Topic 5 of 34
Chirality is non-superimposability on a mirror image
A tetrahedral centre with four different groups is a common cause, but molecular symmetry still matters.
A-Level 9476 (2026-2027)
Chirality is non-superimposability on a mirror image
A tetrahedral centre with four different groups is a common cause, but molecular symmetry still matters.
A chiral centre is commonly a tetrahedral carbon bonded to four different atoms or groups. In butan-2-ol, the OH-bearing carbon is attached to OH, H, CH3 and CH2CH3, giving two non-superimposable mirror-image arrangements called enantiomers. Compare complete groups, not just the first attached atom: methyl and ethyl both begin with carbon but are different groups.
A pair of enantiomers shown with wedges
Each tetrahedral carbon has CH3 and H attached by two ordinary bonds in the plane of the page, with a solid wedge to OH towards the viewer and a hashed wedge to CO2H away. Reflection in the vertical mirror exchanges every left-right position, giving non-superimposable arrangements.
With several chiral centres, inspect the whole molecule. An internal plane of symmetry can make a structure achiral despite containing stereogenic centres. Conversely, a general molecule can be chiral for reasons other than a tetrahedral carbon centre; for the structures supplied, use the actual geometry and symmetry rather than treating a centre count as an infallible rule.
Two chiral centres, but an achiral whole molecule
One stereoisomer of butane-2,3-diol has a plane perpendicular to the page through the midpoint of the central carbon-carbon bond. Reflection exchanges the two central carbons, the two methyl groups, the two OH groups projecting towards the viewer and the two H atoms pointing away. The complete structure maps onto itself.
Worked example
Check centres first, then the whole structure
Is the illustrated stereoisomer of CH3CH(OH)CH(OH)CH3 chiral?
- At either starred carbon the four groups are H, OH, CH3 and CH(OH)CH3. Each is therefore a chiral centre.
- Reflect the entire structure in the marked plane. A front-facing OH maps to the other front-facing OH, and a rear H to the other rear H; the carbon skeleton and methyl groups also match.
- The molecule has an internal plane of symmetry, so its mirror image is superimposable. Counting two centres alone would give the wrong conclusion.
- If the OH and H positions at only one centre are exchanged, this internal plane is lost and a different stereoisomer results. It is essential to inspect the wedges, not only the condensed formula.
This illustrated stereoisomer is achiral despite its two chiral centres. It is often called a meso form; the symmetry reasoning is what establishes the answer.
| Property | Comparison |
|---|---|
| Melting point, boiling point and ordinary solubility | Identical under the same achiral conditions. |
| Plane-polarised light | Equal concentrations of pure enantiomers rotate it by equal magnitudes in opposite directions under matching conditions. |
| Reactions with achiral reagents | The same ordinary chemical properties and rates under matching conditions. |
| Interactions with chiral molecules | Can differ, for example in binding to an enzyme or receptor. |
An optically active sample rotates plane-polarised light and contains chiral molecules with a non-cancelling composition. A racemic mixture contains equal amounts of two enantiomers, so their rotations cancel. Optical inactivity alone therefore does not prove that all molecules in the sample are achiral. The sign of rotation must be measured; it is not read directly from a wedge drawing.
Biological receptors and enzymes are chiral. Two stereoisomers may bind differently and therefore differ in activity, side effects or metabolism. This explains why a molecular formula and functional-group list alone cannot establish a drug's behaviour; three-dimensional arrangement matters. No claim that one particular handedness is always beneficial follows from this principle.