Optical Isomerism and Chirality

This guide covers subtopic 29.4, Isomerism: optical, from Topic 29, An introduction to A Level organic chemistry, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content.

Before studying this

This resource assumes basic isomerism ideas (structural isomerism) and tetrahedral carbon geometry from earlier AS Level organic resources, particularly Hydrocarbons: Alkanes and Alkenes. It does not require A Level Organic Chemistry: Naming, Mechanisms and Aromatic Shape first, though both are part of Topic 29.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 29

29.4 Isomerism: optical — understanding that enantiomers have identical physical and chemical properties apart from their ability to rotate plane polarised light and their potential biological activity; understanding and using the terms optically active and racemic mixture; describing the effect on plane polarised light of the two optical isomers of a substance; explaining the relevance of chirality to synthetic drug preparation, including different biological activity of the two enantiomers, the need to separate a racemic mixture into pure enantiomers, and the use of chiral catalysts to produce a single pure optical isomer. (Meso compounds and diastereoisomer nomenclature are not required, though a compound can contain more than one chiral centre.)

Chirality and enantiomers

A carbon atom bonded to four different groups is a chiral centre — there is no way to rotate one arrangement of the four groups around that carbon to superimpose it exactly onto its mirror image. The molecule and its mirror image are called enantiomers: non-superimposable mirror images of each other, related the way a left hand is related to a right hand.

Worked example. Butan-2-ol, CH₃CH(OH)CH₂CH₃, has a chiral centre at carbon 2. Identify the four different groups attached to it.

The four groups attached to C2 are: —OH, —H, —CH₃, and —CH₂CH₃ — all four are different from each other, confirming C2 is a chiral centre and butan-2-ol exists as a pair of enantiomers.

Enantiomers have identical physical and chemical properties — the same melting point, boiling point, solubility, and the same reactivity with any non-chiral reagent — because those properties depend only on the strength and type of bonding and intermolecular forces present, which are identical between mirror-image structures. The two ways enantiomers differ are:

• Optical activity: each enantiomer rotates the plane of plane polarised light by an equal angle, but in opposite directions — one rotates it clockwise (dextrorotatory), the other anticlockwise (laevorotatory). A single pure enantiomer is described as optically active.
• Biological activity: because biological receptors, enzymes and other chiral molecules in living systems only fit one specific three-dimensional shape, the two enantiomers of a drug can have completely different biological effects from each other.

Plane polarised light and racemic mixtures

Ordinary light vibrates in every possible plane perpendicular to its direction of travel; passing it through a polarising filter selects only the light vibrating in one plane — plane polarised light. Passing plane polarised light through a sample of a single optically active enantiomer rotates the plane of polarisation by a measurable angle; passing it through the other enantiomer rotates the plane by the same angle in the opposite direction.

A racemic mixture is an exactly equal (50:50) mixture of both enantiomers. Because the two equal-and-opposite rotations exactly cancel, a racemic mixture shows no net optical activity — it appears optically inactive even though it’s built entirely from optically active components.

Many laboratory syntheses that create a new chiral centre (for example, by nucleophilic addition of HCN to an unsymmetrical aldehyde or ketone, forming a hydroxynitrile — see Carbonyl Compounds: Aldehydes and Ketones) produce both enantiomers in equal amounts by default: the attacking nucleophile is equally likely to approach the flat, planar carbonyl carbon from either face, giving a racemic mixture rather than a single pure enantiomer.

Chirality in drug synthesis

Chirality matters enormously in pharmaceutical chemistry, for three linked reasons the syllabus specifically expects:

The two enantiomers of a drug can have different biological activity. One enantiomer might produce the intended therapeutic effect while the other is inactive, less effective, or — in more serious cases — harmful. This makes controlling which enantiomer (or mixture) reaches a patient a genuine safety and efficacy question, not just an academic one.

Separating a racemic mixture into its two pure enantiomers is often necessary before a drug can be sold, since regulators increasingly require evidence about each enantiomer’s effects individually. This separation is harder than separating ordinary (non-mirror-image) isomers, precisely because enantiomers share every physical property (boiling point, solubility, etc.) that conventional separation techniques like distillation or simple recrystallisation rely on — specialised chiral separation techniques are needed instead.

Chiral catalysts offer an alternative to separating a racemic mixture after the fact: a catalyst that is itself chiral can favour the formation of one enantiomer over the other during the reaction itself (asymmetric synthesis), producing a single, predominantly pure optical isomer directly and avoiding the need to separate a racemic product afterwards.

(Molecules with more than one chiral centre can exist, and the syllabus expects awareness of this — but classifying the further relationships between such isomers, including meso compounds and the term “diastereoisomer,” is not required.)

Common mistakes

Assuming enantiomers differ in melting point, boiling point or ordinary chemical reactivity. They don’t — those properties are identical between enantiomers; only optical activity and biological activity differ.

Describing a racemic mixture as “not optically active because it isn’t chiral.” A racemic mixture is optically inactive because the equal and opposite rotations of its two chiral components exactly cancel — the individual molecules are still just as chiral as ever.

Forgetting why HCN addition to an unsymmetrical carbonyl gives a racemic product by default. The nucleophile can attack the flat carbonyl carbon from either face with equal probability, unless something in the reaction (such as a chiral catalyst) biases one face over the other.

Assuming any carbon with an OH group attached is automatically chiral. Check all four attached groups are genuinely different — a carbon bonded to two identical groups (e.g. propan-2-ol’s central carbon, bonded to two identical CH₃ groups) is not a chiral centre.

Quick revision checklist

• Chiral centre: carbon bonded to four different groups
• Enantiomers: non-superimposable mirror images; identical physical/chemical properties except optical and biological activity
• Optically active: rotates plane polarised light; opposite enantiomers rotate it in opposite directions by equal amounts
• Racemic mixture: equal amounts of both enantiomers; no net optical activity
• Drug relevance: enantiomers can have different biological activity; racemic mixtures may need separating; chiral catalysts can produce one pure enantiomer directly

Related resources

• A Level Organic Chemistry: Naming, Mechanisms and Aromatic Shape — the rest of Topic 29
• Carbonyl Compounds: Aldehydes and Ketones — HCN addition, a reaction that creates a new chiral centre
• Transition Elements: Colour, Stereoisomerism and Stability Constants — optical isomerism in transition-metal complexes, the same underlying idea applied to a different kind of centre
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.