Optical isomerism

Chemistry· 12 min read

1. Chiral Centres: The Origin of Optical Isomerism★★☆☆☆⏱ 3 min

Optical isomerism arises exclusively from molecules that have no internal plane of symmetry, making them non-superimposable on their mirror image. For CIE A Level 9701, this almost always occurs when a sp³ hybridised carbon atom is bonded to four completely distinct groups.

📘 Definition

Chiral (asymmetric) carbon centre

C*

A tetrahedral sp³ carbon atom covalently bonded to four different atoms or functional groups, with no plane of symmetry passing through the atom.

Example:

The C2 carbon in 2-butanol, bonded to -H, -OH, -CH₃ and -C₂H₅.

Ignore sp² hybridised carbons (double bonded to O or C) as they cannot form tetrahedral 4-group arrangements
Explicitly list all four groups attached to a candidate carbon to check for duplicates
Do not count carbon atoms in alkyl chains with two identical adjacent groups as chiral

📐 Worked Example

Identify all chiral centres in 2,3-dihydroxybutanedioic acid (tartaric acid)

1
Step 1: Draw the full displayed structure: HOOC-CH(OH)-CH(OH)-COOH
2
Step 2: Eliminate the two terminal carboxylic acid carbons, which are sp² hybridised
3
Step 3: Check C2: bonded to -COOH, -H, -OH, and -CH(OH)COOH: all four groups are distinct, so it is chiral
4
Step 4: Check C3: bonded to -COOH, -H, -OH, and -CH(OH)COOH: all four groups are distinct, so it is chiral

2. Properties of Enantiomers★★★☆☆⏱ 3 min

Enantiomers have identical physical properties (boiling point, melting point, solubility) except for their interaction with plane-polarised light. They also have identical chemical properties unless reacting with another chiral substance.

[\alpha]_\lambda^T = \frac{\alpha}{l \times c}

📘 Definition

Specific rotation

A standardised measure of how much an enantiomer rotates plane-polarised light, corrected for path length and concentration.

📐 Worked Example

Pure (S)-lactic acid has a specific rotation of -3.8°. Calculate the observed rotation of a 1 g/cm³ solution in a 1 dm path length cell.

1
Step 1: Rearrange the specific rotation formula to solve for observed rotation $α$
2
3
Step 2: Substitute the given values: $[α] = - 3.8°$ , $l = 1$ dm, $c = 1$ g/cm³
4
Step 3: Calculate result: $α = - 3.8 \times 1 \times 1 = - 3.8°$ , corresponding to 3.8° anticlockwise rotation

3. Racemic Mixtures★★★☆☆⏱ 3 min

A racemic mixture (or racemate) is an equal 50:50 blend of two opposite enantiomers. The equal and opposite rotation of plane-polarised light from each enantiomer cancels out completely, so the mixture shows zero net optical activity.

📐 Worked Example

Explain why nucleophilic addition of HCN to propanal produces an optically inactive product mixture

1
Step 1: The carbonyl group in propanal is planar, so CN⁻ nucleophiles can attack from above or below the plane with equal probability
2
Step 2: Attack from one face generates the (+) enantiomer of 2-hydroxypropanenitrile, attack from the opposite face generates the (-) enantiomer
3
Step 3: Equal quantities of both enantiomers form, creating a perfect 50:50 racemic mixture
4
Step 4: Equal and opposite rotations of plane-polarised light cancel completely, so no net optical activity is observed

4. Meso Compounds★★★★☆⏱ 3 min

Meso compounds are molecules that contain two or more chiral centres, but have an internal plane of symmetry that makes the entire molecule achiral and optically inactive. This is a common trick question in CIE Paper 4.

Property	Pure enantiomer	Racemic mixture	Meso compound
Chiral centres present?	Yes	Yes	Yes
Internal plane of symmetry?	No	No (individual molecules)	Yes
Optically active?	Yes	No	No

✓ Quick check

Test your understanding of core rules:

Which of the following molecules is optically active?
- Pure 2-chlorobutane
- 50:50 mix of (+) and (-) 2-chlorobutane
- Meso tartaric acid
- Propanal
Reveal answer
Pure 2-chlorobutane —
Only the single pure enantiomer has no plane of symmetry and shows net optical activity.

5. Common Pitfalls

Wrong move:

Marking a carbon bonded to two identical groups as chiral

Why:

Students often miss repeated alkyl groups e.g. two -CH₃ groups attached to the same central carbon

Correct move:

Explicitly list all four groups attached to a candidate sp³ carbon before confirming it is chiral

Wrong move:

Drawing enantiomers as flat 2D mirror images

Why:

Markers cannot confirm you understand non-superimposability without 3D representation

Correct move:

Always use wedge notation for groups coming out of the page, dash for groups going behind the page

Wrong move:

Stating racemic mixtures contain no chiral molecules

Why:

Individual molecules in the racemate are fully chiral, their rotations just cancel out

Correct move:

Specify that equal amounts of both enantiomers produce zero net optical activity

Wrong move:

Assuming all molecules with two chiral centres are chiral

Why:

Meso compounds have internal symmetry that cancels out optical activity

Correct move:

Check for a plane of symmetry across the full molecule even if chiral centres are present

Wrong move:

Claiving S configuration = laevorotatory and R configuration = dextrorotatory

Why:

(+)/(-) is an experimental measurement, S/R is an arbitrary naming convention with no direct link

Correct move:

Never connect Cahn-Ingold-Prelog labels to direction of light rotation in exam answers

6. Quick Reference Cheatsheet

Property	Pure single enantiomer	Racemic mixture	Meso compound
Chiral centres present?	Yes	Yes	Yes
Plane of symmetry?	No	No	Yes
Optically active?	Yes	No	No
Melting point	Sharp	Often different from pure enantiomer	Sharp

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 42
Identify chiral centres in amino acid structure
2022 · 33
Explain optical inactivity of racemic product
2021 · 21
Draw 3D structure of 2-butanol enantiomer

What's Next

Mastering optical isomerism is a critical foundation for advanced CIE A Level organic chemistry, as this concept is frequently combined with reaction mechanisms, amino acid structure, and drug stereochemistry to create extended 6-8 mark structured questions. You will regularly be asked to predict the optical activity of products formed from nucleophilic addition or substitution reactions, a standard high-mark question in Paper 4. This topic also overlaps heavily with geometric isomerism, so you will need to distinguish between the two classes of stereoisomerism to avoid losing easy marks. Next, practice applying this knowledge to related stereochemistry topics.