ (–).
Racemic mixture (50-50) → $\alpha = 0$ .
Optical/excess formula: $\text{ee}\,(\%) = \left(\tfrac{|\alpha{\text{mix}}|}{|\alpha{\text{pure}}|}\right)!\times 100$ .
Max isomers formula: $2^n$ (when $n$ = no. of chiral centres).